if語法：IF(expr1,expr2,expr3)
其中，expr1是判斷條件，expr2和expr3分別是符合expr1和不符合expr1的返回結果。
如我們想把銷售量大于100的產品記為熱銷，其他記為不熱銷，

SELECT b.brand,
IF(b.sale > 100 , '熱銷', '不熱銷') AS hot_or_not
FROM db_data_jiadian.t_brand_sale AS b

結果：

這樣只能處理兩種情況，要處理多種情況用case...when。

SELECT b.brand AS '品牌', 

CASE
WHEN b.sale = 0 THEN '未售出'
WHEN b.sale < 100 THEN '非爆款'
ELSE '爆款'
END AS '銷售情況'

FROM db_data_jiadian.t_brand_sale AS b

結果：

注意匹配時匹配到第一個符合的項就結束匹配，所以sale==0時會匹配未售出，而不會再匹配非爆款。

再來一個曾經面試時被問到過的行轉列/列轉行的問題。比如表結構是item_id, param_name, param_value，最后想變成每個item_id一行，取某些param_name值作為列名，則：

SELECT item_id,max(nengxiao),max(neijizaoyin),max(waijizaoyin),max(pishu),max(bianpin) from(
SELECT item_id, 
 CASE WHEN param_name ="能效等級"  THEN param_value  END AS 'nengxiao',
 CASE WHEN param_name="內機噪音"  THEN param_value  END AS  'neijizaoyin' ,
  CASE WHEN param_name ="外機噪音"  THEN param_value  END AS  "waijizaoyin" ,
  CASE WHEN param_name = "匹數"      THEN  param_value  END AS "pishu" ,
  CASE WHEN param_name = "定頻/變頻" THEN   param_value  END AS "bianpin" 

 FROM db_midea_data_jiadian.temp_item_param) a 
GROUP BY item_id