MySQL选择加入何处和何处

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问题描述

我的数据库中有两个表:

I have two tables in my database:

产品

id(int，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

但是它给我的产品具有任何给定的标签，而不是具有所有给定的标签.写入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

这种类型的问题称为关系划分

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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