SQL tables join [closed] - sql

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I am looking for a way to fetch records in a single sql query for the complex joins. I have three tables lets say users, user_projects and appointments. User can be assigned to multiple projects and user can have multiple appointments on different dates. How do I select all the users which is assigned to project1 and project2 and has appointments on date1 and date2? I am using postgres 9.5
Sample Data:
users table:
id, name
1, Steve
2, Bill
3, Emma
user_projects table:
id, user_id, project_id
1, 1, 1
2, 2, 1
3, 3, 1
4, 1, 2
appointments table:
id, user_id, date
1, 1, 2016-10-07
2, 2, 2016-10-07
3, 3, 2016-10-07
4, 1, 2016-11-15
5, 2, 2016-11-15
For this special case lets say I want to find all the users that belongs to project with id 1 and 2 and has appointment fixed on date 2016-10-07 and 2016-11-15. And the expected output is it should only include user with id 1 ie. Steve in this case

Answer 1

Perhaps I'm missing something in the question. Looks like a simple join to me:
select distinct u.id
from users u
join user_projects up1 on u.id=up1.user_id
join user_projects up2 on u.id=up2.user_id
join appointments a1 on u.id=a1.user_id
join appointments a2 on u.id=a2.user_id
where up1.project_id = 1
and up2.project_id = 2
and a1.date = '2016-10-07'
and a2.date = '2016-11-15'
Here is a fiddle: http://sqlfiddle.com/#!15/415fd/1/0

Answer 2

SELECT ID
FROM USERS U
JOIN (
-- Users in two projects
SELECT USER_ID
FROM USER_PROJECT
WHERE PROJECT_ID = 1
INTERSECTION
SELECT USER_ID
FROM USER_PROJECT
WHERE PROJECT_ID = 2
) UP ON U.ID = UP.USER_ID
JOIN (
-- user ids that have appointments on two dates:
SELECT USER_ID
FROM APPOINTMENT
WHERE DATE = '2016-10-07'
INTERSECTION
SELECT USER_ID
FROM APPOINTMENT
WHERE DATE = '2016-11-15'
) A ON U.ID = A.USER_ID
Another way to do it that should have the same performance (maybe this seems better because there are less lines?):
SELECT ID
FROM USERS U
JOIN (
-- Users in two projects
SELECT USER_ID
FROM USER_PROJECT
WHERE PROJECT_ID IN (1,2)
GROUP BY USER_ID
HAVING COUNT(DISTINCT PROJECT_ID) = 2
) UP ON U.ID = UP.USER_ID
JOIN (
-- user ids that have appointments on two dates:
SELECT USER_ID
FROM APPOINTMENT
WHERE DATE IN ('2016-10-07','2016-11-15')
GROUP BY USER_ID
HAVING COUNT(DISTINCT DATE) = 2
) A ON U.ID = A.USER_ID