Analyze the combined use of Aggregate functions and connection tables

Analyze the combination of the aggregate function and the connected table to see such a relation as MERS MERs (cust_id, cust_name, cust_contact) Orders (order_num, cust_id, order_date) If there is such a search statement: SQL code select cust_name, cust_contact, (select count (*) from Orders where Orders. cust_id = MERs. cust_id group by cust_id) from MERs: 1. you can retrieve the order quantity of each cust_id from the Orders table, because the isolated count (*) is aggregated based on the group by customer number. In addition, the join table operation is performed before aggregation. Connect orders and MERs. In this way, only the customer information is agreed internally and externally. 2. The outer layer lists other required customer information. Note: The inner layer is usually used as an isolated aggregate function. Before grouping operations on the inner layer, the connection conditions with the outer layer must be provided. The selection columns behind the outer select are basically in the outer table. To sum up, is this sentence a subquery or a normal connection. Obviously. It is a subquery that contains a select sentence in the select statement. Very special. clauses can be used as a search Column !~ That's it. In the future, we will try to group but involve two different tables.