Question

我遇到SQL查询问题。我想将以下2个查询加入到一个查询中。

此查询应返回0，这是我期望的

SELECT COUNT(*) FROM table3 INNER JOIN table1 ON
    table3.id = table1.id
WHERE table3.somefield = '0' AND table1.field1 = '1' AND table1.field2 = 'something') AS counts

此查询应返回2，这是我期望的

SELECT COUNT(*) FROM table3 INNER JOIN table2 ON
    table3.id = table2.id
WHERE table3.somefield = '0' AND table2.field1 = '1' AND table2.field2 = 'something') AS counts

我合并了查询并将条件移到了JOIN但这会导致查询计数太多行

SELECT COUNT(*) FROM table3 
    LEFT JOIN table1 ON
        table3.id = table1.id AND (table1.field1 = '1'  AND table1.field2 = 'something')
    LEFT JOIN table2 ON
         table3.id = table2.id AND (table2.field1 = '1' AND table2.field2 = 'something')
WHERE table3.somefield = '0'

我期望的是查询的第一部分，它应该返回0，第二部分应该是2，但输出的是148，它必须与JOIN中的AND条件有关。

这里出了什么问题？

Answer 1

在这里找到我的答案

mysql left join returns unexpected amount of rows

SELECT
    (SELECT COUNT(*) FROM table3 INNER JOIN table1 ON
        table3.id = table1.id
    WHERE table3.somefield = '0' AND table1.field1 = '1' AND table1.field2 = 'something') +
    (SELECT COUNT(*) FROM table3 INNER JOIN table2 ON
        table3.id = table2.id
    WHERE table3.somefield = '0' AND table2.field1 = '1' AND table2.field2 = 'something') AS countA,


    (SELECT COUNT(*) FROM table3 INNER JOIN table1 ON
        table3.id = table1.id
    WHERE table3.somefield = '1' AND table1.field1 = '1' AND table1.field2 = 'something') +
    (SELECT COUNT(*) FROM table3 INNER JOIN table2 ON
        table3.id = table2.id
    WHERE table3.somefield = '1' AND table2.field1 = '1' AND table2.field2 = 'something') AS countB

Answer 2

正在进行的是您的个人查询使用INNER JOIN，而您的合并查询使用LEFT JOIN。

您可以通过在table2上使用INNER JOIN并在table1上使用LEFT JOIN来修复它：

SELECT * FROM table3 
    INNER JOIN table2 ON
        table3.id = table2.id and table2.field1 = '1' AND table2.field2 = 'something'      
    LEFT JOIN table1 ON
        table3.id = table1.id and table1.field1 = '1'  AND table1.field2 = 'something'
WHERE table3.somefield = '0';

但这要求您在查询运行之前了解一些有关数据的信息（换句话说，它要求您已经知道table2有匹配而table1没有）。这不是一个好主意。

最好只获取COUNTs的SUM：

SELECT SUM(c) as totalCount
FROM (
  SELECT COUNT(*) as c FROM table3 INNER JOIN table1 ON
    table3.id = table1.id
  WHERE table3.somefield = '0' AND table1.field1 = '1' AND table1.field2 = 'something'
  UNION ALL
  SELECT COUNT(*) as c FROM table3 INNER JOIN table2 ON
    table3.id = table2.id
  WHERE table3.somefield = '0' AND table2.field1 = '1' AND table2.field2 = 'something'
) bothCounts;

MySQL中JOIN条件返回太多行的

2 个答案: