Question

I have 3 tables in sqlite, the first one is about an entity I call bar:

bar
    *id
    -name

and the other one keeps a relationship between two bars, I call this relationship foo:

foo
    *id
    -bar_one (fk->bar)
    -bar_two (fk->bar)
    -baz (fk->baz)

baz holds an attribute that qualifies the KIND relationship that foo represents:

baz
    *id
    -description

I need to show a query of foo that substitutes the foreign keys with the values of bar an baz, something like:

id    bar_one    bar_two    baz
1     bar1       bar2       siblings
2     bar2       bar3       acquaintances
3     bar3       bar4       enemies

I’m not a specialist in DBs as you can see, I tried with a join and it works with foo and baz.

SELECT foo.id, baz.description FROM foo, baz WHERE foo.baz = baz.id;

But to show the two members of bar is difficult for me in this way. Some advice?

Answer 1

Assuming that columns of table foo: bar_one (fk->bar), bar_two (fk->bar), baz are non-null columns, you can use INNER JOIN to combine both tables, but if they are nullable, use LEFT JOIN instead.

SELECT  a.ID,
        b.name as bar_one,
        c.name AS bar_two,
        d.description
FROM    foo a
        INNER JOIN bar b
            ON a.bar_one = b.id
        INNER JOIN bar c
            ON a.bar_two = c.id
        INNER JOIN baz d
            ON a.baz = c.id

basically, INNER JOIN only displays the rows that a record has atleast one match on every table define, while LEFT JOIN displays all rows define on the left hand side whether there is match or none.