如何组合这两个查询？ （在哪里）？

Question

Select * 
FROM Products as a 
join ProductTags as b 
on (a.ProductId = b.ProductId)
join Tags as c 
on (b.TagId = c.TagId)
where c.TagName = 'FISH'

Select * 
FROM Products as a 
join ProductTags as b 
on (a.ProductId = b.ProductId)
join  Tags as c 
on ( b.TagId = c.TagId)
where c.TagName = 'STEAK'

我知道可能有一个相当简单的解决方案，但我无法弄明白

我想获得所有在“标签”的ProductTags表中都有条目的产品。

新查询

原件：

SELECT * 
FROM ProductTags as a 
JOIN Tags as b on (a.TagId = b.TagId)
WHERE b.TagName = 'FISH'

SELECT * 
FROM ProductTags as a 
JOIN Tags as b on (a.TagId = b.TagId)
WHERE b.TagName = 'STEAK'

解     选择a.ProductId     从ProductTags作为     将标签加入为b（a.TagId = b.TagId）     在哪里b.TagName（'FISH'，'STEAK'）

我想我需要一个HAVING条款？ IDK

Answer 1

您只需要一个IN条件：

Select * 
FROM Products as a 
join ProductTags as b 
on (a.ProductId = b.ProductId)
join Tags as c 
on (b.TagId = c.TagId)
where c.TagName IN ('FISH', 'STEAK')

您可以在IN子句中添加任意数量的文字，只要它们以逗号分隔即可。

根据更新的问题进行修改：

如果您想获得多个类别匹配的所有ProductId，您可以这样做：

SELECT ProductTags.ProductId
FROM ProductTags
WHERE 
    -- Pull all matches of FISH
    ProjectTags.TagId IN (SELECT TagId
                          FROM Tags
                          WHERE TagName = 'FISH')
    AND
    -- Pull all matches of STEAK
    ProjectTags.TagId IN (SELECT TagId
                          FROM Tags
                          WHERE TagName = 'STEAK')

Answer 2

如果您想要返回所有STEAK和FISH，您可以使用简单的OR条件：

Select * 
FROM Products as a 
join ProductTags as b 
on (a.ProductId = b.ProductId)
join  Tags as c 
on ( b.TagId = c.TagId)
where c.TagName = 'STEAK' OR c.TagName = 'FISH'

Answer 3

这就是关系分工，我今天早些时候在Meta提出了这个问题。您可以使用子查询。外部查询将拉出与“Fish”匹配的所有行，内部查询将仅选择与“Steak”匹配的行。试试这个：

SELECT a.product_id
FROM producttags a
JOIN tags b ON b.tagid = a.tagid
WHERE b.tagname = 'Fish' AND a.product_id IN(
   SELECT a.product_id
   FROM producttags a
   JOIN tags b ON b.tagid = a.tagid
   WHERE b.tagname = 'Steak'

修改

如果需要添加其他代码，您可以在IN子句中添加更多WHERE个运算符：

SELECT a.product_id
FROM producttags a
JOIN tags b ON b.tagid = a.tagid
WHERE b.tagname = 'Fish' 
   AND a.product_id IN(
      SELECT a.product_id
      FROM producttags a
      JOIN tags b ON b.tagid = a.tagid
      WHERE b.tagname = 'Steak')
   AND a.product_id IN(
      SELECT a.product_id
      FROM producttags a
      JOIN tags b ON b.tagid = a.tagid
      WHERE b.tagname = 'Chicken')