MyBatis见解3
8.MyBatis的关联查询
8.3.一对多查询
需求：查询所有用户信息及用户关联的账户信息。
分析：用户信息和他的账户信息为一对多关系，并且查询过程中如果用户没有账户信息，此时也要将用户信息查询出来，此时左外连接查询比较合适。
8.3.1.pojo
package com.by.pojo;
import java.io.Serializable;
import java.util.Date;
import java.util.List;
public class User implements Serializable {
private Integer id;
private String username;
private Date birthday;
private String sex;
private String address;
private List<Account> account;
@Override
public String toString() {
return "User{" +
"id=" + id +
", username='" + username + '\'' +
", birthday=" + birthday +
", sex='" + sex + '\'' +
", address='" + address + '\'' +
", account=" + account +
'}';
}
public List<Account> getAccount() {
return account;
}
public void setAccount(List<Account> account) {
this.account = account;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public Date getBirthday() {
return birthday;
}
public void setBirthday(Date birthday) {
this.birthday = birthday;
}
public String getSex() {
return sex;
}
public void setSex(String sex) {
this.sex = sex;
}
public String getAddress() {
return address;
}
public void setAddress(String address) {
this.address = address;
}
}
8.3.2.mapper
package com.by.dao;
import com.by.pojo.User;
import java.util.List;
public interface UserDao {
User getUserById(Integer id);
}
<?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">
<resultMap id="getUserByIdResultMap" type="user">
<id column="id" property="id"></id>
<result column="username" property="username"></result>
<result column="birthday" property="birthday"></result>
<result column="sex" property="sex"></result>
<result column="address" property="address"></result>
<!--一对多使用collection标签指定数据的封装规则-->
<collection property="account" ofType="account">
<id column="aid" property="id"></id>
<result column="uid" property="uid"></result>
<result column="money" property="money"></result>
</collection>
</resultMap>
<select id="getUserById" parameterType="int" resultMap="getUserByIdResultMap">
SELECT
u.*,
a.id aid,
a.money money,
a.uid uid
FROM
user u
LEFT JOIN
account a
ON
u.id=a.uid
WHERE
u.id=#{id}
</select>
8.3.3.测试
/**
* 一对多：一个user 对 多个Account
*/
@Test
public void testGetUsertById() throws IOException {
UserDao userDao = sqlSession.getMapper(UserDao.class);
User user = userDao.getUserById(41);
System.out.println(user);
}
8.4.多对多查询
需求：查询角色及角色赋予的用户信息。
分析：一个用户可以拥有多个角色，一个角色也可以赋予多个用户，用户和角色为双向的一对多关系，多对多关系其实我们看成是双向的一对多关系。
user(uid, username)：王贺、万通
user_role(uid, rid)
role(rid, )：校长、老师、学生
8.3.1.pojo
package com.by.pojo;
import java.util.List;
// 一方
public class Role {
private Integer id;
private String roleName;
private String roleDesc;
// 多方
private List<User> user;
@Override
public String toString() {
return "Role{" +
"id=" + id +
", roleName='" + roleName + '\'' +
", roleDesc='" + roleDesc + '\'' +
", user=" + user +
'}';
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getRoleName() {
return roleName;
}
public void setRoleName(String roleName) {
this.roleName = roleName;
}
public String getRoleDesc() {
return roleDesc;
}
public void setRoleDesc(String roleDesc) {
this.roleDesc = roleDesc;
}
public List<User> getUser() {
return user;
}
public void setUser(List<User> user) {
this.user = user;
}
}
8.3.2.mapper
package com.by.dao;
import com.by.pojo.Role;
public interface RoleMapper {
Role getRoleById(Integer id);
}
<?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.by.dao.RoleMapper">
<resultMap id="getRoleByIdResultMap" type="role">
<id column="rid" property="id"></id>
<result column="role_name" property="roleName"></result>
<result column="role_desc" property="roleDesc"></result>
<!--
一对多使用collection标签指定数据的封装规则
property="userList"：pojo的属性
ofType="com.by.pojo.User"：集合的泛型，等价于resultType
-->
<collection property="user" ofType="user">
<id column="id" property="id"></id>
<result column="username" property="username"></result>
<result column="birthday" property="birthday"></result>
<result column="sex" property="sex"></result>
<result column="address" property="address"></result>
</collection>
</resultMap>
<select id="getRoleById" parameterType="int" resultMap="getRoleByIdResultMap">
SELECT
r.id rid,
role_name,
