如何使用两个联接编写此查询

Question

I have 3 table

type

  +----+-------+ | id | type | +----+-------+ | 1 | typeA | | 2 | typeB | | 3 | typeC | +----+-------+ 

brand (contains brands and sub brands with parent brand id, like brandC is a sub brand of brandA)

 +----+--------+--------+ | id | brand | parent | +----+--------+--------+ | 1 | brandA | 0 | | 2 | brandB | 0 | | 3 | brandC | 1 | +----+--------+--------+ 

equipment

 +----+-------+-------+ | id | type | brand | +----+-------+-------+ | 1 | 1 | 2 | | 2 | 2 | 1 | | 3 | 3 | 3 | +----+-------+-------+ 

I wrote this query:

$query = "select
    a.id,
    b.type,
    c.brand

from
    equipment a
        join type b
            on a.type=b.id
        join brand c
            on a.brand=c.id
where
    a.id=3";

it shows me the result below:

 +----+--------+---------+ | id | type | brand | +----+--------+---------+ | 3 | typeC | brandC | +----+--------+---------+ 

How shall I modify my query to show the parent brand as well if a brand has a parent brand. for instance brandC is a sub brand of brandA. So my result should look like:

 +----+--------+---------+----------------+ | id | type | brand | Parent Brand | +----+--------+---------+----------------+ | 3 | typeC | brandC | brandA | +----+--------+---------+----------------+ 

and when there is no parent brand it leaves the cell blank

also How will I modify the above query to see all equipment with their brands and sub brands like below.

 +----+--------+---------+----------------+ | id | type | brand | Parent Brand | +----+--------+---------+----------------+ | 1 | typeA | brandB | | | 2 | typeB | brandA | | | 3 | typeC | brandC | brandA | +----+--------+---------+----------------+ 

Answer 1

SELECT * FROM brands b
JOIN equipment e on e.brand = b.id -- match type to brand
JOIN types t on t.id = e.type -- get type names
JOIN brands p on p.id = b.parent; -- get parent brand

Answer 2

Because not all brands have valid parents, you need a left outer join for the parent:

select e.id, t.type, b.brand, bp.brand as parentBrand
from equipment e join
     type t
     on e.type= t.id join
     brand b
     on e.brand = b.id left outer join
     brand bp
     on b.parent = bp.id
where e.id = 3;

I also changed the aliases to be abbreviations of the table names. This makes the query much easier to follow.