什么是MySQL子查询中的EXIST和EXIST NOT运算符的用途？

EXIST运算符用于测试子查询结果集中的行是否存在。如果找到子查询行值，则EXISTS子查询为TRUE，NOT EXISTS子查询为FALSE。为说明此问题，我们使用以下数据的表Cars，Customers和Reservations-

mysql> Select * from Cars;
+------+--------------+---------+
| ID   | Name         | Price   |
+------+--------------+---------+
|    1 | Nexa         | 750000  |
|    2 | Maruti Swift | 450000  |
|    3 | BMW          | 4450000 |
|    4 | VOLVO        | 2250000 |
|    5 | Alto         | 250000  |
|    6 | Skoda        | 1250000 |
|    7 | Toyota       | 2400000 |
|    8 | Ford         | 1100000 |
+------+--------------+---------+
8 rows in set (0.02 sec)

mysql> Select * from Customers;
+-------------+----------+
| Customer_Id | Name     |
+-------------+----------+
|           1 | Rahul    |
|           2 | Yashpal  |
|           3 | Gaurav   |
|           4 | Virender |
+-------------+----------+
4 rows in set (0.00 sec)

mysql> Select * from Reservations;
+------+-------------+------------+
| ID   | Customer_id | Day        |
+------+-------------+------------+
|    1 |           1 | 2017-12-30 |
|    2 |           2 | 2017-12-28 |
|    3 |           2 | 2017-12-29 |
|    4 |           1 | 2017-12-25 |
|    5 |           3 | 2017-12-26 |
+------+-------------+------------+
5 rows in set (0.00 sec)

以下是使用上述表的MySQL子查询与EXIST的示例-

mysql> Select Name from customers WHERE EXISTS (SELECT * FROM Reservations WHERE Customers.customer_id = Reservations.customer_id);
+---------+
| Name    |
+---------+
| Rahul   |
| Yashpal |
| Gaurav  |
+---------+
3 rows in set (0.06 sec)

上面的查询列出了进行了预订的客户的姓名。

以下是使用上述表的MySQL子查询与NOT EXIST的示例-

mysql> Select Name from customers WHERE NOT EXISTS (SELECT * FROM Reservations WHERE Customers.customer_id = Reservations.customer_id);
+----------+
| Name     |
+----------+
| Virender |
+----------+
1 row in set (0.04 sec)

上面的查询列出了没有进行任何预订的客户的姓名。

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