JavaEE MyBatis之动态SQL（教材学习笔记）

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动态SQL是MyBatis的强大之处，下面一一学习

一、if语句

在MyBatis中<if>元素时最常用到的判断元素，它类似于Java中的if

1.创建chapter08项目，并将chapter06（chaoter06点这里）中的项目文件复制过来

2.修改映射文件CustomerMapper.xml，并使用if元素进行查询

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
    "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.itheima.mapper.CustomerMapper">

    
    <select id="findCustomerByNameAndJobs" parameterType="Integer" resultType="com.itheima.po.Customer">
        select * from t_customer where 1=1
        <if test="username!=null and username !='' ">
            and username like concat('%',#{username},'%')
        </if>
        <if test="jobs!=null and jobs!='' ">
            and jobs #{jobs}
        </if>
    </select>
</mapper>

3.在测试类MybatisTest中编写测试方法findCustomerByNameAndJobsTest()，如下所示：

package com.itheima.test;

import java.util.List;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;
import com.itheima.po.Customer;
import com.itheima.utils.MybatisUtils;

public class MybatisTest {
	
	@Test
	public void findCustomerByNameAndJobsTest() {
		SqlSession session = MybatisUtils.getSession();
		Customer customer = new Customer();
		customer.setUsername("jack");
		customer.setJobs("teacher");
		//执行sqlsession的查询方法，返回结果集
		List<Customer> customers = session.selectList("com.itheima.mapper."
				+"CustomerMapper.findCustomerByNameAndJobs",customer);
		//打印结果
		for(Customer customer2 : customers) {
			System.out.println(customer2);
		}
		//关闭SqlSession
	    session.close();
	}
}

4.查看运行结果

二、<choose> 、<when>、<otherwise>元素

这三个元素其实和java当中的switch.......case.......default作用差不多，

1.在映射文件中使用这三种元素执行动态SQL

<select id="findCustomerByNameOrJobs" parameterType="Integer" resultType="com.itheima.po.Customer">
        select * from t_customer where 1=1
        <choose>
            <when test="username!=null and username !='' ">
                and username like concat('%',#{username},'%')
            </when>
            <when test="jobs!=null and jobs!='' ">
                and jobs=#{jobs}
            </when>
            <otherwise>
                and phone is not null
            </otherwise>
        </choose>
    </select>

2.编写测试方法

@Test
	public void findCustomerByNameOrJobsTest() {
		SqlSession session = MybatisUtils.getSession();
		Customer customer = new Customer();
		customer.setUsername("jack");
		customer.setJobs("teacher");
		//执行sqlsession的查询方法，返回结果集
		List<Customer> customers = session.selectList("com.itheima.mapper."
				+"CustomerMapper.findCustomerByNameOrJobs",customer);
		//打印结果
		for(Customer customer2 : customers) {
			System.out.println(customer2);
		}
		//关闭SqlSession
	    session.close();
	}

3.执行，查看结果

三、<set>元素

set元素用于更新操作，主要作用是在动态包含的sql语句前面加一个set关键字，并将sql语句中最后一个,逗号去掉

1.在映射文件中使用<set>元素使用动态sql

<update id="updateCustomer" parameterType="com.itheima.po.Customer">
        update t_customer
        <set>
            <if test="username!=null and username !='' ">
                username=#{username},
            </if>
            <if test="jobs!=null and jobs!='' ">
                jobs=#{jobs},
            </if>
            <if test="phone!=null and phone!='' ">
                phone=#{phone},
            </if>
        </set>
        where id=#{id}
    </update>

2.编写测试方法

@Test
	public void updateCustomerTest() {
		SqlSession session = MybatisUtils.getSession();
		Customer customer = new Customer();
		customer.setId(3);
		customer.setPhone("13111122223");
		//执行sqlsession的查询方法，返回结果集
		int rows = session.update("com.itheima.mapper."
				+"CustomerMapper.updateCustomer",customer);
		if(rows>0) {
			System.out.println("恭喜您插入了"+rows+"条数据");
		}else {
			System.out.println("插入失败了");
		}
		//提交事务
		session.commit();
		//关闭SqlSession
	    session.close();
	}

3.执行测试方法并查看结果

四、<foreach>元素

foreach用于数组和集合循环遍历。

主要用于构建IN条件语句时使用

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1.在映射文件中使用<foreach>建立基于in条件的查询

<select id="findCustomerByIds" parameterType="List" resultType="com.itheima.po.Customer">
        select * from t_customer where id in
        <foreach item="id" index="index" collection="list" open="(" separator="," close=")">
            #{id}
        </foreach>
    </select>

2.编写测试代码

@Test
	public void findCustomerByIdsTest() {
		SqlSession session = MybatisUtils.getSession();
		List<Integer> ids=new ArrayList<Integer>();
		ids.add(1);
		ids.add(2);
		//执行sqlsession的查询方法，返回结果集
		List<Customer> customers = session.selectList("com.itheima.mapper."
				+"CustomerMapper.findCustomerByIds",ids);
		//打印结果
		for(Customer customer2 : customers) {
			System.out.println(customer2);
		}
		//关闭SqlSession
	    session.close();
	}

3.执行测试方法并查看结果