Table 12.19 Miscellaneous Functions
|Return the default value for a table column|
|Get a named lock|
|Return the numeric value of an IP address|
|Return the IP address from a numeric value|
|Checks whether the named lock is free|
|Checks whether the named lock is in use. Return connection identifier if true.|
|Block until the slave has read and applied all updates up to the specified position|
|Causes the column to have the given name|
|Return a random floating-point value|
|Releases the named lock|
|Sleep for a number of seconds|
|Return a Universal Unique Identifier (UUID)|
|Defines the values to be used during an INSERT|
Returns the default value for a table column. Starting with MySQL 5.0.2, an error results if the column has no default value.
UPDATE t SET i = DEFAULT(i)+1 WHERE id < 100;
Formats the number
X to a format
'#,###,###.##', rounded to
D decimal places, and returns the
result as a string. For details, see
Section 12.5, “String Functions”.
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
Locks obtained with
are not released when transactions commit or roll back.
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,
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
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
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.
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 is a known bug (fixed in MySQL 5.5).
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
NULL if it does not understand its
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
'127.0.0.1'). Because of
INET_ATON()a should not
be used for such addresses.
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”.
Given a numeric IPv4 network address in network byte order,
returns the dotted-quad representation of the address as a
NULL if it does not understand its
SELECT INET_NTOA(167773449);-> '10.0.5.9'
Checks whether the lock named
is free to use (that is, not locked). Returns
1 if the lock is free (no one is using the
0 if the lock is in use, and
NULL if an error occurs (such as an
Checks whether the lock named
is in use (that is, locked). If so, it returns the connection
identifier of the client session that holds the lock.
Otherwise, it returns
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
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.
-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.
timeout value is specified,
MASTER_POS_WAIT() stops waiting
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.
Returns the given value. When used to produce a result set
NAME_CONST() causes the
column to have the given name. The arguments should be
SELECT NAME_CONST('myname', 14);+--------+ | myname | +--------+ | 14 | +--------+
This function was added in MySQL 5.0.12. It 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 18.6, “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:
SELECT 14 AS myname;+--------+ | myname | +--------+ | 14 | +--------+ 1 row in set (0.00 sec)
See Section 13.2.8, “SELECT Syntax”, for more information about column aliases.
Releases the lock named by the string
str that was obtained with
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.
Sleeps (pauses) for the number of seconds given by the
duration argument, then returns 0.
SLEEP() is interrupted, it
returns 1. The duration may have a fractional part. This
function was added in MySQL 5.0.12.
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
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.
SELECT UUID();-> '6ccd780c-baba-1026-9564-0040f4311e29'
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() does not work with
... ON DUPLICATE KEY UPDATE statement, you can use
function in the
to refer to column values from the
INSERT portion of the
statement. In other words,
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
INSERT statements and
NULL otherwise. See
Section 126.96.36.199, “INSERT ... ON DUPLICATE KEY UPDATE Syntax”.
INSERT INTO table (a,b,c) VALUES (1,2,3),(4,5,6)->
ON DUPLICATE KEY UPDATE c=VALUES(a)+VALUES(b);