Basic characteristics of subquery: 1. When using the associated subqueries, subsequent query using the IN clause or the exists clause is executed
It is usually the same 2. EXISTS is generally not used for non-related subquers (because the error result may be returned) 3. When the external query returns a relatively small record, the associated subquerix is faster than the non-associated subquerix. When the child query returns more than a row of data, the query cannot be converted to a connection operation 5, if there is a small amount in the internal query Record, the non-associated purple query is more than the associated query
fast,
