如何與沒有記錄的表聯接？

Question

我有這兩個表產品和願望清單。 這是結構

這是願望清單

這是我的查詢

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，當我刪除顯示產品記錄的條件時，該查詢未顯示任何記錄。

因此，即使要使用條件，我也要顯示產品記錄，該如何解決？

Answer 1

在ON Clause使用b.userId = 'usr001'代替where clause

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

Answer 2

這按預期效果工作

+----+-----------+
| 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)

我看不到我的模型與您的模型之間的區別-如果您將示例數據作為文本與表定義（作為文本）一起添加到問題中，這將有所幫助。