This chapter lists some common problems and error messages that users have run into. You will learn how to figure out what the problem is, and what to do to solve it. You will also find proper solutions to some common problems.
When you run into problems, the first thing you should do is to find out which program / piece of equipment is causing problems:
kbd_mode -aon it.
taskmanager, or some similar program, to check which program is taking all CPU or is locking the machine.
df, or a similar program if you are out of memory, disk space, open files, or some other critical resource.
If after you have examined all other possibilities and you have concluded that it's the MySQL server or a MySQL client that is causing the problem, it's time to do a bug report for our mailing list or our support team. In the bug report, try to give a very detailed description of how the system is behaving and what you think is happening. You should also state why you think it's MySQL that is causing the problems. Take into consideration all the situations in this chapter. State any problems exactly how they appear when you examine your system. Use the 'cut and paste' method for any output and/or error messages from programs and/or log files!
Try to describe in detail which program is not working and all symptoms you see! We have in the past received many bug reports that just state "the system doesn't work". This doesn't provide us with any information about what could be the problem.
If a program fails, it's always useful to know:
top. Let the program run for a while, it may be evaluating something heavy.
mysqldserver that is causing problems, can you do
mysqladmin -u root pingor
mysqladmin -u root processlist?
mysql, for example) when you try to connect to the MySQL server? Does the client jam? Do you get any output from the program?
When sending a bug report, you should of follow the outlines described in this manual. See section 126.96.36.199 Asking Questions or Reporting Bugs.
MySQL server has gone awayerror
Can't connect to [local] MySQL servererror
Host '...' is blockederror
Too many connectionserror
Some non-transactional changed tables couldn't be rolled backError
Out of memoryerror
Packet too largeerror
The table is fullerror
Can't create/write to fileError
Commands out of syncerror in client
Table 'xxx' doesn't existerror
Can't initialize character set xxxerror.
This section lists some errors that users frequently get. You will find descriptions of the errors, and how to solve the problem here.
See section 4.2.5 How the Privilege System Works, and especially. See section 4.2.10 Causes of
Access denied Errors.
MySQL server has gone awayError
This section also covers the related
Lost connection to server
during query error.
The most common reason for the
MySQL server has gone away error
is that the server timed out and closed the connection. By default, the
server closes the connection after 8 hours if nothing has happened. You
can change the time limit by setting the
wait_timeout variable when
Another common reason to receive the
MySQL server has gone away error
is because you have issued a ``close'' on your MySQL connection
and then tried to run a query on the closed connection.
You can check that the MySQL hasn't died by executing
mysqladmin version and examining the uptime.
If you have a script, you just have to issue the query again for the client to do an automatic reconnection.
You normally can get the following error codes in this case (which one you get is OS-dependent):
|The client couldn't send a question to the server.|
|The client didn't get an error when writing to the server, but it didn't get a full answer (or any answer) to the question.|
You can also get these errors if you send a query to the server that is
incorrect or too large. If
mysqld gets a packet that is too large
or out of order, it assumes that something has gone wrong with the client and
closes the connection. If you need big queries (for example, if you are
working with big
BLOB columns), you can increase the query limit by
mysqld with the
-O max_allowed_packet=# option
(default 1M). The extra memory is allocated on demand, so
use more memory only when you issue a big query or when
return a big result row!
Can't connect to [local] MySQL servererror
A MySQL client on Unix can connect to the
mysqld server in two
different ways: Unix sockets, which connect through a file in the file
system (default `/tmp/mysqld.sock') or TCP/IP, which connects
through a port number. Unix sockets are faster than TCP/IP but can only
be used when connecting to a server on the same computer. Unix sockets
are used if you don't specify a hostname or if you specify the special
On Windows you can connect only with TCP/IP if the
is running on Win95/Win98. If mysqld is running on NT and started with
enable-named-pipe, you can also connect with named pipes. The
name of the named pipe is MySQL. If you don't give a hostname when
mysqld, a MySQL client will first try to connect to
the named pipe, and if this doesn't work it will connect to the TCP/IP
port. You can force the use of named pipes on Windows by using
as the hostname.
The error (2002)
Can't connect to ... normally means that there
isn't a MySQL server running on the system or that you are
using a wrong socket file or TCP/IP port when trying to connect to the
Start by checking (using
ps or the task manager on Windows) that
there is a process running named
mysqld on your server! If there
mysqld process, you should start one. See section 2.4.2 Problems Starting the MySQL Server.
mysqld process is running, you can check the server by
trying these different connections (the port number and socket pathname
might be different in your setup, of course):
shell> mysqladmin version shell> mysqladmin variables shell> mysqladmin -h `hostname` version variables shell> mysqladmin -h `hostname` --port=3306 version shell> mysqladmin -h 'ip for your host' version shell> mysqladmin --socket=/tmp/mysql.sock version
Note the use of backquotes rather than forward quotes with the
command; these cause the output of
hostname (that is, the current
hostname) to be substituted into the
Here are some reasons the
Can't connect to local MySQL server
error might occur:
mysqldis not running.
mysqlduses the MIT-pthreads package. See section 2.2.2 Operating Systems Supported by MySQL. However, all MIT-pthreads versions doesn't support Unix sockets. On a system without sockets support you must always specify the hostname explicitly when connecting to the server. Try using this command to check the connection to the server:
shell> mysqladmin -h `hostname` version
mysqlduses (default `/tmp/mysqld.sock'). You might have a
cronjob that removes the MySQL socket (for example, a job that removes old files from the `/tmp' directory). You can always run
mysqladmin versionand check that the socket
mysqladminis trying to use really exists. The fix in this case is to change the
cronjob to not remove `mysqld.sock' or to place the socket somewhere else. See section A.4.5 How to Protect or change the MySQL socket file `/tmp/mysql.sock'.
mysqldserver with the
--socket=/path/to/socketoption. If you change the socket pathname for the server, you must also notify the MySQL clients about the new path. You can do this by providing the socket path as an argument to the client. See section A.4.5 How to Protect or change the MySQL socket file `/tmp/mysql.sock'.
mysqldthreads (for example, with the
mysql_zapscript before you can start a new MySQL server. See section A.4.1 What To Do If MySQL Keeps Crashing.
mysqldso that it uses a directory that you can access.
If you get the error message
Can't connect to MySQL server on
some_hostname, you can try the following things to find out what the
problem is :
telnet your-host-name tcp-ip-port-numberand press
RETURNa couple of times. If there is a MySQL server running on this port you should get a responses that includes the version number of the running MySQL server. If you get an error like
telnet: Unable to connect to remote host: Connection refused, then there is no server running on the given port.
mysqlddaemon on the local machine and check the TCP/IP port that
mysqldit's configured to use (variable
mysqldserver is not started with the
Host '...' is blockedError
If you get an error like this:
Host 'hostname' is blocked because of many connection errors. Unblock with 'mysqladmin flush-hosts'
this means that
mysqld has gotten a lot (
of connect requests from the host
'hostname' that have been interrupted
in the middle. After
max_connect_errors failed requests,
assumes that something is wrong (like an attack from a cracker), and
blocks the site from further connections until someone executes the command
mysqld blocks a host after 10 connection errors.
You can easily adjust this by starting the server like this:
shell> safe_mysqld -O max_connect_errors=10000 &
Note that if you get this error message for a given host, you should first
check that there isn't anything wrong with TCP/IP connections from that
host. If your TCP/IP connections aren't working, it won't do you any good to
increase the value of the
Too many connectionsError
If you get the error
Too many connections when you try to connect
to MySQL, this means that there is already
clients connected to the
If you need more connections than the default (100), then you should restart
mysqld with a bigger value for the
mysqld actually allows (
clients to connect. The last connection is reserved for a user with the
process privilege. By not giving this privilege to normal
users (they shouldn't need this), an administrator with this privilege
can log in and use
SHOW PROCESSLIST to find out what could be
wrong. See section 4.5.5
The maximum number of connects MySQL is depending on how good the thread library is on a given platform. Linux or Solaris should be able to support 500-1000 simultaneous connections, depending on how much RAM you have and what your clients are doing.
Some non-transactional changed tables couldn't be rolled backError
If you get the error/warning:
Warning: Some non-transactional
changed tables couldn't be rolled back when trying to do a
ROLLBACK, this means that some of the tables you used in the
transaction didn't support transactions. These non-transactional tables
will not be affected by the
The most typical case when this happens is when you have tried to create
a table of a type that is not supported by your
mysqld doesn't support a table type (or if the table type is
disabled by a startup option) , it will instead create the table type
with the table type that is most resembles to the one you requested,
You can check the table type for a table by doing:
SHOW TABLE STATUS LIKE 'table_name'. See section 188.8.131.52
SHOW TABLE STATUS.
You can check the extensions your
mysqld binary supports by doing:
show variables like 'have_%'. See section 184.108.40.206
Out of memoryError
If you issue a query and get something like the following error:
mysql: Out of memory at line 42, 'malloc.c' mysql: needed 8136 byte (8k), memory in use: 12481367 bytes (12189k) ERROR 2008: MySQL client ran out of memory
note that the error refers to the MySQL client
reason for this error is simply that the client does not have enough memory to
store the whole result.
To remedy the problem, first check that your query is correct. Is it
reasonable that it should return so many rows? If so,
you can use
mysql --quick, which uses
to retrieve the result set. This places less of a load on the client (but
more on the server).
Packet too largeError
When a MySQL client or the
mysqld server gets a packet bigger
max_allowed_packet bytes, it issues a
Packet too large
error and closes the connection.
If you are using the
mysql client, you may specify a bigger buffer by
starting the client with
If you are using other clients that do not allow you to specify the maximum
packet size (such as
DBI), you need to set the packet size when you
start the server. You cau use a command-line option to
mysqld to set
max_allowed_packet to a larger size. For example, if you are
expecting to store the full length of a
BLOB into a table, you'll need
to start the server with the
You can also get strange problems with large packets if you are using
big blobs, but you haven't given
mysqld access to enough memory
to handle the query. If you suspect this is the case, try adding
ulimit -d 256000 to the beginning of the
MySQL 3.23.40 you only get the
connection error of you start
If you find errors like the following in your error log.
010301 14:38:23 Aborted connection 854 to db: 'users' user: 'josh'
See section 4.9.1 The Error Log.
This means that something of the following has happened:
interactive_timeoutwithout doing any requests. See section 220.127.116.11
When the above happens, the server variable
The server variable
Aborted_connects is incremented when:
connect_timeoutseconds to get a connect package.
Note that the above could indicate that someone is trying to break into your database!
See section 18.104.22.168
Other reasons for problems with Aborted clients / Aborted connections.
max_allowed_packetis too small or queries require more memory than you have alloacated for
mysqld. See section A.2.8
Packet too largeError.
The table is fullError
This error occurs in older MySQL versions when an in-memory temporary
table becomes larger than
tmp_table_size bytes. To avoid this
problem, you can use the
-O tmp_table_size=# option to
mysqld to increase the temporary table size or use the SQL
SQL_BIG_TABLES before you issue the problematic
query. See section 5.5.6
You can also start
mysqld with the
This is exactly the same as using
SQL_BIG_TABLES for all queries.
In MySQL Version 3.23, in-memory temporary tables will automatically be
converted to a disk-based
MyISAM table after the table size gets
Can't create/write to fileError
If you get an error for some queries of type:
Can't create/write to file '\\sqla3fe_0.ism'.
this means that MySQL can't create a temporary file for the
result set in the given temporary directory. (The above error is a
typical error message on Windows, and the Unix error message is similar.)
The fix is to start
--tmpdir=path or to add to your option
assuming that the `c:\\temp' directory exists. See section 4.1.2 my.cnf Option Files.
Check also the error code that you get with
perror. One reason
may also be a disk full error;
shell> perror 28 Error code 28: No space left on device
Commands out of syncError in Client
If you get
Commands out of sync; You can't run this command now
in your client code, you are calling client functions in the wrong order!
This can happen, for example, if you are using
try to execute a new query before you have called
It can also happen if you try to execute two queries that return data without
mysql_store_result() in between.
If you get the following error:
Found wrong password for user: 'some_user@some_host'; Ignoring user
this means that when
mysqld was started or when it reloaded the
permissions tables, it found an entry in the
user table with
an invalid password. As a result, the entry is simply ignored by the
Possible causes of and fixes for this problem:
mysqldwith an old
usertable. You can check this by executing
mysqlshow mysql userto see if the password field is shorter than 16 characters. If so, you can correct this condition by running the
--old-protocoloption. Update the user in the
usertable with a new password or restart
usertable without using the
mysqlto update the user in the
usertable with a new password. Make sure to use the
mysql> update user set password=PASSWORD('your password') where user='XXX';
Table 'xxx' doesn't existError
If you get the error
Table 'xxx' doesn't exist or
find file: 'xxx' (errno: 2), this means that no table exists
in the current database with the name
Note that as MySQL uses directories and files to store databases and tables, the database and table names are case sensitive! (On Windows the databases and tables names are not case sensitive, but all references to a given table within a query must use the same case!)
You can check which tables you have in the current database with
SHOW TABLES. See section 4.5.5
Can't initialize character set xxxerror.
