CASE

Syntax:

--SIMPLE CASE EXPRESSION
SELECT <columns>
    CASE <input_expression>
        WHEN <possible_input_expression_value> THEN <desired_expression>
        WHEN <possible_input_expression_value> THEN <desired_expression>
        ELSE <default_expression>
    END AS <desired_column_name>
FROM…WHERE…ETC

--SEARCHED CASE EXPRESSION
SELECT <columns>
    CASE
        WHEN <evaluation> THEN <desired_expression>
        WHEN <evaluation> THEN <desired_expression>
        ELSE <default_expression>
    END AS <desired_column_name>
FROM…WHERE…ETC

Examples:

SELECT DATEPART(MONTH, ct.TransactionDate),
       CASE
           WHEN DATEPART(MONTH, TransactionDate) BETWEEN 3 AND 5
           THEN 'Spring'
           WHEN DATEPART(MONTH, TransactionDate) BETWEEN 6 AND 8
           THEN 'Summer'
           WHEN DATEPART(MONTH, TransactionDate) BETWEEN 9 AND 11
           THEN 'Fall'
           WHEN DATEPART(MONTH, TransactionDate) = 12
                OR DATEPART(MONTH, TransactionDate) BETWEEN 1 AND 2
           THEN 'Winter'
       END AS [Season],
       *
FROM Sales.CustomerTransactions ct
WHERE ct.TransactionDate BETWEEN '2013-01-01' AND '2013-12-31'
      AND ct.CustomerID = 801;

SELECT CASE ct.TransactionTypeID
           WHEN 1
           THEN 'Invoice'
           WHEN 3
           THEN 'Payment'
       END AS [TransType],
       *
FROM Sales.CustomerTransactions ct
WHERE ct.TransactionDate BETWEEN '2013-01-01' AND '2013-12-31'
      AND ct.CustomerID = 801;