MySQL no results because foreign key is NULL

Question

I want to assign one to three people to a subject. Then I want to retrieve the assignment for a particular subject.

I have one table SUBJECTS in which there is a dozen subjects (columns : id(PK), name). I have one table USERS in which there is a dozen users (columns : id(PK), firstName, lastName, etc.)

I then created a table AS_SUBJECTS in which there are the following columns (all of which are integers):

id(PK), subjectName, user1, user2, user3  

Relations :

• subjectName is FK to subjects.id
• user1 is FK to users.id
• user2 is FK to users.id
• user3 is FK to users.id

I want user2 and user3 to be optional, ie there can be one or two or three people assigned to one subject. Therefore I have allowed NULL on these columns.

Now let's say I enter these values in AS_SUBJECTS :

id = 1
subjectName = 1
user1 = 7
user2 = NULL
user3 = NULL

If I query : SELECT * FROM as_subjects WHERE as_subjects.subjectName = 1; I get the results for all columns, including the ones with NULL values.

The problem : I need to query a subject in AS_SUBJECTS and also retrieve users.firstName and users.lastName for each user assigned to this subject. So I use aliases :

SELECT 
    as_subjects.subjectName
    as_subjects.user1
    u1.firstName as user1FirstName
    u1.lastName as user1LastName
    as_subjects.user2
    u2.firstName as user2FirstName
    u2.lastName as user2LastName
    as_subjects.user3
    u3.firstName as user3FirstName
    u3.lastName as user3LastName
FROM as_subjects
JOIN subjects ON as_subjects.subjectName = subjects.id
JOIN users u1 ON as_subjects.user1 = users.id
JOIN users u2 ON as_subjects.user2 = users.id
JOIN users u3 ON as_subjects.user3 = users.id
WHERE as_subjects.subjectName = 1;

The query is valid, but when as_subjects.user2 or 3 is NULL, the set in empty, no data at all.

Any inputs on how to achieve this? All I could find were posts about how to select columns where values is NULL. What I need is to select columns NO MATTER IF the value is NULL.

Answer 1

Just change the [INNER] JOIN s on users to LEFT JOIN s

...
FROM as_subjects
JOIN subjects ON as_subjects.subjectName = subjects.id
LEFT JOIN users u1 ON as_subjects.user1 = users.id
LEFT JOIN users u2 ON as_subjects.user2 = users.id
LEFT JOIN users u3 ON as_subjects.user3 = users.id
...

See this SO post for a detailed discussion on various join types.

Answer 2

Take a look at different types of joins - it looks like you may be looking for a left outer join for example. (ie replace JOIN with LEFT OUTER JOIN in your examples whenever you want to keep rows where the table on the 'right' doesn't match the tables on the 'left')

https://www.sunnyhoi.com/app/uploads/2017/07/inner-join-vs-cross-join-in-sql-server.jpg