If you get an error like:
MySQL Connection Failed: Can't initialize character set xxx
This means one of the following things:
--with-extra-charsets=xxx. See section 2.3.3 Typical
configureOptions. All standard MySQL binaries are compiled with
--with-extra-character-sets=complexwhich will enable support for all multi-byte character sets. See section 4.6.1 The Character Set Used for Data and Sorting.
mysqldand the character set definition files is not in the place where the client expect to find them. In this case you need to:
If you get
ERROR '...' not found (errno: 23),
file: ... (errno: 24), or any other error with
errno 23 or
errno 24 from MySQL, it means that you haven't allocated
enough file descriptors for MySQL. You can use the
perror utility to get a description of what the error number
shell> perror 23 File table overflow shell> perror 24 Too many open files shell> perror 11 Resource temporarily unavailable
The problem here is that
mysqld is trying to keep open too many
files simultaneously. You can either tell
mysqld not to open so
many files at once or increase the number of file descriptors
mysqld to keep open fewer files at a time, you can make
the table cache smaller by using the
-O table_cache=32 option to
safe_mysqld (the default value is 64). Reducing the value of
max_connections will also reduce the number of open files (the
default value is 90).
To change the number of file descriptors available to
can use the option
-O open-files-limit=# to
mysqld. See section 22.214.171.124
The easiest way to do that is to add the option to your option file.
See section 4.1.2 my.cnf Option Files. If you have an old
mysqld version that
doesn't support this, you can edit the
safe_mysqld script. There
is a commented-out line
ulimit -n 256 in the script. You can
'#' character to uncomment this line, and change the
number 256 to affect the number of file descriptors available to
open-files-limit) can increase the number of
file descriptors, but only up to the limit imposed by the operating
system. There is also a 'hard' limit that can only be overrided if you
mysqld as root (Just remember that
you need to also use the
--user=.. option in this case). If you
need to increase the OS limit on the number of file descriptors
available to each process, consult the documentation for your operating
Note that if you run the
ulimit will not work!
tcsh will also report incorrect values when you ask for the current
limits! In this case you should start
If you are linking your program and you get errors for unreferenced
symbols that start with
mysql_, like the following:
/tmp/ccFKsdPa.o: In function `main': /tmp/ccFKsdPa.o(.text+0xb): undefined reference to `mysql_init' /tmp/ccFKsdPa.o(.text+0x31): undefined reference to `mysql_real_connect' /tmp/ccFKsdPa.o(.text+0x57): undefined reference to `mysql_real_connect' /tmp/ccFKsdPa.o(.text+0x69): undefined reference to `mysql_error' /tmp/ccFKsdPa.o(.text+0x9a): undefined reference to `mysql_close'
you should be able to solve this by adding
-lmysqlclient LAST on your link line.
If you get
undefined reference errors for the
compress function, add
-lz LAST on your link
line and try again!
If you get
undefined reference errors for functions that should
exist on your system, like
connect, check the man page for the
function in question, for which libraries you should add to the link
If you get
undefined reference errors for functions that don't
exist on your system, like the following:
mf_format.o(.text+0x201): undefined reference to `__lxstat'
it usually means that your library is compiled on a system that is not 100 % compatible with yours. In this case you should download the latest MySQL source distribution and compile this yourself. See section 2.3 Installing a MySQL Source Distribution.
If you are trying to run a program and you then get errors for
unreferenced symbols that start with
mysql_ or that the
mysqlclient library can't be found, this means that your system
can't find the share
The fix for this is to tell your system to search after shared libraries where the library is located by one of the following methods:
libmysqlclient.soto some place that is searched by your system, like `/lib', and update the shared library information by executing
Another way to solve this problem is to link your program statically, with
-static, or by removing the dynamic MySQL libraries
before linking your code. In the second case you should be
sure that no other programs are using the dynamic libraries!
The MySQL server
mysqld can be started and run by any user.
In order to change
mysqld to run as a Unix user
user_name, you must
do the following:
user_namehas privileges to read and write files in them (you may need to do this as the Unix
shell> chown -R user_name /path/to/mysql/datadirIf directories or files within the MySQL data directory are symlinks, you'll also need to follow those links and change the directories and files they point to.
chown -Rmay not follow symlinks for you.
user_name, or, if you are using MySQL Version 3.22 or later, start
mysqldas the Unix
rootuser and use the
mysqldwill switch to run as the Unix user
user_namebefore accepting any connections.
userline that specifies the user name to the
[mysqld]group of the `/etc/my.cnf' option file or the `my.cnf' option file in the server's data directory. For example:
At this point, your
mysqld process should be running fine and dandy as
the Unix user
user_name. One thing hasn't changed, though: the
contents of the permissions tables. By default (right after running the
permissions table install script
mysql_install_db), the MySQL
root is the only user with permission to access the
database or to create or drop databases. Unless you have changed those
permissions, they still hold. This shouldn't stop you from accessing
MySQL as the MySQL
root user when you're logged in
as a Unix user other than
root; just specify the
-u root option
to the client program.