role_desc,
u.*
FROM
user_role ur
LEFT JOIN
role r
ON
ur.rid=r.id
LEFT JOIN
user u
ON
ur.uid=u.id
WHERE
r.id=#{id}
</select>
</mapper>
8.3.3.测试
/**
* 多对多：一个user 对 多个role 一个role 对 多个user
*/
@Test
public void testGetRoletById() throws IOException {
RoleMapper roleMapper = sqlSession.getMapper(RoleMapper.class);
Role role = roleMapper.getRoleById(1);
System.out.println(role);
}
9.MyBatis的延迟加载
创建工程：
9.1.什么是延迟加载？
开启延迟加载后，在真正使用数据的时候才发起级联查询，不用的时候不查询。
9.2.mapper
package com.by.dao;
import com.by.pojo.User;
import java.util.List;
public interface UserDao {
User getUserById2(Integer id);
}
<?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.by.dao.UserDao">
<!--
id:和接口方法名保持一致
resultType:和接口返回类型保持一致
-->
<select id="findAll" resultType="com.by.pojo.User">
select * from user
</select>
<select id="getUserById" parameterType="int" resultType="user">
select * from user where id=#{id}
</select>
<resultMap id="getUserById2ResultMap" type="user">
<id column="id" property="id"></id><result column="username" property="username"></result>
<result column="birthday" property="birthday"></result>
<result column="sex" property="sex"></result>
<result column="address" property="address"></result>
<!--
property="accountList"：pojo的属性
ofType="account"：集合的泛型
select="com.by.mapper.AccountMapper.selectAccountByUid"：要调用的select标签的id
column="id"：传递的参数
fetchType="lazy"：局部开启延迟加载
-->
<collection property="accountList" ofType="account" select="com.by.dao.AccountMapper.selectAccountById" column="id" fetchType="lazy">
</collection>
</resultMap>
<select id="getUserById2" parameterType="int" resultMap="getUserById2ResultMap">
SELECT * FROM user WHERE id=#{id}
</select>
</mapper>
9.3.全局开启懒加载
<!-- 全局配置延迟加载策略 -->
<settings>
<!-- 打开延迟加载的开关 -->
<setting name="lazyLoadingEnabled" value="true"/>
</settings>
9.4.测试
@Test
public void testGetUserById2() throws IOException {
UserDao userDao = sqlSession.getMapper(UserDao.class);
User user = userDao.getUserById2(41);
System.out.println(user.getUsername());
List<Account> accountList = user.getAccountList();
for (Account account : accountList) {
System.out.println(account);
}
}
10.MyBatis的动态SQL
创建工程：
10.1.什么是动态SQL?
MyBatis的映射文件中支持在基础SQL上添加一些逻辑操作，并动态拼接成完整的SQL之后再执行，以达到SQL复用、简化编程的效果。
10.2.if标签
我们根据实体类的不同取值，使用不同的SQL语句来进行查询。比如在id如果不为空时可以根据 id查询，如果username不同空时还要加入用户名作为条件。这种情况在我们的多条件组合查询中经常会碰到。
mapper接口
package com.by.dao;
import com.by.pojo.User;
import java.util.List;
public interface UserDao {
User findAll2(User user);
}
mapper映射文件
<?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">
<select id="findAll2" parameterType="user" resultType="user">
SELECT * FROM user
<where>
<if test="id != null">
AND id=#{id}
</if>
<if test="sex != null and sex != ''">
AND sex=#{sex}
</if>
</where>
</select>
测试
public void testFindAll2(){
UserDao userDao = sqlSession.getMapper(UserDao.class);
User user = new User();
user.setId(41);
user.setSex("男");
System.out.println(userDao.findAll2(user));
}
10.3.where标签
为了简化上面where 1=1的条件拼装，我们可以使用where标签将if标签代码块包起来，将1=1条件去掉。
若查询条件的开头为 “AND” 或 “OR”，where 标签会将它们去除。
mapper映射文件
<?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">
<!--namespace：隔离sql，一般是接口名称的全类名-->
<mapper namespace="com.by.dao.UserDao">
<select id="findByUser" resultType="User">
select * from user
<!--where标签将if标签代码块包起来去掉开头 “AND” 或 “OR”-->
<where>
<if test="username!=null and username != ''">
and username=#{username}
</if>
<if test="birthday!=null">
and birthday=#{birthday}
</if>
<if test="sex!=null and sex != ''">
and sex=#{sex}
</if>
<if test="address!=null and address != ''">
and address=#{address}
</if>
</where>
</select>
</mapper>
10.4.set标签
set标签用于动态包含需要更新的列，并会删掉额外的逗号
mapper
package com.by.dao;
import com.by.pojo.User;
import java.util.List;
public interface UserDao {
void updateUserById(User user);
}
<update id="updateUserById" parameterType="user">
UPDATE user
<set>
<if test="username != null and username != ''">
username=#{username},
</if>
</set>
WHERE id=#{id}
</update>
测试
@Test
public void testUpdateUserById(){
UserDao userDao = sqlSession.getMapper(UserDao.class);
User user = new User();
user.setId(41);
user.setUsername("杨过");
userDao.updateUserById(user);
}