SQL - Recursive Query - mysql

Question

I have the following mysql table with sample data as follows:
id location parentid
1 UK 0
2 East Anglia 1
3 Cambridgeshire 2
4 Norfolk 2
5 Suffolk 2
6 East Midlands 1
7 Derbyshire 6
8 Leicestershire 6
9 EU Countries 0
10 Austria 9
11 Belgium 9
I want to generate a query whereby I can get a list of locations by location name but the location should include any parent location. e.g.
A search for folk should return:
id location
4 Norfolk, East Anglia, UK
5 Suffolk, East Anglia, UK
A search for East should return:
id location
2 East Anglia, UK
6 East Midlands, UK
A search for Bel should return:
id location
11 Belgium
In the above we are excluding concatanting EU countries
Obviously the following doesnt work:
select c.id, CONCAT_WS(', ', c.location, p.location, pp.location) as location
from tbl_locations c
outer left join tbl_locations p on p.id = c.parentid
outer left join tbl_locations pp on pp.id = p.parentid
where c.location like '%whatever%'

Answer 1

If you only want a the parent location this is done with a self join:
select c.id, c.location, p.id, p.location
from tbl_locations c
outer left join tbl_locations p on p.id = c.parentid
where c.location like '%whatever%'
This can be extended (via outer joins) to an arbitrary number of levels but the query will get long. Eg. to three levels:
select c.id, c.location, p.id, p.location, pp.id, pp.location
from tbl_locations c
outer left join tbl_locations p on p.id = c.parentid
outer left join tbl_locations pp on pp.id = p.parentid
where c.location like '%whatever%'
More general recursive queries depend on the details of the RDBMS. The easiest approach is to use a Common Table Expression (CTE). But MySQL doesn't support them (at least, not yet). Other approaches can be used: Generating Depth based tree from Hierarchical Data in MySQL (no CTEs).

Answer 2

1) There is no standard SQL query which can calculate the "transitive closure" a transitive relation. If you want to nest select statements you will always have a maximum depth that can be reached.
2) There is no standard SQL query which will return a row with a variable number of columns. So you would have to format your results in some way or another (e.g. csv).
However, you can accomplish that in MySQL with a stored procedure:
1 CREATE DATABASE IF NOT EXISTS test;
2 USE test;
3
4
5 DROP TABLE IF EXISTS location;
6 CREATE TABLE location (id INT UNSIGNED PRIMARY KEY, name VARCHAR(30) NOT NULL, parent_id INT UNSIGNED NULL REFERENCES location(id));
7
8 INSERT INTO location VALUES
9 (1,"UK",0),
10 (2,"East Anglia",1),
11 (3,"Cambridgeshire",2),
12 (4,"Norfolk",2),
13 (5,"Suffolk",2),
14 (6,"East Midlands",1),
15 (7,"Derbyshire",6),
16 (8,"Leicestershire",6);
17
18
19
20
21 DROP FUNCTION IF EXISTS location_with_parents;
22 DELIMITER //
23 CREATE FUNCTION location_with_parents(location_id INT UNSIGNED) RETURNS VARCHAR(255) READS SQL DATA
24 BEGIN
25 DECLARE LOC_STR VARCHAR(255) DEFAULT NULL;
26 DECLARE LOC_ADD VARCHAR(255) DEFAULT NULL;
27 DECLARE PAR_ID INT UNSIGNED DEFAULT location_id;
28
29 SELECT name INTO LOC_STR FROM location where id=PAR_ID;
30 loop_label: LOOP
31 SELECT parent_id INTO PAR_ID FROM location where id=PAR_ID;
32
33 IF PAR_ID = 0 THEN
34 LEAVE loop_label;
35 ELSE
36 SELECT name INTO LOC_ADD FROM location where id=PAR_ID;
37 SET LOC_STR = CONCAT(LOC_STR, ', ', LOC_ADD);
38 ITERATE loop_label;
39 END IF;
40 END LOOP loop_label;
41 RETURN LOC_STR;
42
43 END;
44 //
45
46 DELIMITER ;
47
48
49
50 SELECT location_with_parents(id) FROM location WHERE name LIKE "%folk%";
51
52 DROP DATABASE test;
Works for me with MySQL 5.6.35
Hope this helps!

Answer 3

Below Query gives you exact result which you want using Recursion method.
Select S.ID ,
concat( S.location,',', Group_concat
(distinct A.location ORDER BY A.location SEPARATOR ',' ) ) as location
from
( SELECT distinct #r AS _id ,location,
(
SELECT #r := parentid
FROM tbl_locations
WHERE id = _id
) AS parentid,
#l := #l + 1 AS level
FROM (
SELECT #r := h.ID,
#l := 0,
#cl := 0
from tbl_locations h
where location like '%folk%'
) vars,
tbl_locations h
WHERE #r <> 0
)A , tbl_locations S
where s.location like '%folk%'
group by S.ID
OutPut :
location like '%East%' :
location like '%Folk%'
its good question and check and ask if you have any concerns.