Unqualified column names in correlated references

An unqualified column name can also be a correlated reference if the column:

Unqualified correlated references are not recommended because it makes the SQL statement difficult to understand. The column will be implicitly qualified when the statement is prepared depending on which table the column was found in. Once this implicit qualification is determined it will not change until the statement is re-prepared. When an SQL statement that has an unqualified correlated reference is prepared or executed, a warning is returned (SQLSTATE 01545).