SQL-方法论

写SQL时可以考虑的手段：

1. 行转列
   1. 先分为多个临时表，然后JOIN到一起

   2. select uid,t1.name YuWen,t2.name ShuXue
      from (select uid,namefrom tableAwhere naem = '语文') t1join (select uid,namefrom tableAwhere naem = '数学') t2on t1.uid = t2.uid;

   3. 用sum(if())

   4. select uid,sum(if(name = '语文')) YuWen,sum(if(name = '数学')) ShuXue
      from tableA
      group by uid;

2. 列转行
   1. 先分为多个临时表，然后UNION到一起

   2. select uid,YuWen
      from tableAunion allselect uid,ShuXue
      from tableA;