SQL：选择A而不是B的集合

Question

I have Two tables: left one is users_projects, right one is projects:

I want to select the projects that user 3 is not participating in (only p_ID 5 and 7).

I've tried SELECT * FROM users_projects up INNER JOIN projects p ON p.p_ID=up.p_ID WHERE up.u_ID!=3

but that also returns me p_ID 1 which both user 2 and 3 are a part of.

Thanks for your help!

Answer 1

A solution with LEFT JOIN :

SELECT 
  * 
FROM 
  projects p LEFT JOIN users_projects up ON (p.p_ID = up.p_ID AND up.u_ID = 3)
WHERE 
  up.u_ID IS NULL

Basically select all Projects and join them with the user_projects of the desired user. Left join makes all rows from the project table appear even if the is no corresponding row in the users_projects table. These rows have all fields from the users_projects set to NULL , so we can just select those.

Answer 2

This is not a JOIN query, but a query with a non-correlated sub-select with a NOT IN() predicate.

I hope the columns of the projects table are enough ...

SELECT
  *
FROM 
  (         SELECT 1,'Apple'   -- input data, don't use in 'real' query
  UNION ALL SELECT 5,'Banna'   -- input data, don't use in 'real' query
  UNION ALL SELECT 7,'Carrot'  -- input data, don't use in 'real' query
  UNION ALL SELECT 8,'Durian') -- input data, don't use in 'real' query 
projects(p_id,p_name)
WHERE p_id NOT IN (
  SELECT
    p_id
  FROM 
    (         SELECT 2,1  -- input data, don't use in 'real' query
    UNION ALL SELECT 2,5  -- input data, don't use in 'real' query
    UNION ALL SELECT 2,7  -- input data, don't use in 'real' query
    UNION ALL SELECT 3,1  -- input data, don't use in 'real' query
    UNION ALL SELECT 3,8) -- input data, don't use in 'real' query
  users_projects(u_id,p_id)
  WHERE u_id=3
)
;

p_id|p_name
   7|Carrot
   5|Banna