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,
                   name
            from tableA
            where naem = '语文') t1
               join (select uid,
                            name
                     from tableA
                     where naem = '数学') t2
                    on 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 tableA
       
      union all
       
      select uid,
             ShuXue
      from tableA;