Joe Celko's SQL for Smarties, 3rd Edition

29.5. Date and Time Extraction Functions

No two SQL products agree on the functions that should be available for use with <datetime> data types. In keeping with the practice of overloading functions, the SQL3 proposal has a function for extracting components from a datetime or interval value. The syntax looks like this:

<extract expression> ::=
 EXTRACT (<extract field> FROM <extract source>)

<extract field> ::= <datetime field> | <time zone field>

<time zone field> ::= TIMEZONE_HOUR | TIMEZONE_MINUTE

<extract source> ::= <datetime value expression>
      | <interval value expression>

The interesting feature is that this function always returns a numeric value. For example, EXTRACT (MONTH FROM birthday) will be an INTEGER between 1 and 12. Vendors ...