Question

我有3张桌子，

categories:
ID, category
"1","Cars"
"2","Trucks"
"3","Bikes"
"4","Planes"
"5","Boats"

users:
ID, username
"1","john"
"2","bob"
"3","billy"

users_categories:
ID, userid, categoryid
"1","1","1"
"2","1","2"
"3","2","2"
"4","3","3"

现在我需要选择具有多个类别（我指定的）的用户，所以在这个例子中，我需要一个只选择具有类别1和2的用户“john”的查询。我将传递我的类别需要在查询中。即1和2。

Answer 1

如果需要两个以上的类别，此查询会为所需用户提供：

SELECT userid
FROM user_categories
WHERE 
   categoryid IN (1,2)
GROUP BY userid
HAVING COUNT(*) = 2;

注意：having count(*) = xx必须等于传递给IN ( ... )列表的多个类别。

要检索用户名，请将以上查询加入users表：

SELECT u.*
FROM users u
JOIN (
  SELECT userid
   FROM user_categories
   WHERE 
      categoryid IN (1,2)
   GROUP BY userid
   HAVING COUNT(*) = 2
) q
ON u.id = q.userid;

演示：http://www.sqlfiddle.com/#!2/58651/4

Answer 2

编辑：meh，错过了有关选择用户名的详细信息，而不是用户名约束。

这是正确答案：

SELECT * FROM `users_categories` AS `uc`
INNER JOIN `users` AS `u` ON `u`.`ID`=`uc`.`userid`
WHERE `uc`.`categoryid` IN (1,2)
GROUP BY `uc`.`userid`
HAVING COUNT(`uc`.`userid`) >= 2

小提琴：http://sqlfiddle.com/#!2/8fdca/9

出于历史目的而留下原始答案。

是的，这可以通过表连接实现。

约束为username和categoryid ... john ：

SELECT `c`.* FROM `users` AS `u`
INNER JOIN `users_categories` AS `uc` ON `uc`.`userid`=`u`.`ID` AND `uc`.`categoryid` IN (1,2)
INNER JOIN `categories` AS `c` ON `c`.`ID`=`uc`.`categoryid`
WHERE `u`.`username`='john';

小提琴：http://sqlfiddle.com/#!2/8fdca/1

如果您删除categoryid约束（以获取给定用户名的所有类别），此次删除AND uc.categoryid IN (1,2) ... billy ，查询也会有效：

SELECT `c`.* FROM `users` AS `u`
INNER JOIN `users_categories` AS `uc` ON `uc`.`userid`=`u`.`ID`
INNER JOIN `categories` AS `c` ON `c`.`ID`=`uc`.`categoryid`
WHERE `u`.`username`='billy';

小提琴：http://sqlfiddle.com/#!2/8fdca/3

此外，如果你已经知道用户的ID，那么现在可以进一步简化......对于 bob （id＃2）：

SELECT `c`.* FROM `users_categories` AS `uc`
INNER JOIN `categories` AS `c` ON `c`.`ID`=`uc`.`categoryid`
WHERE `uc`.`userid`=2;

小提琴：http://sqlfiddle.com/#!2/8fdca/5

从映射表中选择

2 个答案: