SQL LIMIT在没有LIMIT返回结果的情况下不返回任何结果

Question

SELECT * FROM mm_tfs 
WHERE product_slug LIKE '%football%' 
    AND schoolid = '8' AND category_id ='21'
LIMIT 4

返回4个值,就像我问的那样,但是下面的语句返回0 - 是否有关于使用我不熟悉的OR语句的规则？我的假设是它应该返回1中的所有值(如果不是限制,则返回更多值).

SELECT * FROM mm_tfs 
WHERE (product_slug LIKE '%football%' 
   AND schoolid = '8' AND category_id ='21') 
OR (product_description LIKE '%football%' 
   AND schoolid = '8' AND category_id ='21') 
LIMIT 4

_{cyberkiwi的注释Q2的第一个OR部分与Q1的WHERE子句完全相同}

product_description LIKE '%football%' AND schoolid = '8' AND category_id ='21

只要没有限制,没有OR语句本身就会产生所需的结果.当OR语句与LIMIT一起使用时,返回0值.

SELECT * FROM mm_tfs 
WHERE product_description LIKE '%football%' 
    AND schoolid = '8' AND category_id ='21'
LIMIT 4

^ - 产生0结果

SELECT * FROM mm_tfs 
WHERE product_description LIKE '%football%' 
    AND schoolid = '8' AND category_id ='21'

^ - 产生结果

最奇怪的是,所有这些查询在我的PHPMYADMIN SQL查询窗口中产生了正确的效果,但在应用程序本身中却没有.

Answer 1

你重复了一些不必要的条件。试试这个：

SELECT * FROM mm_tfs 
WHERE 
   (product_slug LIKE '%football%' 
 OR product_description LIKE '%football%')
   AND schoolid = '8' 
   AND category_id ='21'
LIMIT 4

更新：

我创建了下表：

create table mm_tfs2 (schoolid varchar(2), categoryid varchar(2), 
                      description varchar(20), slug varchar(20));

还有5次：

insert into mm_tfs2 values (8, 21, '', 'football');

最后是查询：

select * from mm_tfs2 where 
(slug like '%football%' and schoolid = 8 and categoryid = 21) 
or (description like '%football%' and schoolid = 8 and categoryid = 21) 
limit 4;
+----------+------------+-------------+----------+
| schoolid | categoryid | description | slug     |
+----------+------------+-------------+----------+
| 8        | 21         |             | football |
| 8        | 21         |             | football |
| 8        | 21         |             | football |
| 8        | 21         |             | football |
+----------+------------+-------------+----------+
4 rows in set (0.00 sec)

所以我很遗憾地说我无法重现这个问题。