How can A left outer join B return more rows than are in A?

This can happen when column b is not unique in table B. Suppose you have this data:

  A      B+---+  +---+---+| b |  | b | c |+---+  +---+---+| 1 |  | 2 | 1 || 2 |  | 2 | 2 |+---+  +---+---+

When you left-join from A to B on column b, you get

+-----+------+------+| A.b | B.b  | B.c  |+-----+------+------+|   1 | NULL | NULL ||   2 |    2 |    1 ||   2 |    2 |    2 |+-----+------+------+

which gives three rows in total, even though both A and B only have two rows each.

There is nothing strange about it (and this situation applies to inner joins too). A left outer join:

• Returns all rows from A cross B where the join condition match
• And returns all rows from A where the join condition does not match

So at minimum the query will return 25766 rows but there could be more. It is possible to have one row in table A that matches many rows in table B. Example:

A             B             Result+----+----+  +-----+----+   +------+-----+-----+------+|  id|   b|  |   id|   b|   |  a.id|  a.b|  b.b|  b.id|+----+----+  +-----+----+   +------+-----+-----+------+|   1|  10|  |  123|  10|   |     1|   10|   10|   123|+----+----+  |  456|  10|   |     1|   10|   10|   456|             +-----+----+   +------+-----+-----+------+

This returns two rows even though there is one row in table A.