Question

I have two tables, tbl_foo and tbl_bar, and I want to join these tables with tbl_foo.foo_id = tbl_bar.foo_id in the on-clause. However, for each tbl_bar.baz_id there should be one row for each tbl_foo.foo_id (even if no such entry in tbl_bar exists). How do I write such query?

There’s more info on the schema and my desired result below.

• Edit: Each row must have a foo_id and baz_id.
• Edit 2: Added tbl_baz below.

Desired result

+--------+--------+--------+------------+
| bar_id | baz_id | foo_id | some_field |
+--------+--------+--------+------------+
|      1 |    101 |      1 | foo        |
|      2 |    101 |      2 | bar        |
|      3 |    101 |      3 | baz        |
|   NULL |    101 |      4 | bin        |
|      4 |    102 |      1 | foo        |
|   NULL |    102 |      2 | bar        |
|      5 |    102 |      3 | baz        |
|   NULL |    102 |      4 | bin        |
+--------+--------+--------+------------+

Table: tbl_foo

+--------+------------+
| foo_id | some_field |
+--------+------------+
|      1 | foo        |
|      2 | bar        |
|      3 | baz        |
|      4 | bin        |
+--------+------------+

Table: tbl_bar

+--------+--------+--------+
| bar_id | baz_id | foo_id |
+--------+--------+--------+
|      1 |    101 |      1 |
|      2 |    101 |      2 |
|      3 |    101 |      3 |
|      4 |    102 |      1 |
|      5 |    102 |      3 |
+--------+--------+--------+

Table: tbl_baz

+--------+
| baz_id |
+--------+
|    101 |
|    102 |
+--------+

SQL Schema

CREATE TABLE tbl_foo (
    foo_id INT,
    some_field VARCHAR(255),
    PRIMARY KEY (foo_id)
);

INSERT INTO tbl_foo VALUES
(1, 'foo'),
(2, 'bar'),
(3, 'baz'),
(4, 'bin');

CREATE TABLE tbl_bar (
    bar_id INT,
    baz_id INT,
    foo_id INT,
    PRIMARY KEY (bar_id, baz_id),
    FOREIGN KEY (baz_id) REFERENCES tbl_baz (baz_id),
    FOREIGN KEY (foo_id) REFERENCES tbl_foo (foo_id)
);

INSERT INTO tbl_bar VALUES
(1, 101, 1),
(2, 101, 2),
(3, 101, 3),
(4, 102, 1),
(5, 102, 3);

CREATE TABLE tbl_baz (
    baz_id INT,
    PRIMARY KEY (baz_id)
);

INSERT INTO tbl_baz VALUES
(101),
(102);

Answer 1

You may be looking for a query like this:

UPDATE

Based upon new tbl_baz:

select y.bar_id, x.baz_id, x.foo_id, x.some_field
from (
    select a.foo_id, a.some_field, b.baz_id
    -- Cross foo_id with all baz_id
    from tbl_foo as a, tbl_baz as b
) as x
    -- Get the bar_id where it exists for each foo_id/baz_id combo
    left join tbl_bar as y on x.foo_id = y.foo_id
        and x.baz_id = y.baz_id
order by x.baz_id, x.foo_id

This is based on the assumption that you want to see each foo_id for each baz_id regardless of what is in your many-to-many table.

EXAMPLE of why you may not want this, or may want to update your many-to-many table instead:

If we replace “foo” and “baz” with “person” and “car”, this query is essentially saying that every person owns every car. This may be the case, but it is certainly not represented in the “ownership” many-to-many table (bar).