Question

我正在尝试编写一个mysql命令来选择每个类别中的6个项目，如最右侧的“category”字段所示。我基本上想要为每个类别值选择6条记录。

下面是从SELECT *命令到数据库

的前几行的示例

myArray = myArray.filter(function(element) {
  return element.state.name !== 'object #1';
})

这是我正在使用的命令。我正在使用while循环继续运行，直到它运行了多次类别（在这种情况下为3），每次循环运行时从每个类别中选择6个项目

+--------------------------------------+------------------------------------+--------------+--------+--------------+---------------------------+
| imageLocation                        | productName                        | manufacturer | price  | availability | category                  |
+--------------------------------------+------------------------------------+--------------+--------+--------------+---------------------------+
| ./images/Strawberry_Pi_3_Model_B.jpg | Camera Module XF181                | Toro         |  35.99 |           10 | Strawberry Pi             |
| ./images/Strawberry_Pi_3_Model_B.jpg |  Strawberry Pi Extension Kit SG218 | Apollo       |  22.99 |            4 | Popular Items             |
| ./images/Strawberry_Pi_Zero.jpg      |  Strawberry Pi Extension Kit AU252 | Gorella      | 194.99 |            1 | Strawberry Pi Accessories |
| ./images/Strawberry_Pi_2_Model_B.jpg |  Strawberry Pi Case ZM942          | Corona       | 182.99 |            7 | Popular Items             |
| ./images/Strawberry_Pi_2_Model_B.jpg |  Compute Module Kit GP664          | Corona       |  16.99 |            1 | Strawberry Pi Accessories |
| ./images/Strawberry_Pi_3_Model_B.jpg |  Camera Module CL638               | Apollo       | 256.99 |            7 | Strawberry Pi Accessories |

我收到错误

SET x = 0; WHILE ( x < COUNT(SELECT UNIQUE category FROM PRODUCTS) DO SELECT * FROM Products WHERE category IN (SELECT UNIQUE category FROM Products LIMIT x-1,1) LIMIT 6; SET x = x + 1; END WHILE;

我想要的输出示例是

ERROR 1193 (HY000): Unknown system variable 'x'
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'WHILE ( x < COUNT(SELECT UNIQUE category FROM PRODUCTS) DO SELECT * FROM Product' at line 1
ERROR 1193 (HY000): Unknown system variable 'x'
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'END WHILE' at line 1

Answer 1

SELECT *
   FROM
     (SELECT *,
            @category_rank := IF(@current_category = category, @category_rank + 1, 1) AS category_rank,
            @current_category := category
       FROM Products
       ORDER BY category
     ) ranked
   WHERE category_rank <= 6;

MySQL：为字段中的每个唯一值选择固定数量的记录

1 个答案: