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Functions
Whenever a column or a constant can be used in a query definition, a
function can be used instead. Functions perform operations on the data,
the usual quoting rules applying to their arguments.
The available function and their names names vary wildly from server to
server, as does their syntax. We'll introduce here the most important MySQL
functions. For further information, consult your server's documentation.
- Arithmetic and math functions
-
- +, -(unary or binary), / (with infix notation), *
- ABS(X)
-
- SIGN(X)
-
- MOD(X Y)
-
modulo, like 'X % Y' in C
- FLOOR(X)
-
- CEILING(X)
-
- ROUND(X)
-
rounding operators
- LEAST(X, Y,...)
-
returns the smallest of its arguments
- GREATEST(X, Y,...)
-
returns the greatest of its arguments
- Comparison and logic functions
-
- =
-
equality
- != or <>
-
dis-equality
- >, >=, <, <=
- IS [NOT] NULL
-
true if the compared value is (not) NULL
- expr IN (value, ...)
-
true if the expression expr appears in the list
- NOT or !
-
- OR or ||
-
- AND or &&
-
logic operators
- String comparison and operations
-
- value LIKE pattern
-
see the Conditions page
- value REGEXP pattern
-
performs a regular-expression match
- CONCAT(str1, str2,...)
-
concatenates the arguments
- LENGTH(str)
-
returns the length of its argument
- LEFT(str,len)
-
return the leftmost len characters
- RIGHT(str,len)
-
return the rightmost len characters
- SUBSTRING(string,start_at,length)
-
returns length characters starting from position start_at
- TRIM([LEADING|TRAILING|BOTH] FROM string)
-
trims leading, trailing or both spaces from string
- LOWER(string)
-
returns the string in lower case
- UPPER(string)
-
returns the string in upper case
- PASSWORD(string)
-
returns a Mysql password that checks against string
- ENCRYPT(string[,salt])
-
same as the Unix crypt(3) function. If supplied, 'salt' should be 2 characters long. Otherwise it uses a random salt.
- Control flow operators
-
- IFNULL(expr1,expr2)
-
if expr1 is not null, returns it, otherwise it returns expr2
- IF(expr1,expr2,expr3)
-
if expr1 is true, returns expr2, else expr3
- Date-related functions
-
- DAYOFWEEK(date)
-
returns the weekday index for date (Sunday=1...Saturday=7)
- DAYOFMONTH(date)
-
returns the day of the month for date (1..31)
- DAYOFYEAR(date)
-
returns the day of the year for date (1..366)
- MONTH(date)
-
returns the month for date (1..12)
- YEAR(date)
-
returns the year from date (1000..9999)
- HOUR(time), MINUTE(time), SECOND(time)
-
time extraction functions
- CURRENT_DATE, CURRENT_TIME, CURRENT_TIMESTAMP
-
'magic' variables, that are treated like functions. They contain the current date, time, and timestamp respectively.
- Miscellaneous functions
-
- LAST_INSERT_ID()
-
returns the last value automatically generated by an 'AUTO_INCREMENT'-type column
- Special functions
-
These functions are somewhat 'special', in that they have different
semantics when used in conjunction with the 'GROUP BY' clause (which is not
covered in this manual).
- COUNT([DISTINCT] expr)
-
if 'expr' is a column name, it returns the number
of non-null rows returned for that column. If it's an asterisk '*', it
gives the number of returned rows. If the DISTINCT keyword is specified,
duplicate values are not counted.
- AVG(expr)
-
Returns the average of the columns matched.
- MIN(expr)
-
Returns the least of the columns matched.
- MAX(expr)
-
Returns the greatest of the columns matched.
- SUM(expr)
-
Returns the sum of the columns matched.
Show the current day of the week:
$ mysql -u user -p password sample
[MySQL Monitor]
> select dayofweek(now()) as day;
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Count the number of rows in a table:
> select count(*) from ids;
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Count the number of countries whose name begins by 'i':
> select count(*) as number from ids where ids.name like 'I%'
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