Question

   Table1            Table2   
id      name    !   id  number
1       cat     !   1   10
2       dog     !   2   null
3       bat     !   3   null
4       rat     !   4   11
5       kong    !   5   null
Select *
FROM Table1 t0 left join Table2 T1 on T1.id = T2.id
id      name        number
1       cat         10
2       dog         null
3       bat         null
4       rat         11
5       kong        null
I want
id      name        number
1       cat         10
2       dog         10
3       bat         10
4       rat         11
5       kong        11

我需要更新sql表中的数字

我的问题：我找不到适合这里的逻辑，以至于where条件应该适用于所有这三种情况，因此它不应该破坏现有的功能。

Answer 1

您可以使用COALESCE和子查询：

Select T1.ID, T1.Name, 
       COALESCE(T2.Number, (SELECT Number
                            FROM Table2 
                            WHERE Table2.Number IS NOT NULL
                             AND  Table2.ID < T1.ID
                            ORDER BY Table2.ID DESC
                            LIMIT 1)) AS Number
FROM Table1 T1 
left join Table2 T2 
    on T1.id = T2.id

Demo

Answer 2

Select t1.id, name, t2.id, 
       if(isnull(t2.number), @n, @n:=t2.number) number
  FROM Table1 t1 
    left join Table2 t2 
      on T1.id = T2.id, 
    (select @n:=0) n

如何输入null是SQL

2 个答案: