尝试利用 https://stackoverflow.com/a/123481 中的技术我正在尝试为每个 name 检索一行，具有特定的排序，即按 active、created<>，然后是 prid。但是，active 列可能包含数字或 NULL 值，这会导致 name=bat 情况下出现重复。任何帮助将不胜感激。

源表:

+------+-------+--------+---------+
| prid | name  | active | created |
+------+-------+--------+---------+
| 1    | bat   | NULL   | 3       |
| 2    | bat   | 1      | 2       |
| 3    | bat   | 2      | 1       |
| 4    | bat   | 3      | 0       |
| 5    | noise | NULL   | 2       |
| 6    | noise | NULL   | 1       |
| 7    | cup   | NULL   | 0       |
| 8    | cup   | NULL   | 0       |
| 9    | egg   | 4      | 4       |
| 10   | egg   | 4      | 2       |
+------+-------+--------+---------+

期望的结果:

+------+-------+--------+---------+
| prid | name  | active | created |
+------+-------+--------+---------+
| 9    | egg   | 4      | 4       |
| 4    | bat   | 3      | 0       |
| 5    | noise | NULL   | 2       |
| 8    | cup   | NULL   | 0       |
+------+-------+--------+---------+

SQL:

SELECT p1.*
FROM source_table p1
LEFT JOIN source_table p2 ON (
  p1.name = p2.name
  AND (
    p1.active < p2.active
    OR (
      (p1.active = p2.active OR (p1.active IS NULL AND p2.active IS NULL))
      AND (
        p1.created < p2.created
        OR (
          p1.created = p2.created AND p1.prid < p2.prid
        )
      )
    )
  )
)
WHERE p2.prid IS NULL
ORDER BY p1.active DESC, p1.created DESC, p1.prid DESC

实际结果:

+------+-------+--------+---------+
| prid | name  | active | created |
+------+-------+--------+---------+
| 9    | egg   | 4      | 4       |
| 4    | bat   | 3      | 0       |
| 1    | bat   | NULL   | 3       |
| 5    | noise | NULL   | 2       |
| 8    | cup   | NULL   | 0       |
+------+-------+--------+---------+

@Gordon Linoff

感谢您的帮助，我尝试使用带有索引 (name, active, created, prid) 和 (active, created, prid) 的第二个版本，但是速度很慢。

这需要 1 秒，返回正确的结果，但顺序错误:

SELECT t1.prid
FROM source_table t1
WHERE t1.prid = (
  SELECT t2.prid
  FROM source_table t2
  WHERE t2.name = t1.name
  ORDER BY t2.active DESC, t2.created DESC, t2.prid DESC
  LIMIT 1
)
LIMIT 50

这需要 55 秒:

SELECT t1.prid
FROM source_table t1
WHERE t1.prid = (
  SELECT t2.prid
  FROM source_table t2
  WHERE t2.name = t1.name
  ORDER BY t2.active DESC, t2.created DESC, t2.prid DESC
  LIMIT 1
)
ORDER BY t1.active DESC, t1.created DESC, t1.prid DESC
LIMIT 50

我真的需要 LIMIT 500，有什么想法吗？

@Rick James

SQL Fiddle 链接:http://sqlfiddle.com/#!9/f9b39/2/0

最佳答案

使用 ANSI 标准的 row_number() 函数:

select t.*
from (select t.*,
             row_number() over (partition by name
                                order by active desc, created desc, pid desc
                               ) as seqnum
      from source_table t
     ) t
where seqnum = 1;

以上适用于 MySQL 8+ 和 MariaDB 10.2+。在早期版本中，您可以:

select t.*
from source_table t
where t.prid = (select t2.prid
                from source_table t2
                where t2.name = t.name
                order by t2.active desc, t2.created desc, t2.pid desc
                limit 1
               );

为了提高性能，您需要在 source_table(name, actdive desc, created desc, pid desc, prid) 上建立索引。

关于mysql - SQL - 每个名字一行，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/51304173/