SQL CASE..WHEN

Simple CASE expression:
CASE input_expression
WHEN when_expression THEN result_expression [ ...n ]
[ ELSE else_result_expression ]
END
Searched CASE expression:
CASE
WHEN Boolean_expression THEN result_expression [ ...n ]
[ ELSE else_result_expression ]
END

SELECT CustomerID, Name,
CASE CountryCode
WHEN 'TH' THEN 'Thailand'
WHEN 'EN' THEN 'English'
WHEN 'US' THEN 'United states'
ELSE 'Unknown'
END AS Country
FROM customer

SELECT CustomerID, Name,
CASE CountryCode
WHEN 'TH' THEN Column1
WHEN 'EN' THEN 'Value'
WHEN 'US' THEN Column3
ELSE Column4
END AS Country
FROM customer