mysql - 如何连接没有记录的表？

Question

我有这两张表产品和愿望 list 。这是结构

这是愿望 list

这是我的问题

SELECT a.productId,productName,isNew,isHot,productImage,a.categoryId,productPrice,productDescription, 
IF(b.UserId = NULL, 1,0) as isLiked
from products a LEFT JOIN  wishlist b on (a.productId = b.productId) and (a.categoryId = b.categoryId)
where b.userId = 'usr001'

但是查询没有显示任何记录，当我删除条件时它显示产品记录。

所以我想即使使用条件也显示产品记录，我该如何解决它？

Answer 1

这按预期工作 产品展示

+----+-----------+
| id | name      |
+----+-----------+
|  1 | product 1 |
|  2 | product 2 |
|  3 | product 3 |
|  4 | product 4 |
|  5 | product 5 |
+----+-----------+
5 rows in set (0.00 sec)

create table wishlist
(userid int, productid int,name varchar(20));
insert into wishlist values
(1,1,'product 1'),
(1,3,'product 3');

select p.id,p.name,if(w.productid is null, 0 , 1 ) isliked
from products p
left join wishlist w on w.productid = p.id and p.name = w.name and w.userid = 1
order by p.id limit 5;

+----+-----------+---------+
| id | name      | isliked |
+----+-----------+---------+
|  1 | product 1 |       1 |
|  2 | product 2 |       0 |
|  3 | product 3 |       1 |
|  4 | product 4 |       0 |
|  5 | product 5 |       0 |
+----+-----------+---------+
5 rows in set (0.00 sec)

我没有看到我的模型和你的模型之间有什么区别 - 如果您将示例数据作为文本与表格定义(作为文本)一起添加到问题中，将会有所帮助。