过滤器如何影响sql中的LEFT JOIN [重复]

Question

此问题已经在这里有了答案：

Left Outer Join doesn't return all rows from my left table?（2个答案）

关闭6分钟前。

我试图理解为什么尽管LEFT JOIN，以下查询仍只返回一行

with t1(day_partition, entity_id, feature_1) AS (values
  ('2020-05-15', 'id_1', 'x'),
  ('2020-05-15', 'id_2', 'y')
),

t2(day_partition, entity_id, feature_2) AS (values
  ('2020-05-15', 'id_1', 1)
)

SELECT
  t1.day_partition AS day_partition_1,
  t2.day_partition AS day_partition_2,
  t1.entity_id AS entity_id_1,
  t2.entity_id AS entity_id_2
FROM
  t1
LEFT JOIN
  t2
ON
  t1.entity_id = t2.entity_id
WHERE
  t2.day_partition = '2020-05-15'
;

返回

 day_partition_1 | day_partition_2 | entity_id_1 | entity_id_2 
-----------------+-----------------+-------------+-------------
 2020-05-15      | 2020-05-15      | o1          | o1          

但是，删除过滤器

WHERE
  t2.day_partition = '2020-05-15'

将返回

 day_partition_1 | day_partition_2 | entity_id_1 | entity_id_2 
-----------------+-----------------+-------------+-------------
 2020-05-15      | 2020-05-15      | id_1        | id_1        
 2020-05-15      | NULL            | id_2        | NULL        

我发现这种行为是不直观的，其背后的规则是什么？

Answer 1

如果将条件放入join，它将按预期工作

SELECT
  t1.day_partition AS day_partition_1,
  t2.day_partition AS day_partition_2,
  t1.entity_id AS entity_id_1,
  t2.entity_id AS entity_id_2
FROM
  t1
LEFT JOIN
  t2
ON
  t1.entity_id = t2.entity_id AND t2.day_partition = '2020-05-15'

查询解析器不知道您在想什么。如果在where子句中过滤数据，它将影响所有记录，而不仅是联接表的记录。

Answer 2

这是设计使然。 where子句中的条件是强制性的，因此在left join ed表上放置一个条件最终将逐出left join返回为空的行。基本上，这会将left join转换为inner join。

您需要将与来自left join ed表的列相关的所有谓词放在连接的on子句中：

FROM t1
LEFT JOIN t2
    ON t1.entity_id = t2.entity_id
    AND t2.day_partition = '2020-05-15'

通过查看结果集，我倾向于认为您实际上想要t1上的条件：

FROM t1
LEFT JOIN t2
    ON t1.entity_id = t2.entity_id
WHERE t1.day_partition = '2020-05-15'