5.5. Cartesian Products: The Black Sheep of the Family
What if you were joining two tables, or even three tables, and you left off the join conditions? The result would be a Cartesian product. Every row of each table in the FROM clause would be joined with every row of the other tables. If one table had 15 rows, and a second table had 21 rows, a Cartesian product of those two tables would produce 315 rows in the result set of the query. Needless to say, it can be a big problem when you have three or more tables with no join conditions specified.
Cartesian product
A join between two tables where no join condition is specified, and as a result, every row in the first table is joined with every row in the second table.
NOTE
Partial Cartesian products ...
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