Question

请求会搜索具有固定＆＃39; id＆＃39;的对象。只有版本较高的行＆＃39;应该返回（最新版本）。但是如果较高版本已删除＆＃39;那么根本不应返回任何行。列真实。

示例：

|id    |version   |deleted |
|0     |1         |false   |
|0     |2         |false   |
|1     |1         |false   |
|1     |2         |false   |
|1     |3         |true    |

id 0的请求应该返回

|1     |2         |false   |

但是id 1的请求应该不返回任何内容

我可以用INTERSECT做到这一点：

SELECT id, version, deleted FROM elements WHERE id = 1 AND deleted = 'false'
INTERSECT
SELECT id, MAX(version), deleted FROM elements WHERE id = 1

但有更好的方法吗？

Answer 1

我倾向于用子查询做到这一点：

select e.*
from (select id, version, deleted
      from elements
      where id = 1
      order by version desc
      limit 1
     ) e
where deleted = 'false';

每id最多返回一行。只有在delete = 'false'。

时，它才会返回具有最高版本的行

Answer 2

这可以通过使用Row_number()功能实现，方法是根据版本对行进行排序，并使用版本最大的行并使用WHERE子句忽略deleted <> false

的行

select id, version, deleted from (
select id, version, deleted, row_number() over 
(partition by id order by version desc) where deleted = 'false'
) A where rown = 1

Answer 3

我会选择：

WITH src AS ( -- subquery gets the newest version for each of the ids
    SELECT DISTINCT ON (id)
        *
    FROM testtab 
    ORDER BY id, version DESC
)
SELECT * FROM SRC
WHERE NOT deleted -- or deleted = 'false' depending on it's type
    AND id = 0 -- your id goes here

如何仅在符合条件的情况下检索具有最大值的行，否则不应返回任何内容

3 个答案: