Table 12.18 Information Functions
|Repeatedly execute an expression|
|Return the character set of the argument|
|Return the collation coercibility value of the string argument|
|Return the collation of the string argument|
|Return the connection ID (thread ID) for the connection|
|The authenticated user name and host name|
|Return the default (current) database name|
|For a SELECT with a LIMIT clause, the number of rows that would be returned were there no LIMIT clause|
|Value of the AUTOINCREMENT column for the last INSERT|
|The number of rows updated|
|Synonym for DATABASE()|
|Synonym for USER()|
|Synonym for USER()|
|The user name and host name provided by the client|
|Return a string that indicates the MySQL server version|
BENCHMARK()function executes the expression
counttimes. It may be used to time how quickly MySQL processes the expression. The result value is always
0. The intended use is from within the mysql client, which reports query execution times:
mysql> SELECT BENCHMARK(1000000,AES_ENCRYPT('hello','goodbye')); +---------------------------------------------------+ | BENCHMARK(1000000,AES_ENCRYPT('hello','goodbye')) | +---------------------------------------------------+ | 0 | +---------------------------------------------------+ 1 row in set (4.74 sec)
The time reported is elapsed time on the client end, not CPU time on the server end. It is advisable to execute
BENCHMARK()several times, and to interpret the result with regard to how heavily loaded the server machine is.
BENCHMARK()is intended for measuring the runtime performance of scalar expressions, which has some significant implications for the way that you use it and interpret the results:
Only scalar expressions can be used. Although the expression can be a subquery, it must return a single column and at most a single row. For example,
BENCHMARK(10, (SELECT * FROM t))will fail if the table
thas more than one column or more than one row.
Ntimes differs from executing
SELECT BENCHMARK(in terms of the amount of overhead involved. The two have very different execution profiles and you should not expect them to take the same amount of time. The former involves the parser, optimizer, table locking, and runtime evaluation
Ntimes each. The latter involves only runtime evaluation
Ntimes, and all the other components just once. Memory structures already allocated are reused, and runtime optimizations such as local caching of results already evaluated for aggregate functions can alter the results. Use of
BENCHMARK()thus measures performance of the runtime component by giving more weight to that component and removing the “noise” introduced by the network, parser, optimizer, and so forth.
Returns the character set of the string argument.
mysql> SELECT CHARSET('abc'); -> 'latin1' mysql> SELECT CHARSET(CONVERT('abc' USING utf8)); -> 'utf8' mysql> SELECT CHARSET(USER()); -> 'utf8'
Returns the collation coercibility value of the string argument.
mysql> SELECT COERCIBILITY('abc' COLLATE latin1_swedish_ci); -> 0 mysql> SELECT COERCIBILITY(USER()); -> 3 mysql> SELECT COERCIBILITY('abc'); -> 4 mysql> SELECT COERCIBILITY(1000); -> 5
The return values have the meanings shown in the following table. Lower values have higher precedence.
Coercibility Meaning Example
Explicit collation Value with
No collation Concatenation of strings with different collations
Implicit collation Column value, stored routine parameter or local variable
Coercible Literal string
Numeric Numeric or temporal value
NULLor an expression derived from
For more information, see Section 10.1.8.4, “Collation Coercibility in Expressions”.
Returns the collation of the string argument.
mysql> SELECT COLLATION('abc'); -> 'latin1_swedish_ci' mysql> SELECT COLLATION(_utf8'abc'); -> 'utf8_general_ci'
Returns the connection ID (thread ID) for the connection. Every connection has an ID that is unique among the set of currently connected clients.
The value returned by
CONNECTION_ID()is the same type of value as displayed in the
IDcolumn of the
SHOW PROCESSLISToutput, and the
PROCESSLIST_IDcolumn of the Performance Schema
mysql> SELECT CONNECTION_ID(); -> 23786
Returns the user name and host name combination for the MySQL account that the server used to authenticate the current client. This account determines your access privileges. The return value is a string in the
mysql> SELECT USER(); -> 'davida@localhost' mysql> SELECT * FROM mysql.user; ERROR 1044: Access denied for user ''@'localhost' to database 'mysql' mysql> SELECT CURRENT_USER(); -> '@localhost'
The example illustrates that although the client specified a user name of
davida(as indicated by the value of the
USER()function), the server authenticated the client using an anonymous user account (as seen by the empty user name part of the
CURRENT_USER()value). One way this might occur is that there is no account listed in the grant tables for
Within a stored program or view,
CURRENT_USER()returns the account for the user who defined the object (as given by its
DEFINERvalue) unless defined with the
SQL SECURITY INVOKERcharacteristic. In the latter case,
CURRENT_USER()returns the object's invoker.
Triggers and events have no option to define the
SQL SECURITYcharacteristic, so for these objects,
CURRENT_USER()returns the account for the user who defined the object. To return the invoker, use
The following statements support use of the
CURRENT_USER()function to take the place of the name of (and, possibly, a host for) an affected user or a definer; in such cases,
CURRENT_USER()is expanded where and as needed:
Returns the default (current) database name as a string in the
utf8character set. If there is no default database,
NULL. Within a stored routine, the default database is the database that the routine is associated with, which is not necessarily the same as the database that is the default in the calling context.
mysql> SELECT DATABASE(); -> 'test'
If there is no default database,
SELECTstatement may include a
LIMITclause to restrict the number of rows the server returns to the client. In some cases, it is desirable to know how many rows the statement would have returned without the
LIMIT, but without running the statement again. To obtain this row count, include an
SQL_CALC_FOUND_ROWSoption in the
SELECTstatement, and then invoke
mysql> SELECT SQL_CALC_FOUND_ROWS * FROM tbl_name -> WHERE id > 100 LIMIT 10; mysql> SELECT FOUND_ROWS();
In the absence of the
SQL_CALC_FOUND_ROWSoption in the most recent successful
FOUND_ROWS()returns the number of rows in the result set returned by that statement. If the statement includes a
FOUND_ROWS()returns the number of rows up to the limit. For example,
FOUND_ROWS()returns 10 or 60, respectively, if the statement includes
LIMIT 50, 10.
The row count available through
FOUND_ROWS()is transient and not intended to be available past the statement following the
SELECT SQL_CALC_FOUND_ROWSstatement. If you need to refer to the value later, save it:
mysql> SELECT SQL_CALC_FOUND_ROWS * FROM ... ; mysql> SET @rows = FOUND_ROWS();
If you are using
SELECT SQL_CALC_FOUND_ROWS, MySQL must calculate how many rows are in the full result set. However, this is faster than running the query again without
LIMIT, because the result set need not be sent to the client.
FOUND_ROWS()can be useful in situations when you want to restrict the number of rows that a query returns, but also determine the number of rows in the full result set without running the query again. An example is a Web script that presents a paged display containing links to the pages that show other sections of a search result. Using
FOUND_ROWS()enables you to determine how many other pages are needed for the rest of the result.
The use of
FOUND_ROWS()is more complex for
UNIONstatements than for simple
LIMITmay occur at multiple places in a
UNION. It may be applied to individual
SELECTstatements in the
UNION, or global to the
UNIONresult as a whole.Important
FOUND_ROWS()is not replicated reliably using statement-based replication. This function is automatically replicated using row-based replication.
With no argument,
LAST_INSERT_ID()returns a 64-bit value representing the first automatically generated value successfully inserted for an
AUTO_INCREMENTcolumn as a result of the most recently executed
INSERTstatement. The value has a type of
BIGINT UNSIGNEDas of MySQL 5.5.29,
BIGINT(signed) before that. The value of
LAST_INSERT_ID()remains unchanged if no rows are successfully inserted.
With an argument,
LAST_INSERT_ID()returns an unsigned integer as of MySQL 5.5.29, a signed integer before that.
For example, after inserting a row that generates an
AUTO_INCREMENTvalue, you can get the value like this:
mysql> SELECT LAST_INSERT_ID(); -> 195
The currently executing statement does not affect the value of
LAST_INSERT_ID(). Suppose that you generate an
AUTO_INCREMENTvalue with one statement, and then refer to
LAST_INSERT_ID()in a multiple-row
INSERTstatement that inserts rows into a table with its own
AUTO_INCREMENTcolumn. The value of
LAST_INSERT_ID()will remain stable in the second statement; its value for the second and later rows is not affected by the earlier row insertions. (However, if you mix references to
LAST_INSERT_ID(, the effect is undefined.)
If the previous statement returned an error, the value of
LAST_INSERT_ID()is undefined. For transactional tables, if the statement is rolled back due to an error, the value of
LAST_INSERT_ID()is left undefined. For manual
ROLLBACK, the value of
LAST_INSERT_ID()is not restored to that before the transaction; it remains as it was at the point of the
Prior to MySQL 5.5.35, this function was not replicated correctly if replication filtering rules were in use. (Bug #17234370, Bug #69861)
Within the body of a stored routine (procedure or function) or a trigger, the value of
LAST_INSERT_ID()changes the same way as for statements executed outside the body of these kinds of objects. The effect of a stored routine or trigger upon the value of
LAST_INSERT_ID()that is seen by following statements depends on the kind of routine:
If a stored procedure executes statements that change the value of
LAST_INSERT_ID(), the changed value is seen by statements that follow the procedure call.
