二、複雜查詢與聯結

2.1 子查詢

/*涉及到三張表的巢狀子查詢*/
SELECT cust_name, cust_contact
FROM Customers 
WHERE cust_id IN (SELECT cust_id 
                  FROM Orders 
                  WHERE order_num IN (SELECT order_num 
                                      
 
FROM OrderItems 
                                      WHERE prod_id = 'RGAN01'));

注意：作為子查詢的 SELECT 語句只能查詢單個列。企圖檢索多個列將返回錯誤。

/*顯示 Customers 表中每個顧客的訂單總數。訂單與相應的顧客 ID 儲存在 Orders 表中*/
SELECT cust_name, 
       cust_state, 
       (SELECT COUNT(*) 
        FROM Orders 
        WHERE Orders.cust_id = Customers.cust_id) AS
 
 orders 
FROM Customers 
ORDER BY cust_name;

2.2 聯結表