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18 Problems and common errors

18.1 What to do if MySQL keeps crashing

All MySQL versions are tested on many platforms before they are released. This doesn't mean that there isn't any bugs in MySQL, but that 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 executing mysqladmin version. If mysqld has died, you may find the reason for this in the file `mysql-data-directory/'hostname'.err'.

Since 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:

  • Take down the mysqld daemon with mysqladmin shutdown, run myisamchk --silent --force */*.MYI on all tables and restart the mysqld daemon. This will ensure that you are running from a clean state. See section 13 Maintaining a MySQL installation.
  • Use mysqld --log and try to determine from the information in the log whether or not some specific query kills the server. 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. You may be able to verify this using the following procedure:
    • Take down the MySQL daemon (with mysqladmin shutdown)
    • Make a backup of files in the MySQL database directory.
    • Check the tables with myisamchk -s */*.MYI to verify that all tables are correct. If any table is corrupted, repair it with myisamchk -r path-to-table.MYI.
    • Remove (or move away) any old log files from the MySQL data directory.
    • Start the server with safe_mysql --log.
    • If mysqld now dies, you can test if the problem is a specific query by restoring the backup and executing mysql < mysql-log-file. You can of course do the last test in some other directory than the standard MySQL database directory by starting another MySQL server with safe_mysqld --data=path-to-backup-directory.
  • Have you tried the benchmarks? They should test MySQL rather well. You can also add code that simulates your application! The benchmarks can be found in the `bench' directory in the source distribution, or, for a binary distribution, in the `sql-bench' directory under your MySQL installation directory.
  • Try fork_test.pl and fork2_test.pl.
  • Check the file `mysql-data-directory/'hostname'.err' for any errors.
  • If you configure MySQL for debugging, it will be much easier to gather information about possible errors if something goes wrong. Reconfigure MySQL with the --with-debug option to configure and then recompile. See section G.1 Debugging a MySQL server.
  • Configuring MySQL for debugging causes a safe memory allocator to be included that can find some errors. It also provides a lot of output about what is happening.
  • Have you applied the latest patches for your operating system?
  • Use the --skip-locking option to mysqld. On some systems, the lockd lock manager does not work properly; the --skip-locking option tells mysqld not to use external locking. (This means that you cannot run 2 mysqld servers 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.)
  • Have you tried mysqladmin -u root processlist when mysqld appears to be running but not responding? Sometimes mysqld is 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 processlist will 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.
  • Run the command mysqladmin -i 5 status in a separate window to produce statistics while you run your other queries.
  • Try the following:
    1. Start mysqld from gdb (or another debugger).
    2. Run your test scripts.
    3. Do back (or the backtrace command in your debugger) when mysqld core dumps.
  • Try to simulate your application with a Perl script to force MySQL to crash or misbehave.
  • Or send a normal bug report. See section 2.3 How to report bugs or problems. But be even more detailed than usual. Since MySQL works for many people, it may be that the crash results from something that exists only on your computer (for example, an error that is related to your particular system libraries).
  • If you have a problem with table with dynamic length rows and you are not using BLOB/TEXT columns (but only VARCHAR columns) you can try to change all VARCHAR to CHAR with ALTER TABLE. This will force MySQL to use fixed size rows. Fixed size rows take a little extra place, but are much more tolerant for corruption! The current dynamic row code has been in use at TCX 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 if the above helps!

18.2 Some common errors when using MySQL

18.2.1 MySQL server has gone away error

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 with by setting the wait_timeout variable when you start mysqld.

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):

CR_SERVER_GONE_ERROR The client couldn't send a question to the server.
CR_SERVER_LOST 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 starting mysqld with the -O max_allowed_packet=# option (default 1M). The extra memory is allocated on demand, so mysqld will use more memory only when you issue a big query or when mysqld must return a big result row!

18.2.2 Can't connect to [local] MySQL server error

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 hostname localhost.

On Windows you can connect only with TCP/IP if the mysqld server is running on Win95/Win98. If it's running on NT, you can also connect with named pipes. The name of the named pipe is MySQL. If you don't give a hostname when connecting to 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 mysqld server.

Start by checking (using ps or the task manager on windows) that there is a process running named mysqld on your server! If there isn't any mysqld process, you should start one. See section 4.15.2 Problems starting the MySQL server.

If a 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 hostname command; these cause the output of hostname (i.e., the current hostname) to be substituted into the mysqladmin command.

Here are some reasons the Can't connect to local MySQL server error might occur:

  • mysqld is not running.
  • You are running on a system that uses MIT-pthreads. If you are running on a system that doesn't have native threads, mysqld uses the MIT-pthreads package. See section 4.2 Operating systems supported by MySQL. However, MIT-pthreads doesn't support Unix sockets, so on such a system 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
  • Someone has removed the Unix socket that mysqld uses (default `/tmp/mysqld.sock'). You might have a cron job that removes the MySQL socket (e.g., a job that removes old files from the `/tmp' directory). You can always run mysqladmin version and check that the socket mysqladmin is trying to use really exists. The fix in this case is to change the cron job to not remove `mysqld.sock' or to place the socket somewhere else. You can specify a different socket location at MySQL configuration time with this command:
    shell> ./configure --with-unix-socket-path=/path/to/socket
    You can also start safe_mysqld with the --socket=/path/to/socket option and set the environment variable MYSQL_UNIX_PORT to the socket pathname before starting your MySQL clients. @item You have started the mysqld server with the --socket=/path/to/socket option. 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 setting the environment variable MYSQL_UNIX_PORT to the socket pathname or by providing the socket path as an argument to the clients. You can test the socket with this command:
    shell> mysqladmin --socket=/path/to/socket version
  • You are using Linux and one thread has died (core dumped). In this case you must kill the other mysqld threads (for example with the mysql_zap script before you can start a new MySQL server. See section 18.1 What to do if MySQL keeps crashing.

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 is the problem:

  • Check if the server up by doing telnet your-host-name tcp-ip-port-number and press RETURN a 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 used port.
  • Try connecting to the mysqld daemon on the local machine and check the TCP/IP port that mysqld it's configured to use (variable port) with mysqladmin variables.
  • Check that your mysqld server is not started with the --skip-networking option.

18.2.3 Host '...' is blocked error

If you get a 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 (max_connect_errors) of connect requests from the host 'hostname' that have been interrupted in the middle. After max_connect_errors failed requests, mysqld assumes that something is wrong (like a attack from a cracker), and blocks the site from further connections until someone executes the command mysqladmin flush-hosts.

By default, 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 max_connect_errors variable!

18.2.4 Too many connections error

If you get the error Too many connections when you try to connect to MySQL, this means that there is already max_connections clients connected to the mysqld server.

If you need more connections than the default (100), then you should restart mysqld with a bigger value for the max_connections variable.

Note that mysqld actually allows (max_connections+1) 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 login and use SHOW PROCESSLIST to find out what could be wrong. See section 7.21 SHOW syntax (Get information about tables, columns,...).

18.2.5 Out of memory error

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 mysql. The 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 mysql_use_result() to retrieve the result set. This places less of a load on the client (but more on the server).

18.2.6 Packet too large error

When a MySQL client or the mysqld server gets a packet bigger than 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 mysql --set-variable=max_allowed_packet=8M.

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 --set-variable=max_allowed_packet=24M option.

18.2.7 The table is full error

This error occurs 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 option SQL_BIG_TABLES before you issue the problematic query. See section 7.25 SET OPTION syntax.

You can also start mysqld with the --big-tables option. This is exactly the same as using SQL_BIG_TABLES for all queries.

18.2.8 Commands out of sync error 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 mysql_use_result() and try to execute a new query before you have called mysql_free_result(). It can also happen if you try to execute two queries that return data without a mysql_use_result() or mysql_store_result() in between.

18.2.9 Ignoring user error

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 permission system.

Possible causes of and fixes for this problem:

  • You may be running a new version of mysqld with an old user table. You can check this by executing mysqlshow mysql user to see if the password field is shorter than 16 characters. If so, you can correct this condition by running the scripts/add_long_password script.
  • The user has an old password (8 chararacters long) and you didn't start mysqld with the --old-protocol option. Update the user in the user table with a new password or restart mysqld with --old-protocol.
  • You have specified a password in the user table without using the PASSWORD() function. Use mysql to update the user in the user table with a new password. Make sure to use the PASSWORD() function:
    mysql> update user set password=PASSWORD('your password')
               where user='XXX';

18.2.10 Table 'xxx' doesn't exist error

If you get the error Table 'xxx' doesn't exist or Can't find file: 'xxx' (errno: 2), this means that no table exists in the current database with the name xxx.

Note that as MySQL uses directories and files to store databases and tables, the database and table names are case sensitive! (On Win32 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 7.21 SHOW syntax (Get information about tables, columns,...).

18.3 How MySQL handles a full disk

When a disk full condition occurs, MySQL does the following:

  • It checks once every minute to see whether or not there is enough space to write the current row. If there is enough space, it continues as if nothing had happened.
  • Every 6 minutes it writes an entry to the log file warning about the disk full condition.

To alleviate the problem, you can take the following actions:

  • To continue, you only have to free enough disk space to insert all records.
  • To abort the thread, you must send a mysqladmin kill to the thread. The thread will be aborted the next time it checks the disk (in 1 minute).
  • Note that other threads may be waiting for the table that caused the ``disk full'' condition. If you have several ``locked'' threads, killing the one thread that is waiting on the disk full condition will allow the other threads to continue.

18.4 How to run SQL commands from a text file

The mysql client typically is used interactively, like this:

shell> mysql database

However, it's also possible to put your SQL commands in a file and tell mysql to read its input from that file. To do so, create a text file `text_file' that contains the commands you wish to execute. Then invoke mysql as shown below:

shell> mysql database < text_file

You can also start your text file with a USE db_name statement. In this case, it is unnecessary to specify the database name on the command line:

shell> mysql < text_file

See section 12.1 Overview of the different MySQL programs.

18.5 Where MySQL stores temporary files

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 have 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 safe_mysqld to set 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 mysqld.

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 located.

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.

For some SELECT queries, MySQL also creates temporary SQL tables. These are not hidden and have names of the form `SQL_*'.

ALTER TABLE and OPTIMIZE TABLE create a temporary table in the same directory as the original table.

18.6 How to protect `/tmp/mysql.sock' from being deleted

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 the 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 (root).

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.

18.7 Access denied error

See section 6.8 How the privilege system works. And especially see section 6.15 Causes of Access denied errors.

18.8 How to run MySQL as a normal user

The MySQL server mysqld can be started and run by any user. In order to change mysqld to run as Unix user user_name, you must do the following:

  1. Stop the server if it's running (use mysqladmin shutdown).
  2. Change the database directories and files so that user_name has privileges to read and write files in them (you may need to do this as the Unix root user):
    shell> chown -R user_name /path/to/mysql/datadir
    If 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 -R may not follow symlinks for you.
  3. Start the server as user user_name, or, if you are using MySQL 3.22 or later, start mysqld as the Unix root user and use the --user=user_name option. mysqld will switch to run as Unix user user_name before accepting any connections.
  4. If you are using the mysql.server script to start mysqld when the system is rebooted, you should edit mysql.server to use su to run mysqld as user user_name, or to invoke mysqld with the --user option. (No changes to safe_mysqld are necessary.)

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 user root is the only user with permission to access the mysql 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 -u root on the command line, has nothing to do with MySQL running as the Unix root user, or, indeed, as other 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 name.

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 mysql -u root db_name and do whatever he likes.

18.9 How to reset a forgotten password.

If you have forgotten the root user password for MySQL, you can restore it with the following procedure.

  1. Take down the mysqld server by sending a kill (not kill -9) to the mysqld server. The pid is stored in a .pid file which is normally in the MySQL database directory:
    kill `cat /mysql-data-directory/hostname.pid`
    You must be either the UNIX root user or the same user the server runs as to do this.
  2. Restart mysqld with the --skip-grant-tables option.
  3. Connect to the mysqld server with mysql -h hostname mysql and change the password with a GRANT command. See section 7.26 GRANT and REVOKE syntax. You can also do this with mysqladmin -h hostname -u user password 'new password'
  4. Load the privilege tables with: mysqladmin -h hostname flush-privileges or with the SQL command FLUSH PRIVILEGES.

18.10 Problems with file permissions

If you have problems with file permissions, for example, if mysql 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 &

18.11 File not found

If you get ERROR '...' not found (errno: 23), Can't open 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 means:

shell> perror 23
File table overflow
shell> perror 24
Too many open files

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 available to mysqld.

To tell 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 mysqld, modify the safe_mysqld script. There is a commented-out line ulimit -n 256 in the script. You can remove the '#' character to uncomment this line, and change the number 256 to change the number of file descriptors available to mysqld.

ulimit can increase the number of file descriptors, but only up to the limit imposed by the operating system. If you need to increase the OS limit on the number of file descriptors available to each process, consult the documentation for your operating system.

Note that if you run the tcsh shell, ulimit will not work! tcsh will also report incorrect values when you ask for the current limits! In this case you should start safe_mysqld with sh!

18.12 Problems using DATE columns

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 UPDATE expressions and in the WHERE clause of SELECT statements. For example:

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 WHERE clause that compares a date to a TIMESTAMP, 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' and '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 NULL in MyODBC 2.50.12 and above, because ODBC can't handle this kind of date.

Since 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 idate to 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 '1998-2-31', the wrong date will be stored. If the date cannot be converted to any reasonable value, a 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 server.

18.13 Timezone problems

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 timezone. This should be done for the environment in which the server runs, for example in safe_mysqld or mysql.server.

18.14 Case sensitivity in searches

By default, MySQL searches are case-insensitive (although there are some character sets that are never case insensitive, such as czech). That means that if you search with col_name LIKE 'a%', you will get all column values that start with A or a. If you want to make this search case-sensitive, use something like INDEX(col_name, "A")=0 to check a prefix. Or use STRCMP(col_name, "A") = 0 if the column value must be exactly "A".

Simple comparison operations (>=, >, = , < , <=, sorting and grouping) are based on each character's ``sort value''. Characters with the same sort value (like E, e and 'e) are treated as the same character!

LIKE comparisons are done on the uppercase value of each character (E == e but E <> 'e)

If you want a column always to be treated in case-sensitive fashion, declare it as BINARY. See section 7.7 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.

18.15 Problems with NULL values

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 empty string ''. 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 inserts a 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 NULL 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 NULL:

mysql> SELECT NULL,1+NULL,CONCAT('Invisible',NULL);

If you want to search for column values that are NULL, you cannot use the =NULL test. The following statement returns no rows, because 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 column.

When reading data with LOAD DATA INFILE, empty columns are updated with ''. 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 7.16 LOAD DATA INFILE syntax.

When using ORDER BY, NULL values are presented first. If you sort in descending order using DESC, NULL values are presented last. When using GROUP BY, all NULL values are regarded as equal.

To help with NULL handling, you can use the IS NULL and IS NOT NULL operators and the IFNULL() function.

For some column types, NULL values are handled specially. If you insert 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.

18.16 Problems with alias

You can use alias to refer to a column in the GROUP BY, ORDER BY or in the HAVING part. Aliases can also be used to give columns more 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 you ANSI SQL doesn't allow you to refer to an alias in a WHERE clause. This is because that when the WHERE code is executed the column value may not yet be determinated. For example the following query is illegal:

SELECT id,COUNT(*) AS cnt FROM table_name WHERE cnt > 0 GROUP BY id;

The WHERE statement is executed to determinate 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.

18.17 Deleting rows from related tables

As MySQL doesn't support sub-selects or use of more than one table in the DELETE statement, you should use the following approach to delete rows from 2 related tables:

  1. SELECT the rows based on some WHERE condition in the main table.
  2. DELETE the rows in the main table based on the same condition.
  3. 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 execute multiple DELETE statements. You will probably get the fastest DELETE by only deleting 100-1000 related_column id's per time 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 IN clause.

18.18 Solving problems with no matching rows

If you have a complicated query with many tables that doesn't return any rows, you should use the following procedure to find out what is wrong with your query:

  1. Test the query with EXPLAIN and check if you can find something that is obviously wrong. See section 7.22 EXPLAIN syntax (Get information about a SELECT).
  2. Select only those fields that are used in the WHERE clause.
  3. Remove one table at a time from the query until it returns some rows. If the tables are big, it's a good idea to use LIMIT 10 with the query.
  4. Do a SELECT for the column that should have matched a row, against the table that was last removed from the query.
  5. If you are comparing FLOAT or DOUBLE columns 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 FLOAT to a DOUBLE will fix this!
  6. If you still can't find out what's wrong, create a minimal test that can be run with mysql test < query.sql that shows your problems. You can create a test file with mysqldump --quick database tables > query.sql. Take the file up in a editor, remove some insert lines (if there are too many of these) and add your select statement last in the file. Test that you still have your problem by doing:
    shell> mysqladmin create test2
    shell> mysql test2 < query.sql
    Post the test file using mysqlbug to mysql@lists.mysql.com.

18.19 Problems with ALTER TABLE.

If 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 ALTER 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 A- or B-. (You may want to move them elsewhere instead of deleting them).

ALTER TABLE works the following way:

  • Create a new table named `A-xxx' with the requested changes.
  • All rows from the old table are copied to `A-xxx'.
  • The old table is renamed `B-xxx'.
  • `A-xxx' is renamed to your old table name.
  • `B-xxx' is deleted.

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 should get your data back.

18.20 How to change the order of columns in a table

The whole point of SQL is to abstract the application from the data storage format. You should always specify the order in wish you wish to retrieve your data. For example:

SELECT col_name1, col_name2, col_name3 FROM tbl_name;

will return columns in the order col_name1, col_name2, col_name3, whereas:

SELECT col_name1, col_name3, col_name2 FROM tbl_name;

will return columns in the order col_name1, col_name3, col_name2.

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 dramatically.

If you want to change the order of columns anyway, you can do it as follows:

  1. Create a new table with the columns in the right order.
  2. Execute INSERT INTO new_table SELECT fields-in-new_table-order FROM old_table.
  3. Drop or rename old_table
  4. ALTER TABLE new_table RENAME old_table

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