SELECT 查询从每组返回 1 行

问题描述

这是一个产品表，有几百万条记录.

This is a product table and have few million of records.

我想列出如下记录:
通常我使用:

I want to list record as below:
Normally I use:

SELECT id, 
       product_name, 
       store_id 
FROM product
GROUP BY store_id 
ORDER BY id.

目前有 SQL 性能问题.我需要 SQL 查询来输出这样的结果.

Currently having SQL performance issue. I need SQL query to output result like this.

推荐答案

有很多替代方案可以解决这个问题，我推荐的一个是加入一个单独获取最新 ID (假设对于每个 store_ID，该列都是 AUTO_INCREMENTed).

There are many alternatives to solves this, one which I recommend is to have joined a subquery which separately gets the latest ID (assuming that the column is AUTO_INCREMENTed) for each store_ID.

SELECT  a.*
FROM    tableName a
        INNER JOIN
        (
            SELECT  store_ID, MAX(ID) max_ID
            FROM    tableName
            GROUP BY store_ID
        ) b ON a.store_ID = b.store_ID AND
                a.ID = b.max_ID

• SQLFiddle 演示

为了获得更好的性能，请确保在这些列上有一个索引:ID 和 store_id.

for better performance, be sure to have an index on these columns: ID and store_id.

更新 1

如果你想对每条记录设置限制，请在下面使用，

if you want to have limit for every records, use this below,

SELECT ID, product_Name, store_ID
FROM   tableName a
WHERE
  (
     SELECT COUNT(*) 
     FROM   tableName b
     WHERE  b.store_ID = a.store_ID AND b.ID >= a.ID
  ) <= 2;

• SQLFiddle 演示

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