Case Statements

With case statements in T-SQL you can in essence insert a bunch of logic and calculation into a multi-faceted case statement with the output being a single result field

SELECT P.Age
		,CASE
			WHEN P.Age > 55	THEN 1
			ELSE 0
		END AS 'RetirementAge'
FROM Person AS P

This case statement examines the Age field and asks if each value i > 55 if it is, the result field is assigned a 1 if Age is not > 55 then the ELSE statement is executed and the result field receives the value of 0.

The ultimate of this being a boolean flag column indicating if each individual is at RetirementAge or not.

You can also nest several WHEN ... THEN statements for a multi faceted Case statement:

SELECT P.Age
		,CASE
			WHEN P.Age > 65	THEN 'Senior Citizen'
			WHEN P.Age > 55	THEN 'Mature Adult'
			WHEN P.Age > 40	THEN 'Mid-Life Crisis'
			ELSE 'No Discount'
		END AS 'DiscountLevel'
FROM Person AS P