Understanding the number of matched rows in LEFT JOIN

Question

Here is my table structure: SQL Fiddle

CREATE TABLE mytable (
    id int,
    related int
);

INSERT into mytable VALUES(1, NULL);
INSERT into mytable VALUES         


        

Answer 1

First query: t1.related = t2.id

t1              joined t2
id   related  | id   related
--------------+-------------
1    NULL     | --   --
2    1        | 1    NULL
3    1        | 1    NULL

An inner join would result in only two rows, but the outer join also preserves the first row that has no match.

Second query: t1.id = t2.related

t1              joined t2
id   related  | id   related
--------------+-------------
1    NULL     | 2    1
1    NULL     | 3    1
2    1        | --   --
3    1        | --   --

Here too, an inner join would result in only two rows, but the outer join also preserves the two rows that have no match.