php - 按条件按列选择行 Mysql

Question

好吧，我有一个表 (MYSQL) 'product'，其中一个名为 'category_id' 的列存储来自另一个表 'category' 的类别 ID。因此，“category_id”列中有多个类别。例如下表。现在我想从每个类别中选择固定数量的产品(比如 2)，例如我想从产品表中的每个类别中选择 2 个最新产品。我应该使用什么 mysql 查询，知道吗？

产品:

+-------------+------+---------------------+
| category_id | name |      timestamp      |
+-------------+------+---------------------+
|           1 | abc  | 2016-02-27 16:04:00 |
|           1 | def  | 2016-02-28 16:10:00 |
|           1 | ghi  | 2016-02-29 16:11:00 |
|           2 | pqr  | 2016-02-27 16:04:00 |
|           2 | stu  | 2016-02-27 16:05:00 |
|           3 | vwx  | 2016-02-28 16:04:00 |
+-------------+------+---------------------+

预期结果:

产品:

+-------------+------+---------------------+
| category_id | name |      timestamp      |
+-------------+------+---------------------+
|           1 | def  | 2016-02-28 16:10:00 |
|           1 | ghi  | 2016-02-29 16:11:00 |
|           2 | pqr  | 2016-02-27 16:04:00 |
|           2 | stu  | 2016-02-27 16:05:00 |
|           3 | vwx  | 2016-02-28 16:04:00 |
+-------------+------+---------------------+

Answer 1

它可能对你有帮助...

检查 SQL Fiddle 链接中的输出:http://sqlfiddle.com/#!9/3942d2/6

SELECT
    category_id,
    name,
    tsDate
FROM
(
    SELECT
        category_id,
        name,
        tsDate,
        @rn := IF(@prev = category_id, @rn + 1, 1) AS rn,
        @prev := category_id
    FROM Product
    JOIN (SELECT @prev := NULL, @rn := 0) AS vars
    ORDER BY category_id, tsDate DESC, name
) AS T1
WHERE rn <= 2