我在c9.io上使用mysql，我想在限制语句中查看一些变量 我本质上想要的是
While (x<count(SELECT DISTINCT category FROM Products){
SELECT * FROM Products WHERE category=(SELECT DISTINCT category FROM Products LIMIT x, 1) LIMIT 6;
}
当我将x设置为等于0-3之间的任何数字时，while循环内的查询工作正常。 恩。 MySQL的＆GT; SELECT * FROM Products WHERE category =（SELECT DISTINCT category FROM
Products LIMIT 2, 1) LIMIT 6;
+--------------------------------------+------------------------------------+--------------+--------+--------------+---------------------------+
| imageLocation | productName | manufacturer | price | availability | category |
+--------------------------------------+------------------------------------+--------------+--------+--------------+---------------------------+
| ./images/Strawberry_Pi_Zero.jpg | Strawberry Pi Extension Kit AU252 | Gorella | 194.99 | 1 | Strawberry Pi Accessories |
| ./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 |
| ./images/Strawberry_Pi_Case.jpg | Strawberry Pi LG178 | Tufts | 26.99 | 10 | Strawberry Pi Accessories |
| ./images/Sense_Hat.jpg | Strawberry Pi OW299 | Darstar | 35.99 | 4 | Strawberry Pi Accessories |
| ./images/Compute_Module.jpg | Compute Module Kit QR216 | Confitura | 41.99 | 6 | Strawberry Pi Accessories |
+--------------------------------------+------------------------------------+--------------+--------+--------------+---------------------------+
6 rows in set (0.00 sec)
我希望查询在x = 0时选择6条记录，在单个查询中x = 1，x = 2和x = 3时。
我尝试过的while循环是
delimiter // CREATE PROCEDURE wholeblock:while_loop() BEGIN DECLARE x INT; SET x = 0; WHILE x < COUNT(SELECT DISTINCT category FROM Products) DO SELECT * FROM Products WHERE category=(SELECT DISTINCT category FROM Products LIMIT x, 1) LIMIT 6; SET x = x + 1; END WHILE; END//
不返回任何内容。
...
...
...
修改
我希望从1个查询的结果输出此输出。不是4个查询
mysql> SELECT * FROM Products WHERE category=(SELECT DISTINCT category FROM Products LIMIT 0, 1) LIMIT 6;+--------------------------------------+------------------------------------+--------------+--------+--------------+---------------+
| imageLocation | productName | manufacturer | price | availability | category |
+--------------------------------------+------------------------------------+--------------+--------+--------------+---------------+
| ./images/Strawberry_Pi_3_Model_B.jpg | Camera Module XF181 | Toro | 35.99 | 10 | Strawberry Pi |
| ./images/Camera_Module_V2.jpg | Compute Module Kit GX416 | Belrubi | 98.99 | 1 | Strawberry Pi |
| ./images/Sense_Hat.jpg | Strawberry Pi Extension Kit JJ556 | Toro | 92.99 | 1 | Strawberry Pi |
| ./images/Strawberry_Pi_Zero.jpg | Camera Module FI378 | Belrubi | 44.99 | 5 | Strawberry Pi |
| ./images/Compute_Module.jpg | Compute Module Kit HP564 | Elsanta | 239.99 | 5 | Strawberry Pi |
| ./images/Strawberry_Pi_1_Model_A.jpg | Compute Module UZ736 | Revada | 24.99 | 10 | Strawberry Pi |
+--------------------------------------+------------------------------------+--------------+--------+--------------+---------------+
6 rows in set (0.00 sec)
mysql> SELECT * FROM Products WHERE category=(SELECT DISTINCT category FROM Products LIMIT 1, 1) LIMIT 6;
+--------------------------------------+------------------------------------+--------------+--------+--------------+---------------+
| imageLocation | productName | manufacturer | price | availability | category |
+--------------------------------------+------------------------------------+--------------+--------+--------------+---------------+
| ./images/Strawberry_Pi_3_Model_B.jpg | Strawberry Pi Extension Kit SG218 | Apollo | 22.99 | 4 | Popular Items |
| ./images/Strawberry_Pi_2_Model_B.jpg | Strawberry Pi Case ZM942 | Corona | 182.99 | 7 | Popular Items |