For stored functions and triggers that change the value, the value is restored when the function or trigger ends, so following statements will not see a changed value.
The ID that was generated is maintained in the server on a per-connection basis. This means that the value returned by the function to a given client is the first
AUTO_INCREMENTvalue generated for most recent statement affecting an
AUTO_INCREMENTcolumn by that client. This value cannot be affected by other clients, even if they generate
AUTO_INCREMENTvalues of their own. This behavior ensures that each client can retrieve its own ID without concern for the activity of other clients, and without the need for locks or transactions.
The value of
LAST_INSERT_ID()is not changed if you set the
AUTO_INCREMENTcolumn of a row to a non-“magic” value (that is, a value that is not
If you insert multiple rows using a single
LAST_INSERT_ID()returns the value generated for the first inserted row only. The reason for this is to make it possible to reproduce easily the same
INSERTstatement against some other server.
mysql> USE test; mysql> CREATE TABLE t ( id INT AUTO_INCREMENT NOT NULL PRIMARY KEY, name VARCHAR(10) NOT NULL ); mysql> INSERT INTO t VALUES (NULL, 'Bob'); mysql> SELECT * FROM t; +----+------+ | id | name | +----+------+ | 1 | Bob | +----+------+ mysql> SELECT LAST_INSERT_ID(); +------------------+ | LAST_INSERT_ID() | +------------------+ | 1 | +------------------+ mysql> INSERT INTO t VALUES (NULL, 'Mary'), (NULL, 'Jane'), (NULL, 'Lisa'); mysql> SELECT * FROM t; +----+------+ | id | name | +----+------+ | 1 | Bob | | 2 | Mary | | 3 | Jane | | 4 | Lisa | +----+------+ mysql> SELECT LAST_INSERT_ID(); +------------------+ | LAST_INSERT_ID() | +------------------+ | 2 | +------------------+
Although the second
INSERTstatement inserted three new rows into
t, the ID generated for the first of these rows was
2, and it is this value that is returned by
LAST_INSERT_ID()for the following
If you use
INSERT IGNOREand the row is ignored, the
LAST_INSERT_ID()remains unchanged from the current value (or 0 is returned if the connection has not yet performed a successful
INSERT) and, for non-transactional tables, the
AUTO_INCREMENTcounter is not incremented. For
AUTO_INCREMENTcounter is incremented if
innodb_autoinc_lock_modeis set to
2, as demonstrated in the following example:
mysql> USE test; mysql> SELECT @@innodb_autoinc_lock_mode; +----------------------------+ | @@innodb_autoinc_lock_mode | +----------------------------+ | 1 | +----------------------------+ mysql> CREATE TABLE `t` ( `id` INT(11) NOT NULL AUTO_INCREMENT, `val` INT(11) DEFAULT NULL, PRIMARY KEY (`id`), UNIQUE KEY `i1` (`val`) ) ENGINE=InnoDB DEFAULT CHARSET=latin1; # Insert two rows mysql> INSERT INTO t (val) VALUES (1),(2); # With auto_increment_offset=1, the inserted rows # result in an AUTO_INCREMENT value of 3 mysql> SHOW CREATE TABLE t\G *************************** 1. row *************************** Table: t Create Table: CREATE TABLE `t` ( `id` int(11) NOT NULL AUTO_INCREMENT, `val` int(11) DEFAULT NULL, PRIMARY KEY (`id`), UNIQUE KEY `i1` (`val`) ) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=latin1 # LAST_INSERT_ID() returns the first automatically generated # value that is successfully inserted for the AUTO_INCREMENT column mysql> SELECT LAST_INSERT_ID(); +------------------+ | LAST_INSERT_ID() | +------------------+ | 1 | +------------------+ # The attempted insertion of duplicate rows fail but errors are ignored mysql> INSERT IGNORE INTO t (val) VALUES (1),(2); Query OK, 0 rows affected (0.00 sec) Records: 2 Duplicates: 2 Warnings: 0 # With innodb_autoinc_lock_mode=1, the AUTO_INCREMENT counter # is incremented for the ignored rows mysql> SHOW CREATE TABLE t\G *************************** 1. row *************************** Table: t Create Table: CREATE TABLE `t` ( `id` int(11) NOT NULL AUTO_INCREMENT, `val` int(11) DEFAULT NULL, PRIMARY KEY (`id`), UNIQUE KEY `i1` (`val`) ) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=latin1 # The LAST_INSERT_ID is unchanged because the previous insert was unsuccessful mysql> SELECT LAST_INSERT_ID(); +------------------+ | LAST_INSERT_ID() | +------------------+ | 1 | +------------------+
For more information, see Section 220.127.116.11, “AUTO_INCREMENT Handling in InnoDB”.
