MySQL. Subqueries - mysql

Question

I have troubles understanding subqueries. Can someone properly explain it to me please?
I have this query:
SELECT substring_index(marca, ' ', 1) AS companie, count(*) AS numar_masini
FROM tip_masina
GROUP BY companie;
That displays:
companie numar_masini
Chevrolet 5
Dacia 1
Dodge 5
Ford 6
And this query:
SELECT substring_index(id_masina, ' ', 1) AS marca, SUM(nr_vehicule) AS nr_masini
FROM proprietate
GROUP BY marca;
That displays:
marca nr_masini
Chevrolet 18
Dodge 9
Ford 11
I want to 'combine' this queries properly and get a result of:
companie numar_marci numar_masini
Chevrolet 5 18
Dacia 1 0 (or null)
Dodge 5 9
Ford 6 11
How can I do that properly? I don't want code, I want a proper explanation of subqueries as I did not understand them properly and I didn't find a proper documentation to explain it to me.

Answer 1

A sub query (in this way) is just returning a set of rows. You can treat the result of a sub query as if it were a table.
For example your queries could be used as follows.
SELECT b.marca, b.nr_masini, c.numar_masini
(
SELECT substring_index(id_masina, ' ', 1) AS marca, SUM(nr_vehicule) AS nr_masini
FROM proprietate
GROUP BY marca
) b
LEFT OUTER JOIN
(
SELECT substring_index(marca, ' ', 1) AS companie, count(*) AS numar_masini
FROM tip_masina
GROUP BY companie
) c
ON b.marca = c.companie
Here it is taking your first query and getting the 4 makes (with the counts to go with them), then joining that result against the result of your 2nd query. You 2nd query only brings back 3 rows.
Possibly that both queries could return rows that the other query doesn't return. In which case you might use a 3rd sub query to get the main rows. Then you can join the other 2 against that:-
SELECT a.companie, b.nr_masini, c.numar_masini
FROM
(
SELECT substring_index(marca, ' ', 1) AS companie
FROM tip_masina
UNION
SELECT substring_index(id_masina, ' ', 1) AS companie
FROM proprietate
) a
LEFT OUTER JOIN
(
SELECT substring_index(id_masina, ' ', 1) AS marca, SUM(nr_vehicule) AS nr_masini
FROM proprietate
GROUP BY marca
) b
ON a.companie = b.marca
LEFT OUTER JOIN
(
SELECT substring_index(marca, ' ', 1) AS companie, count(*) AS numar_masini
FROM tip_masina
GROUP BY companie
) b
ON a.companie = c.companie
In this way you are using one sub query to get a list of all the possible companie values, and treating that as a table to join against the other 2 sub queries.
You could set up each of these sub queries as a view which would make them look even more like tables when you join them.

Answer 2

That is not a subquery. substring_index is a function.
This is how a subquery works.
SELECT *
FROM
(
-- This is the subquery
SELECT id, test1, test2
FROM table
) TableAlias
Basically, mysql collects the result from the inner select:
SELECT id, test1, test2 FROM table
is now the TableAlias
So the syntax can be used like this now
SELECT TableAlias.Id, TableAlias.test1, TableAlias.test2
FROM
(
-- This is the subquery
SELECT id, test1, test2
FROM table
) TableAlias
Subqueries are useful for many things. Like when using aggergated functions like SUM, AVG. Then it can really help to ORDER reuslts.
Try this link: http://www.toadworld.com/platforms/mysql/w/wiki/6355.table-subqueries.aspx
I hope that explains Subqueries a little bit better for you.