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BENCHMARK(count,expr)The BENCHMARK() function executes the expression expr
repeatedly count times. It may be used to time how fast 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,ENCODE('hello','goodbye'));
+----------------------------------------------+
| BENCHMARK(1000000,ENCODE('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.
CHARSET(str)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'
|
CHARSET() was added in MySQL 4.1.0.
COERCIBILITY(str)Returns the collation coercibility value of the string argument.
mysql> SELECT COERCIBILITY('abc' COLLATE latin1_swedish_ci);
-> 0
mysql> SELECT COERCIBILITY('abc');
-> 3
mysql> SELECT COERCIBILITY(USER());
-> 2
|
The return values have the following meanings:
Coercibility | Meaning |
| Explicit collation |
| No collation |
| Implicit collation |
| Coercible |
Lower values have higher precedence.
COERCIBILITY() was added in MySQL 4.1.1.
COLLATION(str)Returns the collation for the character set of the string argument.
mysql> SELECT COLLATION('abc');
-> 'latin1_swedish_ci'
mysql> SELECT COLLATION(_utf8'abc');
-> 'utf8_general_ci'
|
COLLATION() was added in MySQL 4.1.0.
CONNECTION_ID()Returns the connection ID (thread ID) for the connection. Every connection has its own unique ID.
mysql> SELECT CONNECTION_ID();
-> 23786
|
CONNECTION_ID() was added in MySQL 3.23.14.
CURRENT_USER()Returns the username and hostname combination that the current session was authenticated
as. This value corresponds to the MySQL account that determines your
access privileges. It can be different from the value of USER().
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 username 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 username part of the CURRENT_USER() value). One way this might
occur is that there is no account listed in the grant tables for
davida.
CURRENT_USER() was added in MySQL 4.0.6.
DATABASE()Returns the default (current) database name.
mysql> SELECT DATABASE();
-> 'test'
|
If there is no default database, DATABASE() returns NULL as
of MySQL 4.1.1, and the empty string before that.
FOUND_ROWS()A SELECT statement may include a LIMIT clause 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 get this row count, include a SQL_CALC_FOUND_ROWS option in the
SELECT statement, then invoke FOUND_ROWS() afterward:
mysql> SELECT SQL_CALC_FOUND_ROWS * FROM tbl_name
-> WHERE id > 100 LIMIT 10;
mysql> SELECT FOUND_ROWS();
|
The second SELECT will return a number indicating how many rows the
first SELECT would have returned had it been written without the
LIMIT clause.
(If the preceding SELECT statement does not include the
SQL_CALC_FOUND_ROWS option, then FOUND_ROWS() may return
a different result when LIMIT is used than when it is not.)
Note that 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.
SQL_CALC_FOUND_ROWS and 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() allows you to determine how many other pages are needed
for the rest of the result.
The use of SQL_CALC_FOUND_ROWS and FOUND_ROWS() is more complex
for UNION queries than for simple SELECT statements, because
LIMIT may occur at multiple places in a UNION. It may be applied
to individual SELECT statements in the UNION, or global to the
UNION result as a whole.
The intent of SQL_CALC_FOUND_ROWS for UNION is that it should
return the row count that would be returned without a global LIMIT.
The conditions for use of SQL_CALC_FOUND_ROWS with UNION are:
The SQL_CALC_FOUND_ROWS keyword must appear in the first SELECT
of the UNION.
The value of FOUND_ROWS() is exact only if UNION ALL is used.
If UNION without ALL is used, duplicate removal occurs and the
value of FOUND_ROWS() is only approximate.
If no LIMIT is present in the UNION, SQL_CALC_FOUND_ROWS
is ignored and returns the number of rows in the temporary table that is
created to process the UNION.
SQL_CALC_FOUND_ROWS and FOUND_ROWS() are available starting
at MySQL 4.0.0.
LAST_INSERT_ID() LAST_INSERT_ID(expr)Returns the last automatically generated value that was inserted into
an AUTO_INCREMENT column.
mysql> SELECT LAST_INSERT_ID();
-> 195
|
The last ID that was generated is maintained in the server on a
per-connection basis. This means the value the function returns to a given
client is the most recent AUTO_INCREMENT value generated by that
client. The value cannot be affected by other clients, even if they generate
AUTO_INCREMENT values of their own. This behavior ensures that you can
retrieve your 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
update the AUTO_INCREMENT column of a row with a
non-magic value (that is, a value that is not NULL and not 0).
If you insert many rows at the same time with an insert statement,
LAST_INSERT_ID() returns the value for the first inserted row.
The reason for this is to make it possible to easily reproduce
the same INSERT statement against some other server.
If you use INSERT IGNORE and the record is ignored, the
AUTO_INCREMENT counter still is incremented and
LAST_INSERT_ID() returns the new value.
If expr is 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(); |
The UPDATE statement increments the sequence counter and causes the
next call to LAST_INSERT_ID() to return the updated value.
The SELECT statement retrieves that value.
The C API function
mysql_insert_id() can also be used to get the value.
See section 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 UPDATE
statement and get their own sequence value with the SELECT statement
(or mysql_insert_id()), without affecting or being affected by other
clients that generate their own sequence values.
Note that mysql_insert_id() is only updated after INSERT
and UPDATE statements, so you cannot use the C API function to
retrieve the value for LAST_INSERT_ID(expr) after executing other
SQL statements like SELECT or SET.
SESSION_USER()SESSION_USER() is a synonym for USER().
SYSTEM_USER()SYSTEM_USER() is a synonym for USER().
USER()Returns the current MySQL username and hostname.
mysql> SELECT USER();
-> 'davida@localhost'
|
The value indicates the username you specified when connecting to
the server, and the client host from which you connected. The value can be
different than that of CURRENT_USER().
Prior to MySQL 3.22.11, the function value does not include the client hostname. You can extract just the username part, regardless of whether the value includes a hostname part, like this:
mysql> SELECT SUBSTRING_INDEX(USER(),'@',1);
-> 'davida'
|
As of MySQL 4.1, USER() returns a value in the utf8 character
set, so you should also make sure that the '@' string literal is
interpreted in that character set:
mysql> SELECT SUBSTRING_INDEX(USER(),_utf8'@',1);
-> 'davida'
|
VERSION()Returns a string that indicates the MySQL server version.
mysql> SELECT VERSION();
-> '4.1.2-alpha-log'
|
Note that if your version string ends with -log this means that
logging is enabled.
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