MySql - Combine data from three tables into one table - mysql

Question

I have 3 tables for data of divisions, districts and police_stations table. The table data are dependent like
divisions
- districts
-- police_stations
Table format are
divisions
id
name
districts
id
division_id
name
police_stations
id
division_id
district_id
name
Can I design just one table with all of my tables data with current table dependency? If I can, how will be the traversing process?

Answer 1

To group multiple tables together, you should use JOINS. This is a better database practice than creating new tables.
The INNER JOIN keyword selects all rows from both tables as long as there is a match between the columns in both tables.
SELECT *
FROM divisions divis
INNER JOIN districts dis
ON dis.division_id = divis.id
INNER JOIN police_stations pol
ON pol.district_id = dis.id
Note: Apparently the DIV keyword is reserved in mysql, so you you have to use something else to identify divisions.
If you insist on creating a new table with this data, you can use the CREATE TABLE AS recipe.
In order to do this though, we need to modify our original schema. New tables cannot have duplicate names, so we have to change the original tables to all have unique columns.
For example name becomes district_name.
CREATE TABLE combined AS (
SELECT divis.id, divis.division_name, dis.district_name, pol.police_name
FROM divisions divis
LEFT JOIN districts dis
ON divis.id = dis.division_id
LEFT JOIN police_stations pol
ON pol.district_id = dis.id
);
Here is a fiddle to demonstrate it
http://sqlfiddle.com/#!9/7ec3b/1/0
This is really bad database design though. My first solution is recommended

Answer 2

Using a single table: There would be an id for uniquely identifying each row; it would be the PRIMARY KEY. There would be a parent_id to say which other row is its 'parent' (division is parent of district, etc).
The JOIN becomes a self join.
SELECT ...
FROM tbl AS division
JOIN tbl AS district ON division.id = district.parent_id
JOIN tbl AS station ON district.id = station.parent_id
WHERE ...
Be sure to 'qualify' each field in the SELECT and WHERE with the appropriate 'alias'; example: division.name.
This design pattern works for most 'hierarchical' structures of arbitrary depth. Yours is exactly 3 levels deep, so it is somewhat simpler.
Note that the parent_id would be 0 for any "district" rows. And it would be useful to have INDEX(parent_id).

Answer 3

Hopefully it solves your problem
SELECT * from divisions d
join police_stations ps on d.id=ps.division_id
join districts dist on ps.district_id =dist.id;