插入表（var1）从中选择

Question

我想插入最后一栏（那个房间里的人数）和 我想用

Nc

这显然是错误的，因为我需要在people_id上​​加入表格，但我没有。什么是正确的代码？

Answer 1

以下是一种方法，使用内联视图获取N_People_In_Room总数：

我首先选择SELECT：

 SELECT t.peopleid
      , t.room
      , t.n_people_in_room AS `old_npir`
      , s.n_people_in_room AS `new_npir` 
  FROM mytable t
   JOIN ( SELECT c.room
               , COUNT(1) AS n_people_in_room
            FROM mytable c
           GROUP BY c.room
        ) s
     ON s.room = t.room

通过将SELECT ... FROM与UPDATE重新合并，并添加SET条款，将其转换为更新...

 UPDATE mytable t
   JOIN ( SELECT c.room
               , COUNT(1) AS n_people_in_room
            FROM mytable c
           GROUP BY c.room
        ) s
     ON s.room = t.room
    SET t.n_people_in_room = s.n_people_in_room