mysql的case when和java的switch case方法用法相似。如果条件1，就返回结果1。
语法如下：
CASE
WHEN condition1 THEN result1
WHEN condition2 THEN result2
WHEN conditionN THEN resultN
ELSE result
END;
例子：
select OrderId,Quantity,
CASE
WHEN Qualtity>30 THEN "The quantity is greater than 30 "
WHEN Qualtity=30 THEN "The quantity is 30"
ELSE "The quantity is under 30"
END AS QuantityText
FROM OrderDetails;
返回结果：
