MySQL：当一个表上存在多个连接时，计算NULL或0值

Question

I have a query that looks up people's full names based on their record ID's in a table called users . The full names are tied to their roles in another table ( table1 ). This requires multiple joins to the users table:

SELECT table1.id, users.full_name AS "Requester", 
users.full_name AS "Approver," 
users.full_name AS "Ordered By", 
users.full_name AS "Received By" 
FROM table1
JOIN users AS users
ON table1.requester_id = users.id
JOIN users AS users2
ON table1.approver_id = users2.id
JOIN users AS users3 
ON table1.ordered_by = users3.id
JOIN users AS users4
ON table1.received_by = users4.id
WHERE table1.deleted_record !=1;

The problem I'm having is with ordered_by and received_by . Often, they don't yet exist, because the order has neither been ordered nor received, so the ID for each can be 0, which has no corresponding value in the users table. When I run this query, I should get back all 475 records that exist, but I only get back 365, because of those 0 values. How can I modify this query to make sure all rows are returned, even if ordered_by and/or received_by = 0?

Answer 1

First, your primary table driving the query should be table1 . Then, you are using JOIN instead of LEFT JOIN . LEFT JOIN will give you a null result if no link, but not fail. In which case, you might have to use an IF for your fields value

SELECT table1.id, req.full_name AS "Requester", 
   app.full_name AS "Approver", 
   ordr.full_name AS "Ordered By", 
   rec.full_name AS "Received By" 
FROM table1
LEFT JOIN users AS req
   ON table1.requester_id = req.id
LEFT JOIN users AS app
   ON table1.approver_id = app.id
LEFT JOIN users AS ordr
   ON table1.ordered_by = ordr.id
LEFT JOIN users AS rec
   ON table1.received_by = rec.id
WHERE table1.deleted_record !=1;

This should do it

Answer 2

You are looking for left join :

SELECT t1.id, ur.full_name AS "Requester", 
       ua.full_name AS "Approver," 
       uo.full_name AS "Ordered By", 
       urv.uo AS "Received By" 
FROM table1 t1 LEFT JOIN
     users ur
     ON t1.requester_id = ur.id LEFT JOIN
     users ua
     ON t1.approver_id = ua.id LEFT JOIN
     users uo 
     ON t1.ordered_by = uo.id LEFT JOIN
     users urv
     ON t1.received_by = urv.id
WHERE t1.deleted_record <> 1;

Note that I changed the aliases on the users references from fairly meaningless u1 , u2 , etc. to ua , uo , and so on. Also, these need to be used in the SELECT to get the right full name.