MySQL - IF within SELECT - mysql

Question

I have the following MySQL query:
SELECT id, name, parent_id AS pID,
(SELECT parent_id
FROM category
WHERE id = pID)
AS grandparent_id,
(SELECT parent_id
FROM category
WHERE id = grandparent_id)
AS greatgrandparent_id,
(SELECT name
FROM category
WHERE id = pID)
AS parent,
(SELECT name
FROM category
WHERE id = grandparent_id)
AS grandparent,
(SELECT name
FROM category
WHERE id = greatgrandparent_id)
AS greatgrandparent
FROM category
WHERE active = '1'
HAVING grandparent IS NOT NULL
ORDER BY grandparent_id, parent_id, sort, id ASC
When the greatgrandparent IS NULL I want it to display the grandparent value instead. Is this possible?
I've tried CASE WHEN but must be getting the syntax wrong somewhere as it keeps throwing an error.

Answer 1

Simple solution: IFNULL(greatgrandparent, grandparent).

Answer 2

You cannot have a having clause without a group by clause.

Answer 3

May be this code helps you.
SELECT id, name, parent_id AS pID,
case when grandparent IS NOT NULL then
(SELECT parent_id FROM category WHERE id = grandparent_id)
else
(SELECT parent_id FROM category WHERE id = pID)
end as _parent_id,
case when grandparent IS NOT NULL then
(SELECT name FROM category WHERE id = grandparent_id)
else
(SELECT name FROM category WHERE id = pID)
end AS _parent_name
FROM category WHERE active = '1'
ORDER BY grandparent_id, parent_id, sort, id ASC

Answer 4

I would rewrite this using left joins with aliases and coalesce.
Given this
/*
CREATE TABLE CATEGORY(ID INT, NAME VARCHAR(20), PARENT_ID INT);
TRUNCATE TABLE CATEGORY;
INSERT INTO CATEGORY VALUES
(1,'AAAA',2),(2,'AAA',3),(3,'AA',4),(4,'A',5),
(10,'BBBB',11),(11,'BBB',12)
;
*/
This code
SELECT C.ID,C.NAME,C.PARENT_ID ,
CP.NAME AS PARENTNAME,CP.PARENT_ID AS PARENT_PARENT_ID,
CG.NAME AS GRANDPARENTNAME,
CG.PARENT_ID AS GRANDPARENT_PARENT_ID,
CGG.NAME AS GREATGRANDPARENTNAME ,
CGG.PARENT_ID AS GREATGRANDPARENT_PARENT_ID
FROM CATEGORY C
LEFT JOIN CATEGORY CP ON CP.ID = C.PARENT_ID
LEFT JOIN CATEGORY CG ON CG.ID = CP.PARENT_ID
LEFT JOIN CATEGORY CGG ON CGG.ID = CG.PARENT_ID
ORDER BY C.ID;
Results in
+------+------+-----------+------------+------------------+-----------------+-----------------------+----------------------+----------------------------+
| ID | NAME | PARENT_ID | PARENTNAME | PARENT_PARENT_ID | GRANDPARENTNAME | GRANDPARENT_PARENT_ID | GREATGRANDPARENTNAME | GREATGRANDPARENT_PARENT_ID |
+------+------+-----------+------------+------------------+-----------------+-----------------------+----------------------+----------------------------+
| 1 | AAAA | 2 | AAA | 3 | AA | 4 | A | 5 |
| 2 | AAA | 3 | AA | 4 | A | 5 | NULL | NULL |
| 3 | AA | 4 | A | 5 | NULL | NULL | NULL | NULL |
| 4 | A | 5 | NULL | NULL | NULL | NULL | NULL | NULL |
| 10 | BBBB | 11 | BBB | 12 | NULL | NULL | NULL | NULL |
| 11 | BBB | 12 | NULL | NULL | NULL | NULL | NULL | NULL |
+------+------+-----------+------------+------------------+-----------------+-----------------------+----------------------+----------------------------+
and this code
SELECT C.ID,C.NAME,C.PARENT_ID ,
CP.NAME AS PARENTNAME,CP.PARENT_ID AS PARENT_PARENT_ID,
COALESCE(CG.NAME,CP.NAME) AS GRANDPARENTNAME,
COALESCE(CG.PARENT_ID,CP.PARENT_ID) AS GRANDPARENT_PARENT_ID,
COALESCE(CGG.NAME,CG.NAME) AS GREATGRANDPARENTNAME ,
COALESCE(CGG.PARENT_ID,CG.PARENT_ID) AS GREATGRANDPARENT_PARENT_ID
FROM CATEGORY C
LEFT JOIN CATEGORY CP ON CP.ID = C.PARENT_ID
LEFT JOIN CATEGORY CG ON CG.ID = CP.PARENT_ID
LEFT JOIN CATEGORY CGG ON CGG.ID = CG.PARENT_ID
ORDER BY C.ID
results in
+------+------+-----------+------------+------------------+-----------------+-----------------------+----------------------+----------------------------+
| ID | NAME | PARENT_ID | PARENTNAME | PARENT_PARENT_ID | GRANDPARENTNAME | GRANDPARENT_PARENT_ID | GREATGRANDPARENTNAME | GREATGRANDPARENT_PARENT_ID |
+------+------+-----------+------------+------------------+-----------------+-----------------------+----------------------+----------------------------+
| 1 | AAAA | 2 | AAA | 3 | AA | 4 | A | 5 |
| 2 | AAA | 3 | AA | 4 | A | 5 | A | 5 |
| 3 | AA | 4 | A | 5 | A | 5 | NULL | NULL |
| 4 | A | 5 | NULL | NULL | NULL | NULL | NULL | NULL |
| 10 | BBBB | 11 | BBB | 12 | BBB | 12 | NULL | NULL |
| 11 | BBB | 12 | NULL | NULL | NULL | NULL | NULL | NULL |
+------+------+-----------+------------+------------------+-----------------+-----------------------+----------------------+----------------------------+