The Cartesian product repeats the entirety of the second table for each row of the first table. The Cartesian product is often created by mistake in SQL queries. In a Join, we would only be interested in the rows where some key in table 1 equals a key in the second table. Without this constraint, we get an unqualified join or Cartesian product. Aggregation functions will be inaccurate if mistakenly run on the Cartesian product. Typically this occurs when joining more than two tables and forgetting to include the join condition for one or more joins. For $M$ joins, ensure you have $M-1$ join conditions.