Cartesian product appears in SQL statements

No join condition results in Cartesian product

People who have studied linear algebra know that the Cartesian product is popularly said that each member of the two set is associated with a random member of the other party's collection. It can be imagined that, in SQL queries, the Cartesian product is produced when there is no join condition for two table join queries. This is the most common case of performance problems caused by our Cartesian product: The developer misses the join condition when writing the code.

SQL in which the Cartesian product occurs:

This statement is actually only a part of the SQL statement, the problem is that there is a part of the table OrderForm, so from the OrderForm, but the above part of the statement is not practical to OrderForm, but not set the conditions led to the Cartesian product.

Workaround: Use the left JOIN

Cartesian product appears in SQL statements