MySQL select join where AND where
我的数据库中有两个表:
I have two tables in my database:
产品
- id(整数,主键)
- 名称(varchar)
产品标签
- product_id (int)
- tag_id (int)
我想选择具有所有给定标签的产品.我试过了:
I would like to select products having all given tags. I tried:
SELECT
*
FROM
Products
JOIN ProductTags ON Products.id = ProductTags.product_id
WHERE
ProductTags.tag_id IN (1, 2, 3)
GROUP BY
Products.id
但它为我提供了具有任何给定标签的产品,而不是所有给定的标签.编写 WHERE tag_id = 1 AND tag_id = 2
是没有意义的,因为不会返回任何行.
But it gives me products having any of given tags, instead of having all given tags. Writing WHERE tag_id = 1 AND tag_id = 2
is pointless, because no rows will be returned.
推荐答案
这种类型的问题被称为 关系划分
This type of problem is known as relational division
SELECT Products.*
FROM Products
JOIN ProductTags ON Products.id = ProductTags.product_id
WHERE ProductTags.tag_id IN (1,2,3)
GROUP BY Products.id /*<--This is OK in MySQL other RDBMSs
would want the whole SELECT list*/
HAVING COUNT(DISTINCT ProductTags.tag_id) = 3 /*Assuming that there is a unique
constraint on product_id,tag_id you
don't need the DISTINCT*/
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