这篇文章将为大家详细讲解有关Mybatis中association和collection怎么用，小编觉得挺实用的，因此分享给大家做个参考，希望大家阅读完这篇文章后可以有所收获。
association和collection用法
1.单个关联查询association
1.1实体之间的关联表示
package com.worldly.config.entity;import java.io.Serializable;public class Employee implements Serializable { private Integer id; private String name; private String email; private String tel; //关联的部门实体，查询某个人的时候可以把所在部门信息查询出来 private Department dep; public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } public String getEmail() { return email; } public void setEmail(String email) { this.email = email; } public String getTel() { return tel; } public void setTel(String tel) { this.tel = tel; } public Department getDep() { return dep; } public void setDep(Department dep) { this.dep = dep; } @Override public String toString() { return "{\"Employee\":{" + "\"id\":\"" + id + "\"" + ", \"name\":\"" + name + "\"" + ", \"email\":\"" + email + "\"" + ", \"tel\":\"" + tel + "\"" + ", \"dep\":" + dep + "}}"; }}1.2 两种关联查询方式
//第一中方式:直接进行关联查询把关联实体的属性在xml中配置//然后关联查出来<resultMap id="emp2ResultMap" type="com.worldly.config.entity.Employee"> <id column="emp_id" property="id"></id> <result column="emp_name" property="name"/> <result column="emp_email" property="email"/> <result column="emp_tel" property="tel"/> <association property="dep" column="emp_dep" javaType="com.worldly.config.entity.Department"> <id column="dep_id" property="id"/> <result column="dep_name" property="name"/> <result column="dep_addr" property="addr"/> </association> </resultMap> <select id="selectEmployAll" resultMap="emp2ResultMap"> SELECT * FROM t_emp e INNER JOIN t_dep d ON e.emp_dep = d.dep_id </select>//第二中查询方式,采用 association中的select来查询<resultMap id="empResultMap" type="com.worldly.config.entity.Employee"> <id column="emp_id" property="id"></id> <result column="emp_name" property="name"/> <result column="emp_email" property="email"/> <result column="emp_tel" property="tel"/> <association column="emp_dep" property="dep" javaType="com.worldly.config.entity.Department" select="selectDepByCondition"></association> </resultMap> <select id="selectEmployeeList" resultMap="empResultMap" databaseId="mysql"> select * from t_emp </select> <resultMap id="depResultMap" type="com.worldly.config.entity.Department"> <id column="dep_id" property="id"></id> <result column="dep_name" property="name"/> <result column="dep_addr" property="addr"/> </resultMap> <select id="selectDepByCondition" resultMap="depResultMap"> SELECT * FROM t_dep d WHERE d.dep_id = #{emp_dep} </select>1.3 两种方式的优劣
a.查询条件相同，所用的时间:从测试结果显示，关联查询要比嵌套查询快一点（结果不一定准确，可能关联表多的时候，结果会有所变化）
a.查询条件相同，所用的时间:从测试结果显示，关联查询要比嵌套查询快一点（结果不一定准确，可能关联表多的时候，结果会有所变化）
b.适用的情况
2.多个关联查询 collection
2.1实体之间的关联表示
package com.worldly.config.entity;import java.util.List;public class Department { private int id; private String name; private String addr; List<Employee> employeeList; public int getId() { return id; } public void setId(int id) { this.id = id; } public String getNamel() { return name; } public void setNamel(String name) { this.name = name; } public String getAddr() { return addr; } public void setAddr(String addr) { this.addr = addr; } public List<Employee> getEmployeeList() { return employeeList; } public void setEmployeeList(List<Employee> employeeList) { this.employeeList = employeeList; } @Override public String toString() { return "{\"Department\":{" + "\"id\":\"" + id + "\"" + ", \"name\":\"" + name + "\"" + ", \"addr\":\"" + addr + "\"" + ", \"employeeList\":" + employeeList + "}}"; }}2.2 两种关联查询方式
//第一种方式嵌套查询<resultMap id="depResultMap2" type="com.worldly.config.entity.Department"> <id column="dep_id" property="id"></id> <result column="dep_name" property="name"/> <result column="dep_addr" property="addr"/> <collection column="dep_id" property="employeeList" javaType="java.util.List" ofType="com.worldly.config.entity.Employee" select="selectEmpBydepId"/> </resultMap> <select id="selectDepByCondition" resultMap="depResultMap2"> SELECT * FROM t_dep d WHERE d.dep_id = #{param} </select> <resultMap id="empResultMap" type="com.worldly.config.entity.Employee"> <id column="emp_id" property="id"></id> <result column="emp_name" property="name"/> <result column="emp_email" property="email"/> <result column="emp_tel" property="tel"/> </resultMap> <select id="selectEmpBydepId" resultMap="empResultMap"> SELECT * FROM t_emp e WHERE e.emp_dep = #{dep_id} </select>//第二中方式关联查询<resultMap id="dep2ResultMap" type="com.worldly.config.entity.Department"> <id column="dep_id" property="id"></id> <result column="dep_name" property="name"/> <result column="dep_addr" property="addr"/> <collection property="employeeList" ofType="com.worldly.config.entity.Employee"> <id column="emp_id" property="id"></id> <result column="emp_name" property="name"/> <result column="emp_email" property="email"/> <result column="emp_tel" property="tel"/> </collection> </resultMap> <select id="selectDepWithEmp" resultMap="dep2ResultMap"> SELECT * FROM t_dep d INNER JOIN t_emp e ON d.dep_id = e.emp_dep WHERE d.dep_id = #{param} </select>2.3 多条件查询
<resultMap id="depResultMap2" type="com.worldly.config.entity.Department"> <id column="dep_id" property="id"></id> <result column="dep_name" property="name"/> <result column="dep_addr" property="addr"/> <result column="dep_status" property="status"/> <collection column="{depId=dep_id,status=dep_status}" property="employeeList" javaType="java.util.List" ofType="com.worldly.config.entity.Employee" select="selectEmpBydepId"/> </resultMap> <select id="selectDepByCondition" resultMap="depResultMap2"> SELECT * FROM t_dep d WHERE d.dep_id = #{param} </select> <resultMap id="empResultMap" type="com.worldly.config.entity.Employee"> <id column="emp_id" property="id"></id> <result column="emp_name" property="name"/> <result column="emp_email" property="email"/> <result column="emp_tel" property="tel"/> </resultMap> <select id="selectEmpBydepId" resultMap="empResultMap"> SELECT * FROM t_emp e WHERE e.emp_dep = #{depId} AND e.emp_status=#{status} </select>多条件查询,用{}来包装方法
3.鉴别器discriminator
3.1 鉴别器适用的场景
3.2 鉴别器的实现
association和collection关联查询用法
这里只做最简单的用法，其它方法请自行查询；
一对多 collection
<collection property="要查询的实体集合" javaType="java.util.List" ofType="要查询的实体所在包路径" select="要查询的mapper方法" column="关联的实体中的字段=关联的数据库中的字段"/>举例
<collection property="stsManageStudentList" javaType="java.util.List" ofType="com.crm.project.domain.StsManageStudent" select="com.crm.project.mapper.StsManageStudentMapper.selectStsManageStudentList" column="manageId=manage_id"/>一对一 & 多对一
<association property="要查询的实体" column="数据库中的关联字段" javaType="要查询的实体所在包路径" select="要查询的mapper方法"/>举例
<association property="stsStudent" column="student_id" javaType="com.crm.project.domain.StsStudent" select="com.crm.project.mapper.StsStudentMapper.selectStsStudentById"/>关于“Mybatis中association和collection怎么用”这篇文章就分享到这里了，希望以上内容可以对大家有一定的帮助，使各位可以学到更多知识，如果觉得文章不错，请把它分享出去让更多的人看到。
