标记一个表中的行，其中值存在于连接表中？

Question

For my query, I have two tables, as defined below:

permissions table:

| permission_id | permission_description |
|---------------|------------------------|
|             1 | Create User            |
|             2 | Edit User              |
|             3 | Delete User            |

users_permissions table:

| permission_id | user_id |
|---------------|---------|
|             1 |       1 |
|             1 |       2 |
|             3 |       2 |
|             3 |       5 |
|             1 |       3 |
|             3 |       1 |

Now, I need to retrieve a list of all permissions in the permissions table, with a column to indicate if the user with user_id of 1 exists for each permission in the users_permissions table.

So, my desired output for the query would be:

| permission_id | permission_description | has_permission |
|---------------|------------------------|----------------|
|             1 | Create User            | TRUE           |
|             2 | Edit User              | FALSE          |
|             3 | Delete User            | TRUE           |

---

So far, I have tried the following query, but it returns entries for all permissions and all user_id values:

SELECT permissions.permission_id,
       permission_description,
       CASE WHEN user_id = 1 THEN 'TRUE' ELSE 'FALSE' END AS has_permission
FROM permissions
       INNER JOIN users_permissions ON permission.permission_id = users_permissions.permissions_id;

How do I limit that to just one entry per permission ?

---

For clarity, the end goal is to get a list of available permissions and mark the ones the user already has.

Answer 1

If you only want to know the answer for one user then an exists subquery will do the job - no need for a join.

SELECT P.permission_id
  , P.permission_description
  , CASE WHEN exists (select 1 from users_permissions UP where UP.permission_id = P.permission_id and UP.user_id = 1) THEN 'TRUE' ELSE 'FALSE' END AS has_permission
FROM [permissions] P

PS - I wouldn't recommend having a table called permissions as its a reserved word in SQL Server.

Answer 2

Use LEFT JOIN instead of INNER JOIN and check if it is null

select permissions.permission_id,
       permission_description,
       case when user_id is null then 'FALSE' else 'TRUE' END as has_permission
FROM permissions
LEFT JOIN users_permissions 
  ON permission.permission_id = users_permissions.permissions_id and user_id = 1