Question

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（例如2）中选择固定数量的产品，例如我想从产品表中选择每个类别的2个最新产品。我应该使用什么mysql查询，任何想法？

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小提琴链接中的输出：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

Answer 2

试试这个：

File theFile=new File(selectedPath);
            Wini ini=new Wini(theFile);
            System.out.println("path== "+selectedPath);
            String field = "soc.gif" + ",0";
            ini.put(".ShellClassInfo", "IconResource", field);
            ini.store();
            System.out.println("path2== "+selectedPath);

            Runtime.getRuntime().exec("attrib -h +s " + theFile.getAbsolutePath());

使用条件Mysql按列选择行

2 个答案: