Day15——使用MyBatis查询的几种情况

青旅半醒 2023-07-04 12:57 6阅读 0赞

# 一. 例子 #

EmployeeMapper.java

package com.atguigu.mybatis.dao;
    
    import java.util.List;
    import java.util.Map;
    
    import org.apache.ibatis.annotations.MapKey;
    import org.apache.ibatis.annotations.Param;
    
    import com.atguigu.mybatis.beans.Employee;
    
    public interface EmployeeMapper { 
        //1. 查询单条数据返回单个对象
    	public Employee selectEmployeeById(Integer id);
    	
    	//2. 查询多条数据返回多个对象的集合
    	public List<Employee> selectAllEmployees();
    	
    	//3.查询单条数据返回一个Map
    	public Map<String, Object> selectEmployeeByIdReturnMap(Integer id);
    	
    	//4.查询多条数据返回一个Map
    	@MapKey("id")  //指定使用对象的哪个属性作为Map中的key
    	public Map<Integer, Employee> selectEmployeesReturnMap();
    	
    }

EmployeeMapper.xml

<select id="selectEmployeeById" resultType="com.atguigu.mybatis.beans.Employee">
    	    select id, last_name, email, gender from tbl_employee where id = #{id}
    	</select>
    
    	<select id="selectAllEmployees" resultType="com.atguigu.mybatis.beans.Employee">
    	     select id, last_name, email, gender from tbl_employee
    	</select>
    	
    	
    	
    	<select id="selectEmployeeByIdReturnMap" resultType="java.util.HashMap">
    	   select id, last_name, email, gender from tbl_employee where id = #{id}
    	</select>
    	
    	
    	<select id="selectEmployeesReturnMap" resultType="com.atguigu.mybatis.beans.Employee">
    	   select id, last_name, email, gender from tbl_employee
    	</select>

TestMyBatis.java

@Test
    	public void testSelect() throws Exception{ 
    		SqlSessionFactory ssf = getSqlSessionFactory();
    		SqlSession session = ssf.openSession();
    		//ssf.openSession(true);带自动提交功能
    		try { 
    			EmployeeMapper mapper = session.getMapper(EmployeeMapper.class);
    			
    // List<Employee> emps = mapper.selectAllEmployees();
    // System.out.println(emps);
    			
    // Map<String, Object> map = mapper.selectEmployeeByIdReturnMap(1001);
    // System.out.println(map);
    			
    			  Map<Integer, Employee> emps = mapper.selectEmployeesReturnMap();
    			  System.out.println(emps);			
    				
    			session.commit();
    		} finally { 
    			// TODO: handle finally clause
    			session.close();
    		}
    		
    	}