如何在MySQL数据库进行子查询

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create database TestDB;

创建测试表table1和table2；
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CREATE TABLE table1
(
customer_id VARCHAR(10) NOT NULL,
city VARCHAR(10) NOT NULL,
PRIMARY KEY(customer_id)
)ENGINE=INNODB DEFAULT CHARSET=UTF8;

CREATE TABLE table2
(
order_id INT NOT NULL auto_increment,
customer_id VARCHAR(10),
PRIMARY KEY(order_id)
)ENGINE=INNODB DEFAULT CHARSET=UTF8;

插入测试数据；
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INSERT INTO table1(customer_id,city) VALUES('163','hangzhou');
INSERT INTO table1(customer_id,city) VALUES('9you','shanghai');
INSERT INTO table1(customer_id,city) VALUES(