The CASE statement evaluates a list of conditions and returns one of multiple possible result expressions.
The CASE expression has two formats:
The simple CASE expression compares an expression to a set of simple expressions to determine the result.
The searched CASE expression evaluates a set of Boolean expressions to determine the result.
Both formats support an optional ELSE argument.
CASE can be used in any statement or clause that allows a valid expression.
References: https://docs.microsoft.com/en-us/sql/t-sql/language-elements/case-transact-sql?view=sql-server-2017