12.17 Miscellaneous Functions

Table 12.21 Miscellaneous Functions

Name	Description
DEFAULT()	Return the default value for a table column
GET_LOCK()	Get a named lock
INET_ATON()	Return the numeric value of an IP address
INET_NTOA()	Return the IP address from a numeric value
IS_FREE_LOCK()	Whether the named lock is free
IS_USED_LOCK()	Whether the named lock is in use; return connection identifier if true
MASTER_POS_WAIT()	Block until the slave has read and applied all updates up to the specified position
NAME_CONST()	Causes the column to have the given name
RAND()	Return a random floating-point value
RELEASE_LOCK()	Releases the named lock
SLEEP()	Sleep for a number of seconds
UUID()	Return a Universal Unique Identifier (UUID)
UUID_SHORT()	Return an integer-valued universal identifier
VALUES()	Defines the values to be used during an INSERT

• DEFAULT(col_name)

  Returns the default value for a table column. An error results if the column has no default value.

  mysql> UPDATE t SET i = DEFAULT(i)+1 WHERE id < 100;

• FORMAT(X,D)

  Formats the number X to a format like '#,###,###.##', rounded to D decimal places, and returns the result as a string. For details, see Section 12.5, “String Functions”.

• GET_LOCK(str,timeout)

  Tries to obtain a lock with a name given by the string str, using a timeout of timeout seconds. Before MySQL 5.5.8, a negative timeout value means infinite timeout on Windows. As of 5.5.8, this behavior applies on all platforms.

  Returns 1 if the lock was obtained successfully, 0 if the attempt timed out (for example, because another client has previously locked the name), or NULL if an error occurred (such as running out of memory or the thread was killed with mysqladmin kill).

  A lock obtained with GET_LOCK() is released explicitly by executing RELEASE_LOCK() or implicitly when your session terminates (either normally or abnormally).

  Locks obtained with GET_LOCK() are not released when transactions commit or roll back.

  Important

  The behavior of GET_LOCK() changes in MySQL 5.7. In consideration of future upgrades, limit the str value to 64 characters or less and do not rely on subsequent calls to GET_LOCK() releasing previous locks.

  GET_LOCK() can be used to implement application locks or to simulate record locks. Names are locked on a server-wide basis. If a name has been locked within one session, GET_LOCK() blocks any request by another session for a lock with the same name. This enables clients that agree on a given lock name to use the name to perform cooperative advisory locking. But be aware that it also enables a client that is not among the set of cooperating clients to lock a name, either inadvertently or deliberately, and thus prevent any of the cooperating clients from locking that name. One way to reduce the likelihood of this is to use lock names that are database-specific or application-specific. For example, use lock names of the form db_name.str or app_name.str.

  mysql> SELECT GET_LOCK('lock1',10);
          -> 1
  mysql> SELECT IS_FREE_LOCK('lock2');
          -> 1
  mysql> SELECT GET_LOCK('lock2',10);
          -> 1
  mysql> SELECT RELEASE_LOCK('lock2');
          -> 1
  mysql> SELECT RELEASE_LOCK('lock1');
          -> NULL

  The second RELEASE_LOCK() call returns NULL because the lock 'lock1' was automatically released by the second GET_LOCK() call.

  If multiple clients are waiting for a lock, the order in which they will acquire it is undefined. Applications should not assume that clients will acquire the lock in the same order that they issued the lock requests.

  Note

  Before MySQL 5.5.3, if a client attempts to acquire a lock that is already held by another client, it blocks according to the timeout argument. If the blocked client terminates, its thread does not die until the lock request times out.

  GET_LOCK() is unsafe for statement-based replication. Beginning with MySQL 5.5.1, a warning is logged if you use this function when binlog_format is set to STATEMENT. (Bug #47995)

• INET_ATON(expr)

  Given the dotted-quad representation of an IPv4 network address as a string, returns an integer that represents the numeric value of the address in network byte order (big endian). INET_ATON() returns NULL if it does not understand its argument.

  mysql> SELECT INET_ATON('10.0.5.9');
          -> 167773449

  For this example, the return value is calculated as 10×256³ + 0×256² + 5×256 + 9.

  INET_ATON() may or may not return a non-NULL result for short-form IP addresses (such as '127.1' as a representation of '127.0.0.1'). Because of this, INET_ATON()a should not be used for such addresses.

  Note

  To store values generated by INET_ATON(), use an INT UNSIGNED column rather than INT, which is signed. If you use a signed column, values corresponding to IP addresses for which the first octet is greater than 127 cannot be stored correctly. See Section 11.2.6, “Out-of-Range and Overflow Handling”.

• INET_NTOA(expr)

  Given a numeric IPv4 network address in network byte order, returns the dotted-quad representation of the address as a string. INET_NTOA() returns NULL if it does not understand its argument.

  As of MySQL 5.5.3, the return value is a nonbinary string in the connection character set. Before 5.5.3, the return value is a binary string.

  mysql> SELECT INET_NTOA(167773449);
          -> '10.0.5.9'

• IS_FREE_LOCK(str)

  Checks whether the lock named str is free to use (that is, not locked). Returns 1 if the lock is free (no one is using the lock), 0 if the lock is in use, and NULL if an error occurs (such as an incorrect argument).

  This function is unsafe for statement-based replication. Beginning with MySQL 5.5.1, a warning is logged if you use this function when binlog_format is set to STATEMENT. (Bug #47995)

• IS_USED_LOCK(str)

  Checks whether the lock named str is in use (that is, locked). If so, it returns the connection identifier of the client session that holds the lock. Otherwise, it returns NULL.

  This function is unsafe for statement-based replication. Beginning with MySQL 5.5.1, a warning is logged if you use this function when binlog_format is set to STATEMENT. (Bug #47995)

• MASTER_POS_WAIT(log_name,log_pos[,timeout])

  This function is useful for control of master/slave synchronization. It blocks until the slave has read and applied all updates up to the specified position in the master log. The return value is the number of log events the slave had to wait for to advance to the specified position. The function returns NULL if the slave SQL thread is not started, the slave's master information is not initialized, the arguments are incorrect, or an error occurs. It returns -1 if the timeout has been exceeded. If the slave SQL thread stops while MASTER_POS_WAIT() is waiting, the function returns NULL. If the slave is past the specified position, the function returns immediately.

  If a timeout value is specified, MASTER_POS_WAIT() stops waiting when timeout seconds have elapsed. timeout must be greater than 0; a zero or negative timeout means no timeout. The lock is exclusive. While held by one session, other sessions cannot obtain a lock of the same name.

  This function is unsafe for statement-based replication. Beginning with MySQL 5.5.1, a warning is logged if you use this function when binlog_format is set to STATEMENT. (Bug #47995)

• NAME_CONST(name,value)

  Returns the given value. When used to produce a result set column, NAME_CONST() causes the column to have the given name. The arguments should be constants.

  mysql> SELECT NAME_CONST('myname', 14);
  +--------+
  | myname |
  +--------+
  |     14 |
  +--------+

  This function is for internal use only. The server uses it when writing statements from stored programs that contain references to local program variables, as described in Section 20.7, “Binary Logging of Stored Programs”, You might see this function in the output from mysqlbinlog.

  For your applications, you can obtain exactly the same result as in the example just shown by using simple aliasing, like this:

  mysql> SELECT 14 AS myname;
  +--------+
  | myname |
  +--------+
  |     14 |
  +--------+
  1 row in set (0.00 sec)

  See Section 13.2.9, “SELECT Syntax”, for more information about column aliases.

• RELEASE_LOCK(str)

  Releases the lock named by the string str that was obtained with GET_LOCK(). Returns 1 if the lock was released, 0 if the lock was not established by this thread (in which case the lock is not released), and NULL if the named lock did not exist. The lock does not exist if it was never obtained by a call to GET_LOCK() or if it has previously been released.

  The DO statement is convenient to use with RELEASE_LOCK(). See Section 13.2.3, “DO Syntax”.

  This function is unsafe for statement-based replication. Beginning with MySQL 5.5.1, a warning is logged if you use this function when binlog_format is set to STATEMENT. (Bug #47995)

• SLEEP(duration)

  Sleeps (pauses) for the number of seconds given by the duration argument, then returns 0. If SLEEP() is interrupted, it returns 1. The duration may have a fractional part.

  When sleep returns normally (without interruption), it returns 0:

  mysql> SELECT SLEEP(1000);
  +-------------+
  | SLEEP(1000) |
  +-------------+
  |           0 |
  +-------------+

  When SLEEP() is the only thing invoked by a query that is interrupted, it returns 1 and the query itself returns no error. This statement is interrupted using KILL QUERY from another session:

  mysql> SELECT SLEEP(1000);
  +-------------+
  | SLEEP(1000) |
  +-------------+
  |           1 |
  +-------------+

  When SLEEP() is only part of a query that is interrupted, the query returns an error. This statement is interrupted using KILL QUERY from another session:

  mysql> SELECT 1 FROM t1 WHERE SLEEP(1000);
  ERROR 1317 (70100): Query execution was interrupted

  This function is unsafe for statement-based replication. Beginning with MySQL 5.5.1, a warning is logged if you use this function when binlog_format is set to STATEMENT. (Bug #47995)

• UUID()

  Returns a Universal Unique Identifier (UUID) generated according to RFC 4122, “A Universally Unique IDentifier (UUID) URN Namespace” (http://www.ietf.org/rfc/rfc4122.txt).

  A UUID is designed as a number that is globally unique in space and time. Two calls to UUID() are expected to generate two different values, even if these calls are performed on two separate devices not connected to each other.

  Warning

  Although UUID() values are intended to be unique, they are not necessarily unguessable or unpredictable. If unpredictability is required, UUID values should be generated some other way.

  UUID() returns a value that conforms to UUID version 1 as described in RFC 4122. The value is a 128-bit number represented as a utf8 string of five hexadecimal numbers in aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee format:

  • The first three numbers are generated from the low, middle, and high parts of a timestamp. The high part also includes the UUID version number.

  • The fourth number preserves temporal uniqueness in case the timestamp value loses monotonicity (for example, due to daylight saving time).

  • The fifth number is an IEEE 802 node number that provides spatial uniqueness. A random number is substituted if the latter is not available (for example, because the host device has no Ethernet card, or it is unknown how to find the hardware address of an interface on the host operating system). In this case, spatial uniqueness cannot be guaranteed. Nevertheless, a collision should have very low probability.

    The MAC address of an interface is taken into account only on FreeBSD and Linux. On other operating systems, MySQL uses a randomly generated 48-bit number.

  mysql> SELECT UUID();
          -> '6ccd780c-baba-1026-9564-5b8c656024db'

  Note

  UUID() does not work with statement-based replication.

• UUID_SHORT()

  Returns a “short” universal identifier as a 64-bit unsigned integer. Values returned by UUID_SHORT() differ from the string-format 128-bit identifiers returned by the UUID() function and have different uniqueness properties. The value of UUID_SHORT() is guaranteed to be unique if the following conditions hold:

  • The server_id value of the current server is between 0 and 255 and is unique among your set of master and slave servers

  • You do not set back the system time for your server host between mysqld restarts

  • You invoke UUID_SHORT() on average fewer than 16 million times per second between mysqld restarts

  The UUID_SHORT() return value is constructed this way:

    (server_id & 255) << 56
  + (server_startup_time_in_seconds << 24)
  + incremented_variable++;

  mysql> SELECT UUID_SHORT();
          -> 92395783831158784

  Note

  UUID_SHORT() does not work with statement-based replication.

• VALUES(col_name)

  In an INSERT ... ON DUPLICATE KEY UPDATE statement, you can use the VALUES(col_name) function in the UPDATE clause to refer to column values from the INSERT portion of the statement. In other words, VALUES(col_name) in the UPDATE clause refers to the value of col_name that would be inserted, had no duplicate-key conflict occurred. This function is especially useful in multiple-row inserts. The VALUES() function is meaningful only in the ON DUPLICATE KEY UPDATE clause of INSERT statements and returns NULL otherwise. See Section 13.2.5.3, “INSERT ... ON DUPLICATE KEY UPDATE Syntax”.

  mysql> INSERT INTO table (a,b,c) VALUES (1,2,3),(4,5,6)
      -> ON DUPLICATE KEY UPDATE c=VALUES(a)+VALUES(b);