Question

尝试利用https://stackoverflow.com/a/123481中的技术，我尝试使用特定的排序方式为每个name检索一行，即按{{1降序排序后的第一行/第一行}}，active，然后是created。但是，prid列可能包含数字或active值，这导致在NULL情况下重复。任何帮助将不胜感激。

源表：

name=bat

所需结果：

+------+-------+--------+---------+
| 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       |
+------+-------+--------+---------+

SQL：

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

实际结果：

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

@Gordon Linoff

感谢您的帮助，我尝试使用索引为+------+-------+--------+---------+ | prid | name | active | created | +------+-------+--------+---------+ | 9 | egg | 4 | 4 | | 4 | bat | 3 | 0 | | 1 | bat | NULL | 3 | | 5 | noise | NULL | 2 | | 8 | cup | NULL | 0 | +------+-------+--------+---------+和(name, active, created, prid)的第二个版本，但是运行起来很慢。

这需要1秒钟，返回正确的结果，但是顺序错误：

(active, created, prid)

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
)
LIMIT 50

我真的需要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，有什么想法吗？

@Rick James

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

Answer 1

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;

以上内容适用于MySQL 8+和MariaDB 10.2+。在早期版本中，您可以执行以下操作：

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，您希望在source_table(name, actdive desc, created desc, pid desc, prid)上建立索引。

Answer 2

ORDER BY IFNULL(active,  -1) DESC,
         IFNULL(created, -1) DESC,
         IFNULL(pid,     -1) DESC,

完整提示

SELECT  prid, name, active, created
    FROM  
        ( SELECT  GROUP_CONCAT(prid
                    ORDER BY  IFNULL(active,  -1) DESC,
                              IFNULL(created, -1) DESC,
                              IFNULL(prid,    -1) DESC 
                              ) AS ids
            FROM  source_table
            GROUP BY  name 
        ) AS s1
    JOIN  source_table AS s2
        ON s2.prid = SUBSTRING_INDEX(s1.ids, ',', 1)
    ORDER BY  IFNULL(active,  -1) DESC,
              IFNULL(created, -1) DESC,
              IFNULL(prid,    -1) DESC ;

SQL-每个名称一行

2 个答案: