简化 LEFT JOIN

Question

所以我有这个查询：

  SELECT c1.field1, c2.field2
  FROM table1 c1
  LEFT JOIN table1 c2 ON c1.field1 = c2.field1 AND c1.field2 = :arg1 AND c2.field2 <> :arg1
  LEFT JOIN table2 cc ON c2.field2 = cc.field2 AND cc.field3 = :arg2
  WHERE (c2.field2 IS NULL OR cc.field2 IS NOT NULL)
  ORDER BY ISNULL(c2.field2) ASC

这有两件事：

为给定的 field2 值选择 table1.field1。
从第二个 table 中选取一个 table1.field2，其中 field1 与前一点相同，也有一个特定的属性。（因为归一化才这么复杂）如果这不存在那么 return NULL.

虽然这行得通，但它真的很难看。有没有更简单的版本做同样的事情？我最烦恼的是第二个 JOIN 需要是一个 LEFT，然后是这个 WHERE。我觉得 INNER 应该更简单，但它不起作用。

Answer 1

尝试将左连接转移到union和inner join，然后你可以得到一个新的查询：

SELECT c1.field1, null as field2
FROM table1 c1
  UNION ALL
SELECT c1.field1, c2.field2
FROM table1 c1
INNER JOIN table1 c2 ON c1.field1 = c2.field1 
INNER JOIN table2 cc ON c2.field2 = cc.field2 
WHERE c1.field2 = :arg1 AND c2.field2 <> :arg1 AND cc.field3 = :arg2

首先，我通过拆分 where 子句将您的原始查询从 join 转移到 union：

SELECT c1.field1, c2.field2
FROM table1 c1
LEFT JOIN table1 c2 ON c1.field1 = c2.field1 AND c1.field2 = :arg1 AND c2.field2 <> :arg1
WHERE (c2.field2 IS NULL )
  UNION ALL
SELECT c1.field1, c2.field2
FROM table1 c1
INNER JOIN table1 c2 ON c1.field1 = c2.field1 AND c1.field2 = :arg1 AND c2.field2 <> :arg1
LEFT JOIN table2 cc ON c2.field2 = cc.field2 AND cc.field3 = :arg2
WHERE (c2.field2 IS NOT NULL AND cc.field2 IS NOT NULL)

但是 union 的第一部分可以像这样保持传输：

SELECT c1.field1, null
FROM table1 c1
WHERE c1.field2 <> :arg1 --c2.field2 must be null
  UNION ALL
SELECT c1.field1, c2.field2
FROM table1 c1
LEFT JOIN table1 c2 ON c1.field1 = c2.field1  AND c2.field2 <> :arg1
WHERE c1.field2 = :arg1 AND c2.field2 IS NULL

最后，我得到了将所有左连接转移到联合和内连接的顶级查询。

我没有任何数据，所以我不确定我是否正确。希望对你有帮助。

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