MySQL: Join with 4 tables and common field: row_number() - mysql

Question

I have 4 tables -
Tab: d
Name | ID
----------
A | 1
B | 2
C | 3
Tab: p
Name | ID
----------
AX | 1
B | 2
X | 3
Y | 4
Z | 5
Tab: s
Name | ID
----------
A | 1
BL | 2
V | 3
M | 4
Tab: a
Name | ID
----------
K | 1
J | 2
H | 3
N | 4
Now I am using MySQL and today I found out that MySQL does not support FULL join. So, I am using left join with all 4tables and then using "union" and right join to merge all the 4 tables' records.
The query I am using is like -
(select d.Name, p.Name, s.Name, a.Name from doc d
left join
prof p
on d.id = p.id
left join
sing s
on d.id = s.id
left join
act a
on d.id = a.id)
union
(select d.Name, p.Name, s.Name, a.Name from doc d
right join
prof p
on d.id = p.id
right join
sing s
on d.id = s.id
right join
act a
on d.id = a.id)
But this is not giving the intended output. It is giving something like -
D | P | S | A
---------------------------------
A | AX | A | K
B | B | BL | J
C | X | V | H
NULL | NULL | NULL | N
Actual output should be -
D | P | S | A
---------------------------------
A | AX | A | K
B | B | BL | J
C | X | V | H
NULL | Y | M | N
NULL | Z | NULL | NULL
Please help me to figure out what I am missing! And also to help me to get the result...

Answer 1

The behavior you really want here is a full outer join, but MySQL does not directly support that (and the workaround is fairly ugly). One approach I can suggest here would be to maintain a fifth table containing all ID values which you expect in the result set. Consider:
SELECT c.ID, d.Name, p.Name, s.Name, a.Name
FROM
(SELECT 1 AS ID UNION ALL SELECT 2 UNION ALL SELECT 3 UNION ALL
SELECT 4 UNION ALL SELECT 5) c
LEFT JOIN d ON d.ID = c.ID
LEFT JOIN p ON p.ID = c.ID
LEFT JOIN s ON s.ID = c.ID
LEFT JOIN a ON a.ID = c.ID
ORDER BY c.ID;

Answer 2

Assuming that you're using MySQL v8, you might be able to do it this way:
WITH RECURSIVE cte AS (
SELECT 1 AS rn, MAX(cnt) AS mxcnt
FROM
( SELECT COUNT(ID) cnt FROM doc UNION
SELECT COUNT(ID) FROM prof UNION
SELECT COUNT(ID) FROM sing UNION
SELECT COUNT(ID) FROM act ) v UNION ALL
SELECT rn+1, mxcnt FROM cte WHERE rn+1 <= mxcnt)
SELECT c.rn, d.Name, p.Name, s.Name, a.Name
FROM cte c
LEFT JOIN doc d ON d.ID = c.rn
LEFT JOIN prof p ON p.ID = c.rn
LEFT JOIN sing s ON s.ID = c.rn
LEFT JOIN act a ON a.ID = c.rn
ORDER BY c.rn;
Using WITH RECURSIVE to generate numbering sequence based on the largest count result from all related table then use it as the reference in LEFT JOIN. I agree with Tim about having a master table for all of the ids.