| ./images/placeholder.png | Compute Module VO511 | Darstar | 188.99 | 3 | Popular Items |
| ./images/Strawberry_Pi_2_Model_B.jpg | Strawberry Pi DB112 | Tufts | 79.99 | 1 | Popular Items |
| ./images/Compute_Model_Kit.jpg | Compute Module DX828 | Aliso | 83.99 | 3 | Popular Items |
| ./images/Strawberry_Pi_Zero.jpg | Camera Module SZ841 | Glasso | 115.99 | 6 | Popular Items |
+--------------------------------------+------------------------------------+--------------+--------+--------------+---------------+
6 rows in set (0.00 sec)
mysql> SELECT * FROM Products WHERE category=(SELECT DISTINCT category FROM Products LIMIT 2, 1) LIMIT 6;
+--------------------------------------+------------------------------------+--------------+--------+--------------+---------------------------+
| imageLocation | productName | manufacturer | price | availability | category |
+--------------------------------------+------------------------------------+--------------+--------+--------------+---------------------------+
| ./images/Strawberry_Pi_Zero.jpg | Strawberry Pi Extension Kit AU252 | Gorella | 194.99 | 1 | Strawberry Pi Accessories |
| ./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 |
| ./images/Strawberry_Pi_Case.jpg | Strawberry Pi LG178 | Tufts | 26.99 | 10 | Strawberry Pi Accessories |
| ./images/Sense_Hat.jpg | Strawberry Pi OW299 | Darstar | 35.99 | 4 | Strawberry Pi Accessories |
| ./images/Compute_Module.jpg | Compute Module Kit QR216 | Confitura | 41.99 | 6 | Strawberry Pi Accessories |
+--------------------------------------+------------------------------------+--------------+--------+--------------+---------------------------+
6 rows in set (0.00 sec)
mysql> SELECT * FROM Products WHERE category=(SELECT DISTINCT category FROM Products LIMIT 3, 1) LIMIT 6;
+--------------------------------------+---------------------------+--------------+--------+--------------+--------------------------+
| imageLocation | productName | manufacturer | price | availability | category |
+--------------------------------------+---------------------------+--------------+--------+--------------+--------------------------+
| ./images/Strawberry_Pi_Zero.jpg | Strawberry Pi DS812 | Tufts | 172.99 | 6 | Industrial Compute Model |
| ./images/Strawberry_Pi_Case.jpg | Compute Module Kit CL851 | Confitura | 219.99 | 5 | Industrial Compute Model |
| ./images/placeholder.png | Compute Module Kit CM175 | Tago | 298.99 | 3 | Industrial Compute Model |
| ./images/Strawberry_Pi_3_Model_B.jpg | Camera Module MY484 | Tago | 33.99 | 8 | Industrial Compute Model |
| ./images/Sense_Hat.jpg | Compute Module Kit EB177 | Glasso | 66.99 | 5 | Industrial Compute Model |
| ./images/Strawberry_Pi_3_Model_B.jpg | Compute Module PW477 | Elsanta | 29.99 | 5 | Industrial Compute Model |
+--------------------------------------+---------------------------+--------------+--------+--------------+--------------------------+
6 rows in set (0.01 sec)
答案 0 :(得分：1)
您尝试构建的查询类型称为＆＃34;每个组的前n个＆＃34;并且通常使用ROW_NUMBER窗口函数在其他RDMBS中有效地完成，遗憾的是在MySQL中它仍然缺失。好消息是MySQL中的会话变量可以用来模拟它。
SELECT *
FROM (
SELECT p.*,
@i := IF(@p = p.category, @i + 1, 1) rn, -- increment a row number if it's still the same category
@p = p.category -- preserve the category
FROM products p JOIN (
SELECT DISTINCT category
FROM products
ORDER BY category -- always explicitly set an order when using limit
LIMIT 4
) c ON p.category = c.category CROSS JOIN (
SELECT @i := 0, @p := NULL -- Initialize vars in-line
) i
ORDER BY p.category, id -- order by category first, then by something else i.e. id, productname
) q
WHERE rn < 7 -- return only top 6 per group (category in your case)