What is the syntax of case when in SQL? Can someone please suggest?

[Bobdang]

In what situation is case when used and what are alternatives of using case when?

 

[AngleWyrm]

from MySQL

SELECT OrderID, Quantity,
CASE
    WHEN Quantity > 30 THEN "The quantity is greater than 30"
    WHEN Quantity = 30 THEN "The quantity is 30"
    ELSE "The quantity is under 30"
END
FROM OrderDetails;