Note that accessing MySQL as
root, by supplying
root on the command line, has nothing to do with MySQL running
as the Unix
root user, or, indeed, as another Unix user. The access
permissions and user names of MySQL are completely separate from
Unix user names. The only connection with Unix user names is that if you
don't provide a
-u option when you invoke a client program, the client
will try to connect using your Unix login name as your MySQL user
If your Unix box itself isn't secured, you should probably at least put a
password on the MySQL
root users in the access tables.
Otherwise, any user with an account on that machine can run
root db_name and do whatever he likes.
If you have problems with file permissions, for example, if
issues the following error message when you create a table:
ERROR: Can't find file: 'path/with/filename.frm' (Errcode: 13)
then the environment variable
UMASK might be set incorrectly when
mysqld starts up. The default umask value is
0660. You can
change this behavior by starting
safe_mysqld as follows:
shell> UMASK=384 # = 600 in octal shell> export UMASK shell> /path/to/safe_mysqld &
By default MySQL will create database and
directories with permission type 0700. You can modify this behavior by
UMASK_DIR variable. If you set this, new
directories are created with the combined
UMASK_DIR. For example, if you want to give group access to
all new directories, you can do:
shell> UMASK_DIR=504 # = 770 in octal shell> export UMASK_DIR shell> /path/to/safe_mysqld &
In MySQL Version 3.23.25 and above, MySQL assumes that the
UMASK_DIR is in octal if it starts
with a zero.
See section H Environment Variables.
All MySQL versions are tested on many platforms before they are released. This doesn't mean that there aren't any bugs in MySQL, but it means if there are bugs, they are very few and can be hard to find. If you have a problem, it will always help if you try to find out exactly what crashes your system, as you will have a much better chance of getting this fixed quickly.
First, you should try to find out whether the problem is that the
mysqld daemon dies or whether your problem has to do with your
client. You can check how long your
mysqld server has been up by
mysqladmin version. If
mysqld has died, you may
find the reason for this in the file
`mysql-data-directory/`hostname`.err'. See section 4.9.1 The Error Log.
Many crashes of MySQL are caused by corrupted index / data
files. MySQL will update the data on disk, with the
write() system call, after every SQL statement and before the
client is notified about the result. (This is not true if you are running
delayed_key_writes, in which case only the data is written.)
This means that the data is safe even if
mysqld crashes, as the OS will
ensure that the not flushed data is written to disk. You can force
MySQL to sync everything to disk after every SQL command by
The above means that normally you shouldn't get corrupted tables unless:
mysqldor the machine in the middle of an update.
mysqldthat caused it to die in the middle of an update.
mysqldservers on the same data on a system that doesn't support good file system locks (normally handled by the
lockddaemon ) or if you are running multiple servers with
ALTER TABLEon a repaired copy of the table!
Because it is very difficult to know why something is crashing, first try to check whether or not things that work for others crash for you. Please try the following things:
mysqladmin shutdown, run
myisamchk --silent --force */*.MYIon all tables, and restart the
mysqlddaemon. This will ensure that you are running from a clean state. See section 4 MySQL Database Administration.
mysqld --logand try to determine from the information in the log whether or not some specific query kills the server. About 95% of all bugs are related to a particular query! Normally this is one of the last queries in the log file just before MySQL restarted. See section 4.9.2 The General Query Log. If you can repeatadly kill MySQL with one of the queries, even when you have checked all tables just before doing the query, then you have been able to locate the bug and should do a bug report for this! See section 126.96.36.199 How to Report Bugs or Problems.
configureand then recompile. See section G.1 Debugging a MySQL server.
mysqld. On some systems, the
lockdlock manager does not work properly; the
mysqldnot to use external locking. (This means that you cannot run 2
mysqldservers on the same data and that you must be careful if you use
myisamchk, but it may be instructive to try the option as a test.)
mysqladmin -u root processlistwhen
mysqldappears to be running but not responding? Sometimes
mysqldis not comatose even though you might think so. The problem may be that all connections are in use, or there may be some internal lock problem.
mysqladmin processlistwill usually be able to make a connection even in these cases, and can provide useful information about the current number of connections and their status.
mysqladmin -i 5 statusor
mysqladmin -i 5 -r statusor in a separate window to produce statistics while you run your other queries.
gdb(or in another debugger). See section G.1.3 Debugging mysqld under gdb.
mysqldhas crashed inside gdb:
backtrace info local up info local up info localWith gdb you can also examine which threads exist with
info threadsand switch to a specific thread with
thread #, where
#is the thread id.
