SQL查询返回另一个表中不匹配的行

Question

I have a users country table where I hold a list of users country's- a user can have many addresses and therefore many country's

it is joined to the user table via an address table

if I do a select distinct CountryId from UserCountry Data returned is:

1
2
3
4
5
6
etc

If I run the following query I get a User returned

select * from User u join Address a
on u.AddressId = a.Id
where a.CountryId = 1

If I then run

select * from User u join Address a
on u.AddressId = a.Id
where a.CountryId = 2

I get no data returned which is fine. What I need to to do is pass in a list of all the distinct Country Ids and produce a output of which set return a User Object and which set don't return a User object

Answer 1

The following query:

SELECT CountryId, COALESCE(t2.cnt, 0) AS usersPerCountry 
FROM (
   SELECT DISTINCT CountryId 
   FROM UserCountry) AS t1
LEFT JOIN (
   SELECT a.CountryId, COUNT(*) AS cnt
   FROM User u 
   INNER JOIN Address a ON u.AddressId = a.Id
   GROUP BY a.CountryId
) AS t2 ON t1.CountryId = t2.CountryId
ORDER BY COALESCE(t2.cnt, 0) DESC

will give you the number of users associated with each country. If a country is not related to any users at all then 0 is returned.

Explanation:

The above query makes use of two derived tables, t1 and t2 :

• The first derived table of the query, t1 , contains a list of all distinct CountryId values contained in UserCountry .
• The second derived table, t2 , returns the number of users per CountryId . Ids with no users are not returned by this sub-query.

Performing a LEFT JOIN between t1 and t2 with t1 as the first table returns all values of t1 , ie all CountryId values contained in UserCountry . If there is no match in t2 , then t2.cnt is NULL . COALESCE used in the SELECT clause converts this NULL value into a 0 .

Answer 2

This will give you all items in User and Address that do not have a Country entry. Is this what you are looking for?

SELECT *
FROM User U
JOIN Address a
ON a.Id = u.AddressId
WHERE NOT EXISTS
(
   SELECT 1
   FROM UserCountry uc
   WHERE uc.CountryId = a.CountryId
);

For the list of Country items that do not have a User, you can alternate the statements and find where the CountryId NOT EXISTS in the User and Address tables