expris given as an argument to
LAST_INSERT_ID(), the value of the argument is returned by the function and is remembered as the next value to be returned by
LAST_INSERT_ID(). This can be used to simulate sequences:
Create a table to hold the sequence counter and initialize it:
mysql> CREATE TABLE sequence (id INT NOT NULL); mysql> INSERT INTO sequence VALUES (0);
Use the table to generate sequence numbers like this:
mysql> UPDATE sequence SET id=LAST_INSERT_ID(id+1); mysql> SELECT LAST_INSERT_ID();
UPDATEstatement increments the sequence counter and causes the next call to
LAST_INSERT_ID()to return the updated value. The
SELECTstatement retrieves that value. The
mysql_insert_id()C API function can also be used to get the value. See Section 18.104.22.168, “mysql_insert_id()”.
You can generate sequences without calling
LAST_INSERT_ID(), but the utility of using the function this way is that the ID value is maintained in the server as the last automatically generated value. It is multi-user safe because multiple clients can issue the
UPDATEstatement and get their own sequence value with the
mysql_insert_id()), without affecting or being affected by other clients that generate their own sequence values.
mysql_insert_id()is only updated after
UPDATEstatements, so you cannot use the C API function to retrieve the value for
LAST_INSERT_ID(after executing other SQL statements like
Before MySQL 5.5.5,
ROW_COUNT()returns the number of rows changed, deleted, or inserted by the last statement if it was an
INSERT. For other statements, the value may not be meaningful.
As of MySQL 5.5.5,
ROW_COUNT()returns a value as follows:
DML statements other than
SELECT: The number of affected rows. This applies to statements such as
DELETE(as before), but now also to statements such as
LOAD DATA INFILE.
SELECT: -1 if the statement returns a result set, or the number of rows “affected” if it does not. For example, for
SELECT * FROM t1,
ROW_COUNT()returns -1. For
SELECT * FROM t1 INTO OUTFILE ',
ROW_COUNT()returns the number of rows written to the file.
UPDATEstatements, the affected-rows value by default is the number of rows actually changed. If you specify the
mysql_real_connect()when connecting to mysqld, the affected-rows value is the number of rows “found”; that is, matched by the
REPLACEstatements, the affected-rows value is 2 if the new row replaced an old row, because in this case, one row was inserted after the duplicate was deleted.
INSERT ... ON DUPLICATE KEY UPDATEstatements, the affected-rows value per row is 1 if the row is inserted as a new row, 2 if an existing row is updated, and 0 if an existing row is set to its current values. If you specify the
CLIENT_FOUND_ROWSflag, the affected-rows value is 1 (not 0) if an existing row is set to its current values.
mysql> INSERT INTO t VALUES(1),(2),(3); Query OK, 3 rows affected (0.00 sec) Records: 3 Duplicates: 0 Warnings: 0 mysql> SELECT ROW_COUNT(); +-------------+ | ROW_COUNT() | +-------------+ | 3 | +-------------+ 1 row in set (0.00 sec) mysql> DELETE FROM t WHERE i IN(1,2); Query OK, 2 rows affected (0.00 sec) mysql> SELECT ROW_COUNT(); +-------------+ | ROW_COUNT() | +-------------+ | 2 | +-------------+ 1 row in set (0.00 sec)Important
ROW_COUNT()is not replicated reliably using statement-based replication. This function is automatically replicated using row-based replication.
This function is a synonym for
Returns the current MySQL user name and host name as a string in the
mysql> SELECT USER(); -> 'davida@localhost'
The value indicates the user name you specified when connecting to the server, and the client host from which you connected. The value can be different from that of
Returns a string that indicates the MySQL server version. The string uses the
utf8character set. The value might have a suffix in addition to the version number. See the description of the
versionsystem variable in Section 5.1.5, “Server System Variables”.
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_formatis set to
STATEMENT. (Bug #47995)
mysql> SELECT VERSION(); -> '5.5.59-standard'