BLOB/TEXTcolumns (but only
VARCHARcolumns), you can try to change all
ALTER TABLE. This will force MySQL to use fixed-size rows. Fixed-size rows take a little extra space, but are much more tolerant to corruption! The current dynamic row code has been in use at MySQL AB for at least 3 years without any problems, but by nature dynamic-length rows are more prone to errors, so it may be a good idea to try the above to see if it helps!
If you have forgotten the
root user password for MySQL, you
can restore it with the following procedure:
mysqldserver by sending a
kill -9) to the
mysqldserver. The pid is stored in a
.pidfile, which is normally in the MySQL database directory:
kill `cat /mysql-data-directory/hostname.pid`You must be either the Unix
rootuser or the same user the server runs as to do this.
mysql -h hostname mysqland change the password with a
GRANTcommand. See section 4.3.1
REVOKESyntax. You can also do this with
mysqladmin -h hostname -u user password 'new password'
mysqladmin -h hostname flush-privilegesor with the SQL command
Note that after you started
any usage of
GRANT commands will give you an
error until you have executed
When a disk-full condition occurs, MySQL does the following:
To alleviate the problem, you can take the following actions:
mysqladmin killto the thread. The thread will be aborted the next time it checks the disk (in 1 minute).
Exceptions to the above behaveour is when you use
OPTIMIZE or when the indexes are created in a batch after an
LOAD DATA INFILE or after an
ALTER TABLE statement.
All of the above commands may use big temporary files that left to
themself would cause big problems for the rest of the system. If
MySQL gets disk full while doing any of the above operations,
it will remove the big temporary files and mark the table as crashed
ALTER TABLE, in which the old table will be left
MySQL uses the value of the
TMPDIR environment variable as
the pathname of the directory in which to store temporary files. If you don't
TMPDIR set, MySQL uses the system default, which is
normally `/tmp' or `/usr/tmp'. If the file system containing your
temporary file directory is too small, you should edit
TMPDIR to point to a directory in a file system where you have
enough space! You can also set the temporary directory using the
--tmpdir option to
MySQL creates all temporary files as hidden files. This ensures
that the temporary files will be removed if
mysqld is terminated. The
disadvantage of using hidden files is that you will not see a big temporary
file that fills up the file system in which the temporary file directory is
When sorting (
ORDER BY or
GROUP BY), MySQL normally
uses one or two temporary files. The maximum disk-space needed is:
(length of what is sorted + sizeof(database pointer)) * number of matched rows * 2
sizeof(database pointer) is usually 4, but may grow in the future for
really big tables.
SELECT queries, MySQL also creates temporary SQL
tables. These are not hidden and have names of the form `SQL_*'.
ALTER TABLE creates a temporary table in the same directory as
the original table.
If you have problems with the fact that anyone can delete the
MySQL communication socket `/tmp/mysql.sock', you can,
on most versions of Unix, protect your `/tmp' file system by setting
sticky bit on it. Log in as
root and do the following:
shell> chmod +t /tmp
This will protect your `/tmp' file system so that files can be deleted
only by their owners or the superuser (
You can check if the
sticky bit is set by executing
ls -ld /tmp.
If the last permission bit is
t, the bit is set.
You can change the place where MySQL uses / puts the socket file the following ways:
[client] socket=path-for-socket-file [mysqld] socket=path-for-socket-fileSee section 4.1.2 my.cnf Option Files.
safe_mysqldand most clients with the
--with-unix-socket-path=path-for-socket-file. See section 2.3.3 Typical
You can test that the socket works with this command:
shell> mysqladmin --socket=/path/to/socket version
If you have a problem with
SELECT NOW() returning values in GMT and
not your local time, you have to set the
TZ environment variable to
your current time zone. This should be done for the environment in which
the server runs, for example, in
See section H Environment Variables.
By default, MySQL searches are case-insensitive (although there are
some character sets that are never case insensitive, such as
That means that if you search with
col_name LIKE 'a%', you will get all
column values that start with
a. If you want to make this
search case-sensitive, use something like
INSTR(col_name, "A")=1 to
check a prefix. Or use
STRCMP(col_name, "A") = 0 if the column value
must be exactly
Simple comparison operations (
>=, >, = , < , <=, sorting and
grouping) are based on each character's ``sort value''. Characters with
the same sort value (like E, e and é) are treated as the same character!
