Add support for EXTRACT time/date function.

mysql/functions.go

@@ -224,6 +224,12 @@ var REGEXP_LIKE = jet.REGEXP_LIKE
 
 //----------------- Date/Time Functions and Operators ------------//
 
+// EXTRACT function retrieves subfields such as year or hour from date/time values
+//          EXTRACT(DAY, User.CreatedAt)
+func EXTRACT(field unitType, from Expression) IntegerExpression {
+	return IntExp(jet.EXTRACT(string(field), from))
+}
+
 // CURRENT_DATE returns current date
 var CURRENT_DATE = jet.CURRENT_DATE