MySQL code includes many predefined functions. The list below contains those used more often:

MySQL functions
FUNCTION TYPE DESCRIPTION
ABS() math Returns and absolute number
ADDDATE() date & time Adds the interval of the days to the time
ADDTIME() date & time Adds the time to the datetime
AVG() aggregate Returns average value from columns
BIN() string Returns a string representation of binary value
BIT_COUNT() math Returns the number of active bits from a specified number
BIT_LENGTH() string Returns the length of a string as bits
CEIL() or CEILING() math Returns rounded number to the higher value
CHAR() string Returns the string for code values of an integer passed to function
CHAR_LENGTH() string Returns string the length of a string as chars
CHARSET() admin Returns the character set of a specified string
COLLATION() admin Returns the collation set of a specified string
CONCAT() string Concatenates input string returning a resultant string
CONCAT_WS() string Concatenates input strings with separator value
COUNT() aggregate Returns number of rows from a column
CURRENT_DATE() or CURDATE() date & time Displays the current date
CURRENT_TIME() or CURTIME() date & time Displays current time
DATABASE() admin Returns the current database name
DATE() date & time Returns the date part of the specified datetime expression
DATE_FORMAT() date & time Formats the date according to specified format argument
DAY() or DAYOFMONTH() date & time Returns the day of the month from specified expression
ELT() string Returns a string at the indexed number
EXP() math Returns value raised to the given power
FLOOR() math Returns rounded number to the lower value
HEX() string Returns a string representation of a hexadecimal value
IF() flow Evaluates expression and returns a value
IFNULL() flow Checks if the expression is NULL, and returns appropriate value based on the result
INSERT() string Inserts a new string into a string
INSTR() string Returns the position of the first occurrence of a substring
LCASE() string Returns all lower case
MAX() aggregate Returns maximum value from a column
MIN() aggregate Returns minimum value from a column
MINUTE() date & time Returns the minute from specified expression
MOD() math Returns the modulo value from the specified number
MONTH() date & time Returns the month of the current date
NOW() date & time Shows the current date and time
NULLIF() flow Returns NULL if the condition is met, otherwise returns the first passed expression
OCT() string Gives a string representation of an octal value
POWER() math Returns the value of 'X' to the power of 'Y'
RAND() math Produces a random number
REPEAT() string Repeats a string for a number of times specified
REPLACE() string Replaces the string passed from to another one
REVERSE() string Returns a reverse string of an input one
SECOND() date & time Returns the second of the current time
SPACE() string Introduces 'n' spaces into a string
SQRT() math Returns the square root
STR_TO_DATE() date & time Converts specified string (a date and/or time) into the datetime format
STRCMP() string Compares two strings and if they are equal returns '0'
SUBSTR() string Returns substring from a string, given by the position specified
SUM() aggregate Returns sum of values from two or more columns
TIME() date & time Extract time or datetime from specified expression and returns it as a string
TIMESTAMP() date & time Returns the date or datetime expression as a datetime value
TRIM() string Removes leading or trailing characters from a string
TRUNCATE() math Returns the value of specified number up to the desired decimal places
UPPER() string Returns all upper case
USER() admin Returns the current MySQL username and hostname
VERSION() admin Returns the used MySQL version
YEAR() date & time Returns the year from specified expression