Last modified on 10 August 2013, at 18:31

MySQL/Language/Functions

SyntaxEdit

Function names are case insensitive. You can write them as you prefer:

 SELECT DATABASE() -- ok
 SELECT DATABASE() -- ok
 SELECT DATABASE() -- ok

If the IGNORE_SPACE SQL_MODE is not set, you can not put a space between the function name and the first parenthesis. It would return a 1064 error. IGNORE_SPACE is usually 0. The reason is that the parser is faster if that flag is disabled. So:

 SELECT DATABASE () -- usually not accepted
 SELECT DATABASE() -- always works fine

However, this restriction only applies to the native MySQL functions. UDFs and stored functions may be written with a space after the name.

You can't use a value calculated in the SELECT clause as a constraint in the WHERE clause (its a chicken & egg problem); the WHERE clause is what determines the values in the SELECT clause. What you want is the HAVING clause which is applied *after* all matching rows have been found.

General functionsEdit

Type-indipendent functions.

BENCHMARK(times, expression)Edit

Executes expression n times and returns how time it spent. Useful to find bottlenecks in SQL expressions.

 SELECT BENCHMARK(10000, 'hello');   -- Treatment in 0.0010 sec

CAST(value AS type)Edit

Returns value converted in the specified type.

SELECT CAST(20130101 AS DATE);  -- 2013-01-01

CHARSET(string)Edit

Returns the CHARACTER SET used by string.

SELECT CHARSET(20130101);  -- binary
SHOW CHARACTER SET;       -- displays all the different installed CHARACTER SET

COALESCE(value, ...)Edit

Returns the first argument which is not NULL. If all arguments are NULL, returns NULL. There must be at least one argument.

SELECT COALESCE(NULL, 'hello', NULL);   -- hello

COERCIBILITY(string)Edit

Returns the coercibility (between 0 to 5):

SELECT COERCIBILITY('hello');   -- 4
Coercibility[1] Meaning Example
0 Explicit collation Value with COLLATE clause
1 No collation Concatenation of strings with different collations
2 Implicit collation Column value
3 System constant USER() return value
4 Coercible Literal string
5 Ignorable NULL or an expression derived from NULL

COLLATION(string)Edit

Returns the COLLATION used by the string.

SELECT COLLATION('hello');   -- utf8_general_ci

CONNECTION_ID()Edit

Returns the id of the current thread.

SELECT CONNECTION_ID();   -- 31

CONVERT(value, type)Edit

Returns value converted to the specified type.

 SELECT CONVERT ('666', UNSIGNED INTEGER)

CONVERT(string USING charset)Edit

Converts the passed string to the specified CHARACTER SET.

 SELECT CONVERT ('This is a text' USING utf8)

CURRENT_USER()Edit

Returns the username and the hostname used in the current connection.

 SELECT CURRENT_USER()
 SELECT CURRENT_USER -- it's correct

DATABASE()Edit

Returns the current database's name, set with the USE command.

 SELECT DATABASE()

FOUND_ROWS()Edit

After a SELECT with a LIMIT clause and the SQL_CALC_FOUND_ROWS keyword, you can run another SELECT with the FOUND_ROWS() function. It returns the number of rows found by the previous query if it had no LIMIT clause.

 SELECT SQL_CALC_FOUND_ROWS * FROM stats ORDER BY id LIMIT 10 OFFSET 50
 SELECT FOUND_ROWS() AS n

GREATEST(value1, value2, ...)Edit

Returns the greatest argument passed.

IF(val1, val2, val3)Edit

If val1 is TRUE, returns val2. If val1 is FALSE or NULL, returns val3.

IFNULL(val1, val2)Edit

If val1 is NULL, returns val2; else, returns val1.

ISNULL(value)Edit

If the value passed is NULL returns 1, else returns 0.

INTERVAL(val1, val2, val3, ...)Edit

Returns the location of the first argument which is greater than the first one, beginning by zero in the integers in parameter:

SELECT INTERVAL(10, 20, 9, 8, 7);  -- 0
SELECT INTERVAL(10, 9, 20, 8, 7);  -- 1
SELECT INTERVAL(10, 9, 8, 20, 7);  -- 2
SELECT INTERVAL(10, 9, 8, 7, 20);  -- 3

NULLIF(val1, val2)Edit

If val1 = val2, returns NULL; else, returns val1.

LEAST(value1, value2, ...)Edit

Returns the minimum argument passed.

Date and timeEdit

 SELECT * FROM mytable
  WHERE datetimecol >= (CURDATE() - INTERVAL 1 YEAR)  AND
  datetimecol < (CURDATE() - INTERVAL 1 YEAR) INTERVAL 1 DAY;
 
 SELECT IF(DAYOFMONTH(CURDATE()) <= 15,
  DATE_FORMAT(CURDATE(), '%Y-%m-15'),
  DATE_FORMAT(CURDATE() + INTERVAL 1 MONTH, '%Y-%m-15')) AS next15
 FROM TABLE;
 
 SELECT YEAR('2002-05-10'), MONTH('2002-05-10'), DAYOFMONTH('2002-05-10')
 
 SELECT PurchaseDate FROM TABLE WHERE YEAR(PurchaseDate) <= YEAR(CURDATE())
 
 SELECT COLUMNS FROM TABLE
 WHERE start_time >= '2004-06-01 10:00:00' AND end_time <= '2004-06-03 18:00:00'
 
 SELECT * FROM t1
 WHERE DATE_FORMAT(datetime_column, '%T') BETWEEN 'HH:MM:SS' AND 'HH:MM:SS'
 
 SELECT Start_time, End_time FROM TABLE
 WHERE Start_time >= NOW() - INTERVAL 4 HOUR
 
 SELECT NOW() + INTERVAL 60 SECOND
 
 SELECT UNIX_TIMESTAMP('2007-05-01'); -- 1177970400
 SELECT FROM_UNIXTIME(1177970400); -- 2007-05-01 00:00:00

Aggregate functionsEdit

COUNT(field)Edit

If * is given, instead of the name of a field, COUNT() returns the number of rows found by the query. It's commonly used to get the number of rows in a table.

 SELECT COUNT(*) FROM `antiques`

If the DISTINCT keyword is used, identical rows are counted only once.

 SELECT COUNT(DISTINCT *) FROM `antiques`

If a field name is given, returns the number of non-NULL values.

 SELECT COUNT(`cost`) FROM `antiques`

If a field name is given and the DISTINCT keyword is given, returns the number of non-NULL values, and identical values are counted only once.

 SELECT COUNT(DISTINCT `cost`) FROM `antiques`

You can count non-NULL values for an expression:

 SELECT COUNT(`longitude` + `latitude`) FROM `cities`

This returns the number of rows where longitude and latitude are both non-NULL.

MAX(field)Edit

MAX() can be used to get the maximum value for an expression in the rows matching to a query. If no row matches the query, returns NULL.

 SELECT MAX(`cost`) FROM `antiques`
 SELECT MAX(LENGTH(CONCAT(`first_name`, ' ', `last_name`))) FROM `subscribers`

MIN(field)Edit

MIN() can be used to get the minimum value for an expression in the rows matching to a query. If no row matches the query, returns NULL.

 SELECT MIN(`cost`) FROM `antiques`

AVG(field)Edit

AVG() can be used to get the average value for an expression in the rows matching to a query. If no row matches the query, returns NULL.

 SELECT AVG(`cost`) FROM `antiques`

SUM(field)Edit

SUM() can be used to get the sum of the values for an expression in the rows matching to a query. If no row matches the query, returns NULL.

If SUM(DISTINCT expression) is used, identical values are added only once. It has been added in MySQL 5.1.

 SELECT SUM(`cost`) FROM `antiques`

GROUP_CONCAT(field)Edit

GROUP_CONCAT() can be used to concatenate values from all records for a group into a single string separated by comma or any additional token you like.

 CREATE TEMPORARY TABLE p (
        id INTEGER, ptype VARCHAR(10), pname VARCHAR(50)
    );
 
 INSERT INTO p VALUES 
     (1,'mp3','iPod'),
     (2,'mp3','Zune'),
     (3,'mp3','ZEN'),
     (4,'notebook','Acer Eee PC'),
     (4,'notebook','Everex CloudBook');
 
 SELECT * FROM p;
 
 SELECT ptype,group_concat(pname) 
 FROM p 
 GROUP BY ptype;
 
 SELECT ptype,group_concat(' ',pname) 
 FROM p 
 GROUP BY ptype
 ;

Aggregate bit functionsEdit

General syntax:

 FUNCTION_NAME(''expression'')

These functions calculate expression for each row of the result set and perform the calculation between all the expressions. These are bitwise functions. The precision used is 64 bit.

ANDEdit

 SELECT BIT_AND(ip) FROM log

OREdit

 SELECT BIT_OR(ip) FROM log

(returns 0 if there are no rows)

XOREdit

 SELECT BIT_XOR(ip) FROM log

(returns 0 if there are no rows)

ReferencesEdit