When joining tables, it is necessary to join each table mentioned in the FROM
clause by specifying joins in the WHERE clause. If you list a table
name in the FROM clause, but fail to join it in the WHERE
clause, the effect is to mark that table as unjoined, and cause it to be paired
with every row in the query result. Figure illustrates
The first SELECT performs an ordinary join. The second SELECT
does not join a column from wild_a to a column in wild_b, causing
every value in wild_a to be matched with every value in wild_b.
This effect is usually not intended and is called a Cartesian product.
When a query returns many more rows than expected, look for an unjoined table
in the query.