使用 mysql 选择前 N 个 *groups* [英] Select the first N *groups* using mysql

问题描述

为了简单起见，假设我有一个包含 2 个字段的表 - PERSON_NAME 和 COMPANY_ID 与这些对:

To make it simple, let say I have a table with 2 fields - PERSON_NAME and COMPANY_ID with those pairs:

('a',1) ('b',1) ('c',2) ('d',2) ('e',3)

PERSON_NAME a 和 b 在 COMPANY_ID 1 等工作.

PERSON_NAME a and b are working in COMPANY_ID 1 and so on.

我想选择前 2 个(或 N 个)公司的所有行 -

I would like to select all rows for the first 2 (or N) COMPANIES -

('a',1) ('b',1) ('c',2) ('d',2)

我不知道每个 COMPANY_ID 有多少行.

I don't know how many rows there are for each COMPANY_ID.

GROUP BY 在这里不起作用，因为我需要每个 COMPANY_ID 中的所有行.
这个有效-

GROUP BY won't work here as I need all rows within each COMPANY_ID.
This one works -

SELECT * FROM T
WHERE COMPANY_ID in (SELECT DISTINCT COMPANY_ID
                     FROM T
                     ORDER BY 1 DESC
                     LIMIT N)

但由于性能是一个问题，这还不够(大约有 25k 家公司，每家公司大约有 5-15 人).N 通常为 500.

But since performance is an issue, it won't suffice (there are about 25k companies and about 5-15 persons in each company). N is usually 500.

推荐答案

改为以连接方式执行此操作:

Do this as a join instead:

SELECT T.*
FROM T join
     (SELECT DISTINCT COMPANY_ID
      FROM T
      ORDER BY 1 DESC
      LIMIT N
     ) TN
     on TN.COMPANY_ID = T.COMPANY_ID;

显式连接通常比 where 子句中的 in 表现更好.

Explicit joins often perform better than in in the where clause.

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