(My)SQL与三个表完全连接

问题描述：

我有树表

ID    A
-----------
1     10

ID    B
-----------
1     20
2     30

ID    C
-----------
2     40
3     50

有人可以告诉我如何进行这样的视图或查询打印吗?

Can anybody please tell how to make a view or query prints like this?

ID     A      B      C      R (A + B - C)
-----------------------------------
1     10     20      0     30
2      0     30     40    -10
3      0      0     50    -50

谢谢.

答

据我所知，MySql中没有完整的外部联接.因此，要执行所需的操作，您应该在派生表中获得不同的ID，并保留原始表的左联接:

As far as I know there is no full outer join in MySql. So, to do what you require you should get distinct IDs in derived table and left join original tables:

select ids.id,
       ifnull(table1.A, 0) A,
       ifnull(table2.B, 0) B,
       ifnull(table3.C, 0) C,
       ifnull(table1.A, 0) + ifnull(table2.B, 0) - ifnull(table3.C, 0) R
  from 
  (
    select id
      from table1
    union
    select id
      from table2
    union
    select id
      from table3
  ) ids
  left join table1
    on ids.id = table1.id
  left join table2
    on ids.id = table2.id
  left join table3
    on ids.id = table3.id

(My)SQL与三个表完全连接

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