In older MySQL versions
LIKE comparisons where done on
the uppercase value of each character (E == e but E <> é). In newer
LIKE works just like the other comparison
If you want a column always to be treated in case-sensitive fashion,
declare it as
BINARY. See section 6.5.3
CREATE TABLE Syntax.
If you are using Chinese data in the so-called big5 encoding, you want to
make all character columns
BINARY. This works because the sorting
order of big5 encoding characters is based on the order of ASCII codes.
The format of a
DATE value is
'YYYY-MM-DD'. According to ANSI
SQL, no other format is allowed. You should use this format in
expressions and in the WHERE clause of
SELECT statements. For
mysql> SELECT * FROM tbl_name WHERE date >= '1997-05-05';
As a convenience, MySQL automatically converts a date to a number if
the date is used in a numeric context (and vice versa). It is also smart
enough to allow a ``relaxed'' string form when updating and in a
clause that compares a date to a
DATE, or a
DATETIME column. (Relaxed form means that any punctuation character
may be used as the separator between parts. For example,
'1998#08#15' are equivalent.) MySQL can also convert a
string containing no separators (such as
'19980815'), provided it
makes sense as a date.
The special date
'0000-00-00' can be stored and retrieved as
'0000-00-00'. When using a
'0000-00-00' date through
MyODBC, it will automatically be converted to
MyODBC Version 2.50.12 and above, because ODBC can't handle this kind of
Because MySQL performs the conversions described above, the following statements work:
mysql> INSERT INTO tbl_name (idate) VALUES (19970505); mysql> INSERT INTO tbl_name (idate) VALUES ('19970505'); mysql> INSERT INTO tbl_name (idate) VALUES ('97-05-05'); mysql> INSERT INTO tbl_name (idate) VALUES ('1997.05.05'); mysql> INSERT INTO tbl_name (idate) VALUES ('1997 05 05'); mysql> INSERT INTO tbl_name (idate) VALUES ('0000-00-00'); mysql> SELECT idate FROM tbl_name WHERE idate >= '1997-05-05'; mysql> SELECT idate FROM tbl_name WHERE idate >= 19970505; mysql> SELECT mod(idate,100) FROM tbl_name WHERE idate >= 19970505; mysql> SELECT idate FROM tbl_name WHERE idate >= '19970505';
However, the following will not work:
mysql> SELECT idate FROM tbl_name WHERE STRCMP(idate,'19970505')=0;
STRCMP() is a string function, so it converts
a string and performs a string comparison. It does not convert
'19970505' to a date and perform a date comparison.
Note that MySQL does no checking whether or not the date is
correct. If you store an incorrect date, such as
wrong date will be stored. If the date cannot be converted to any reasonable
0 is stored in the
DATE field. This is mainly a speed
issue and we think it is up to the application to check the dates, and not
The concept of the
NULL value is a common source of confusion for
newcomers to SQL, who often think that
NULL is the same thing as an
''. This is not the case! For example, the following
statements are completely different:
mysql> INSERT INTO my_table (phone) VALUES (NULL); mysql> INSERT INTO my_table (phone) VALUES ("");
Both statements insert a value into the
phone column, but the first
NULL value and the second inserts an empty string. The
meaning of the first can be regarded as ``phone number is not known'' and the
meaning of the second can be regarded as ``she has no phone''.
In SQL, the
NULL value is always false in comparison to any
other value, even
NULL. An expression that contains
always produces a
NULL value unless otherwise indicated in
the documentation for the operators and functions involved in the
expression. All columns in the following example return
mysql> SELECT NULL,1+NULL,CONCAT('Invisible',NULL);
If you want to search for column values that are
cannot use the
=NULL test. The following statement returns no
expr = NULL is FALSE, for any expression:
mysql> SELECT * FROM my_table WHERE phone = NULL;
To look for
NULL values, you must use the
IS NULL test.
The following shows how to find the
NULL phone number and the
empty phone number:
mysql> SELECT * FROM my_table WHERE phone IS NULL; mysql> SELECT * FROM my_table WHERE phone = "";
In MySQL, as in many other SQL servers, you can't index
columns that can have
NULL values. You must declare such columns
NOT NULL. Conversely, you cannot insert
NULL into an indexed
When reading data with
LOAD DATA INFILE, empty columns are updated
''. If you want a
NULL value in a column, you should use
\N in the text file. The literal word
'NULL' may also be used
under some circumstances.
See section 6.4.8
LOAD DATA INFILE Syntax.
NULL values are presented first. If you
sort in descending order using
NULL values are presented
last. When using
GROUP BY, all
NULL values are regarded as
To help with
NULL handling, you can use the
IS NULL and
IS NOT NULL operators and the
For some column types,
NULL values are handled specially. If you
NULL into the first
TIMESTAMP column of a table, the
current date and time is inserted. If you insert
NULL into an
AUTO_INCREMENT column, the next number in the sequence is inserted.
You can use an alias to refer to a column in the
ORDER BY, or in the
HAVING part. Aliases can also be used
to give columns better names:
SELECT SQRT(a*b) as rt FROM table_name GROUP BY rt HAVING rt > 0; SELECT id,COUNT(*) AS cnt FROM table_name GROUP BY id HAVING cnt > 0; SELECT id AS "Customer identity" FROM table_name;
Note that ANSI SQL doesn't allow you to refer to an alias in a
WHERE clause. This is because when the
WHERE code is
executed the column value may not yet be determined. For example, the
following query is illegal:
SELECT id,COUNT(*) AS cnt FROM table_name WHERE cnt > 0 GROUP BY id;
WHERE statement is executed to determine which rows should
be included in the
GROUP BY part while
HAVING is used to
decide which rows from the result set should be used.
As MySQL doesn't support sub-selects or use of more than one table
DELETE statement, you should use the following approach to
delete rows from 2 related tables:
SELECTthe rows based on some
WHEREcondition in the main table.
DELETEthe rows in the main table based on the same condition.
DELETE FROM related_table WHERE related_column IN (selected_rows).
If the total number of characters in the query with
related_column is more than 1,048,576 (the default value of
max_allowed_packet, you should split it into smaller parts and
DELETE statements. You will probably get the
DELETE by only deleting 100-1000
id's per query if the
related_column is an index. If the
related_column isn't an index, the speed is independent of the
number of arguments in the
If you have a complicated query that has many tables and that doesn't return any rows, you should use the following procedure to find out what is wrong with your query:
EXPLAINand check if you can find something that is obviously wrong. See section 5.2.1
EXPLAINSyntax (Get Information About a
LIMIT 10with the query.
SELECTfor the column that should have matched a row against the table that was last removed from the query.
DOUBLEcolumns with numbers that have decimals, you can't use
=! This problem is common in most computer languages because floating-point values are not exact values:
mysql> SELECT * FROM table_name WHERE float_column=3.5; -> mysql> SELECT * FROM table_name WHERE float_column between 3.45 and 3.55;In most cases, changing the
DOUBLEwill fix this!
mysql test < query.sqlthat shows your problems. You can create a test file with
mysqldump --quick database tables > query.sql. Open the file in an editor, remove some insert lines (if there are too many of these), and add your select statement at the end of the file. Test that you still have your problem by doing:
shell> mysqladmin create test2 shell> mysql test2 < query.sqlPost the test file using
ALTER TABLE changes a table to the current character set.
If you during
ALTER TABLE get a duplicate key error, then the cause
is either that the new character sets maps to keys to the same value
or that the table is corrupted, in which case you should run
REPAIR TABLE on the table.
ALTER TABLE dies with an error like this:
Error on rename of './database/name.frm' to './database/B-a.frm' (Errcode: 17)
the problem may be that MySQL has crashed in a previous
TABLE and there is an old table named `A-something' or
`B-something' lying around. In this case, go to the MySQL data
directory and delete all files that have names starting with
B-. (You may want to move them elsewhere instead of deleting them.)
ALTER TABLE works the following way:
If something goes wrong with the renaming operation, MySQL tries to undo the changes. If something goes seriously wrong (this shouldn't happen, of course), MySQL may leave the old table as `B-xxx', but a simple rename on the system level should get your data back.
The whole point of SQL is to abstract the application from the data storage format. You should always specify the order in which you wish to retrieve your data. For example:
SELECT col_name1, col_name2, col_name3 FROM tbl_name;
will return columns in the order
SELECT col_name1, col_name3, col_name2 FROM tbl_name;
will return columns in the order
You should NEVER, in an application, use
SELECT * and
retrieve the columns based on their position, because the order in which
columns are returned CANNOT be guaranteed over time. A simple
change to your database may cause your application to fail rather
If you want to change the order of columns anyway, you can do it as follows:
INSERT INTO new_table SELECT fields-in-new_table-order FROM old_table.
ALTER TABLE new_table RENAME old_table.
The following are a list of the limitations with
select * from temporary_table, temporary_table as t2;We plan to fix the above in 4.0.
TEMPORARYtable. Note that
ALTER TABLE org_name RENAME new_nameworks! We plan to fix the above in 4.0.
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