Question

在前一个问题的后续内容中，假设我有3个表，A，B和C.表A有一个ID，用作表B和C上的外键，每个表都有一些值作为属性。对于表A中的每个ID，我想得到表B和C之间的值差异，可以按如下方式进行：

CREATE VIEW T1 AS
    SELECT B.A_ID AS ID, SUM(B.VALUE) AS VAL
    FROM B
    GROUP BY B.A_ID;
CREATE VIEW T2 AS
    SELECT C.A_ID AS ID, SUM(C.VALUE) AS VAL
    FROM C
    GROUP BY C.A_ID;
SELECT T1.ID, T1.VAL, T2.VAL FROM T1, T2 WHERE T1.ID = T2.ID;

问题是，如果表B具有特定ID的某些值，但表C没有，反之亦然。在这种情况下，我的select语句不会返回该行。有没有办法让我创建一个单独的视图，基本上如下所示：

CREATE VIEW T3 AS
    SELECT B.A_ID AS ID, SUM(B.VALUE) AS VAL1, SUB(C.VAL) AS VAL2
    FROM B, C
    WHERE B.A_ID = C.A_ID
    GROUP BY B.A_ID;

此类视图的创建脚本示例将不胜感激。

Answer 1

您可以使用此

CREATE VIEW myView AS
SELECT test_a.id, name, IFNULL( (
  SELECT SUM( value ) 
  FROM test_b
  WHERE test_b.a_id = test_a.id
  GROUP BY test_b.a_id ) , 0
) - IFNULL( (
  SELECT SUM( value ) 
  FROM test_c
  WHERE test_c.a_id = test_a.id
  GROUP BY test_c.a_id ) , 0
)
FROM test_a

但如果c的总和高于b，这将导致负数。如果你想要绝对差异，请使用：

CREATE VIEW myView AS
SELECT test_a.id, name, ABS( IFNULL( (
  SELECT SUM( value ) 
  FROM test_b
  WHERE test_b.a_id = test_a.id
  GROUP BY test_b.a_id ) , 0 ) - IFNULL( (
SELECT SUM( value ) 
  FROM test_c
  WHERE test_c.a_id = test_a.id
  GROUP BY test_c.a_id ) , 0
  )
)
FROM test_a

Answer 2

SELECT ID, MAX(VAL1), MAX(VAL2) FROM 
((SELECT B.A_ID AS ID, SUM(B.VALUE) AS VAL1, 0 as VAL2
FROM B GROUP BY B.A_ID) as T1
UNION
(SELECT C.A_ID AS ID, 0 VAL1, SUM(C.VALUE) AS VAL2
FROM C GROUP BY C.A_ID) as T2) as Foo
GROUP BY FOO.ID

它有点hacky。另外，您无法创建具有联合的视图。

MySQL中的视图基于多个计算值

2 个答案: