简化LEFT JOIN

Question

所以我有这个问题：

  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：

1. 获取给定table1.field1值的field2。
2. 获取table1.field2，其中field1与上一个点中的第一个点相同，该点也具有来自第二个表的特定属性。 （由于规范化，这只是复杂的）如果不存在则返回NULL。

虽然这很有效但真的很难看。是否有更简单的版本做同样的事情？我很担心第二个JOIN需要LEFT后跟WHERE。我觉得INNER应该更简单但不起作用。

Answer 1

尝试将左连接转移到union和inner join，然后您可以获得这样的新查询：

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，我将原始查询从join转移到union，方法是将where子句拆分为：

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)

但是工会的第一部分可以像这样继续转移：

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union和内连接的顶级查询。

我没有任何数据，所以我不确定我是对还是不对。希望它有所帮助。