11.15. Miscellaneous Functions

Table 11.19. Miscellaneous Functions

NameDescription
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()Checks whether the named lock is free
IS_USED_LOCK()Checks 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_SHORT()Return an integer-valued universal identifier
UUID()Return a Universal Unique Identifier (UUID)
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 11.5, “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. 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). If you have a lock obtained with GET_LOCK(), it is released when you execute RELEASE_LOCK(), execute a new GET_LOCK(), or your connection terminates (either normally or abnormally). Locks obtained with GET_LOCK() do not interact with transactions. That is, committing a transaction does not release any such locks obtained during the transaction.

    This function 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 by one client, GET_LOCK() blocks any request by another client 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 and depends on factors such as the thread library in use. In particular, 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.

    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)

  • INET_ATON(expr)

    Given the dotted-quad representation of a network address as a string, returns an integer that represents the numeric value of the address. Addresses may be 4- or 8-byte addresses.

    mysql> SELECT INET_ATON('209.207.224.40');
            -> 3520061480
    

    The generated number is always in network byte order. For the example just shown, the number is calculated as 209×2563 + 207×2562 + 224×256 + 40.

    INET_ATON() also understands short-form IP addresses:

    mysql> SELECT INET_ATON('127.0.0.1'), INET_ATON('127.1');
            -> 2130706433, 2130706433
    
    Note

    When storing values generated by INET_ATON(), it is recommended that you use an INT UNSIGNED column. If you use a (signed) INT column, values corresponding to IP addresses for which the first octet is greater than 127 cannot be stored correctly. See Section 10.6, “Out-of-Range and Overflow Handling”.

  • INET_NTOA(expr)

    Given a numeric network address in network byte order (4 or 8 byte), returns the dotted-quad representation of the address as a string.

    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(3520061480);
            -> '209.207.224.40'
    
  • 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 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.

    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 19.7, “Binary Logging of Stored Programs”, You might see this function in the output from mysqlbinlog.

  • 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 12.2.3, “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 given in microseconds.

    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 “DCE 1.1: Remote Procedure Call” (Appendix A) CAE (Common Applications Environment) Specifications published by The Open Group in October 1997 (Document Number C706, http://www.opengroup.org/public/pubs/catalog/c706.htm).

    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 computers that are not connected to each other.

    A UUID is a 128-bit number represented by a utf8 string of five hexadecimal numbers in aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee format:

    • The first three numbers are generated from a timestamp.

    • 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 computer has no Ethernet card, or we do not know how to find the hardware address of an interface on your operating system). In this case, spatial uniqueness cannot be guaranteed. Nevertheless, a collision should have very low probability.

      Currently, 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-1026-9564-0040f4311e29'
    
    Warning

    The UUID() function returns a string using the character set defined by the character_set_server parameter. If you are using UUID values in your tables and these columns are indexed the character set of your column or table should match the character set used when the UUID() was called. If you do not use the same character set for the column and the UUID value, the indexes on those columns will not be used, which may lead to a reduction in performance and locked tables during operations as the table is searched sequentially for the value.

    You can convert between different character sets when using UUID-based strings using the CONVERT() function.

    Note

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

  • UUID_SHORT()

    Returns a “short” universal identifier as a 64-bit unsigned integer (rather than a string-form 128-bit identifier as returned by the UUID() function).

    The value of UUID_SHORT() is guaranteed to be unique if the following conditions hold:

    • The server_id of the current host is unique among your set of master and slave servers

    • server_id is between 0 and 255

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

    • You do not invoke UUID_SHORT() on average more 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 that 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 12.2.5.3, “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);
    
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