There are 2 ways to use the CASE expression. The first way is to return one of the predefined values based on the comparison of a given value to a list of target values. The second way is to return one of the predefined values based on a list of conditions. Here is the syntax of both types of CASE expressions:
CASE value WHEN target_value THEN result
WHEN target_value THEN result
WHEN target_value THEN result
...
ELSE result
END
CASE WHEN condition THEN result
WHEN condition THEN result
WHEN condition THEN result
...
ELSE result
END
The tutorial exercise below gives two good examples:
SELECT CASE 'Sun' WHEN 'Mon' THEN 'Open'
WHEN "Fri" THEN "Open" ELSE 'Closed' END FROM DUAL;
Closed
SELECT CASE WHEN HOUR(CURRENT_TIME())<9 THEN
'Closed'
WHEN HOUR(CURRENT_TIME())>17 THEN 'Closed'
ELSE 'Open' END FROM DUAL;
Closed