MyBatis多表管理查询
多表关联关系分析
多表关联：至少两个表关联。分析多表关系的经验技巧：从一条记录出发，不要从表整体去分析，比如分析A表和B表关系，A表中的一条记录对应B表中的几条记录，如果是一条，那么A到B就是一对一；如果多条，那么A到B就是一对多
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一对一，从订单表出发，到用户表，是一对一的关系
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一对多，从用户表出发，到订单表，一条用户数据可以在订单表中存在多条记录，这就是一对多。通过主外键来体现一对多的表结构。一的一方是主表，多的一方是从表，从表当中有一个字段是外键，指向了主表的主键
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多对多，用户和角色表，一个用户可以有很多角色，一个角色有很多用户，多对多通过中间表来体现
多表关联的SQL语句表达分析
- 笛卡尔积
- SELECT *FROM USER,orders
- 关联查询
- 内关联 innder join on
- SELECT * FROM USER u,orders o WHERE u.id=o.user_id
- SELECT * FROM USER u INNER JOIN orders o ON u.id=o.user_id;
- 左外连接
- SELECT * FROM USER u LEFT JOIN orders o ON u.id=o.user_id
- 右外连接
- SELECT * FROM USER u RIGHT JOIN orders o ON u.id=o.user_id
- 内关联 innder join on
一对一查询
需求：查询订单表全部数据，关联查询出订单对应的用户数据（username address）。
Sql语句自己来写，Mybatis只是帮我们执行sql语句同时封装结果集。
SQL语句：
SELECT o.id,o.user_id,o.number,o.note,u.address,u.username FROM orders o LEFT JOIN USER u ON o.user_id=u.id
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对于查询结果集，没有对应的pojo对象，因此MyBatis框架也不能封装结果集，但是可以将User对象放在Orders对象中，因为一对一关系，一个Orders对象可以对应一个User对象。需要手动映射方式，实现查询结果集封装。
- pojo对象
public class Orders {
private Integer id;
private Integer userId;
private String number;
private Date createtime;
private String note;
private User user;
public User getUser() {
return user;
}
public void setUser(User user) {
this.user = user;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public Integer getUserId() {
return userId;
}
public void setUserId(Integer userId) {
this.userId = userId;
}
public String getNumber() {
return number;
}
public void setNumber(String number) {
this.number = number;
}
public Date getCreatetime() {
return createtime;
}
public void setCreatetime(Date createtime) {
this.createtime = createtime;
}
public String getNote() {
return note;
}
public void setNote(String note) {
this.note = note;
}
@Override
public String toString() {
return "Orders{" +
"id=" + id +
", userId=" + userId +
", number='" + number + '\'' +
", createtime=" + createtime +
", note='" + note + '\'' +
", user=" + user +
'}';
}
}
- association（联合）标签，实现手动映射
- propery属性：封装的pojo对象
- javaType属性：封装的pojo对象类型
<mapper namespace="com.itheima.mapper.OrdersMapper">
<select id="queryOrdersUser" resultMap="ordersUserResultType" >
SELECT o.id,o.user_id,o.number,o.note,u.address,u.username FROM orders o LEFT JOIN USER u
ON o.user_id=u.id
</select>
<resultMap id="ordersUserResultType" type="orders">
<id column="id" property="id"></id>
<result column="user_id" property="userId"></result>
<result column="number" property="number"></result>
<result column="createtime" property="createtime"></result>
<result column="note" property="note"></result>
<!-- 手动映射，配置User对象-->
<association property="user" javaType="user">
<id column="user_id" property="id"></id>
<result column="address" property="address"></result>
<result column="username" property="username"></result>
</association>
</resultMap>
</mapper>
@Test
public void queryOrdersUser(){
SqlSession sqlSession = sqlSessionFactory.openSession();
OrdersMapper mapper = sqlSession.getMapper(OrdersMapper.class);
List<Orders> list = mapper.queryOrdersUser();
if(list != null && list.size() > 0){
for (Orders orders : list){
System.out.println(orders);
}
}
sqlSession.close();
}
- 往往还有另外一种方式：新创建一个pojo，这个pojo里面增加两个字段username和address，然后使用resultType自动映射即可，但是这种方式不推荐，因为pojo多了之后很混乱，不容易做技术管理，而且这点方式也有点low
一对多查询
查询全部用户数据，关联查询出订单数据。
SQL语句：
SELECT u.id,u.username,u.sex,u.birthday,u.address,o.user_id,o.id,o.number,o.createtime,o.note FROM USER u LEFT JOIN orders o ON u.id=o.user_id
一个用户对应多个订单数据，因此在pojo中，订单对象是存储在集合中，并保存在User对象中
- pojo对象
public class User {
private int id;
private String username;
private String sex;
private Date birthday;
private String address;
private List<Orders> ordersList;
public List<Orders> getOrdersList() {
return ordersList;
}
public void setOrdersList(List<Orders> ordersList) {
this.ordersList = ordersList;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getSex() {
return sex;
}
public void setSex(String sex) {
this.sex = sex;
}
public Date getBirthday() {
return birthday;
}
public void setBirthday(Date birthday) {
this.birthday = birthday;
}
public String getAddress() {
return address;
}
public void setAddress(String address) {
this.address = address;
}
@Override
public String toString() {
return "User{" +
"id=" + id +
", username='" + username + '\'' +
", sex='" + sex + '\'' +
", birthday=" + birthday +
", address='" + address + '\'' +
", ordersList=" + ordersList +
'}';
}
}
- collection标签：
- property属性：封装的对应的属性名
- ofType属性：已经指定为一个集合List，需要指定集合中的泛型
<select id="queryUserOrders" resultMap="userOrdersResultMap">
SELECT u.id,u.username,u.sex,u.birthday,u.address,o.user_id,o.id,
o.number,o.createtime,o.note
FROM USER u LEFT JOIN orders o
ON u.id=o.user_id
</select>
<resultMap id="userOrdersResultMap" type="user">
<id column="id" property="id"></id>
<result column="username" property="username"></result>
<result column="sex" property="sex"></result>
<result column="birthday" property="birthday"></result>
<result column="address" property="address"></result>
<collection property="ordersList" ofType="orders">
<id column="id" property="id"></id>
<result column="user_id" property="userId"></result>
<result column="number" property="number"></result>
<result column="createtime" property="createtime"></result>
<result column="note" property="note"></result>
</collection>
</resultMap>
@Test
public void queryUserOrders(){
SqlSession sqlSession = sqlSessionFactory.openSession();
UserMapper mapper = sqlSession.getMapper(UserMapper.class);
List<User> list = mapper.queryUserOrders();
if(list != null && list.size() > 0){
for(User user : list){
System.out.println(user);
}
}
sqlSession.close();
}
多对多查询
多对多：双向的一对多，从A表到B表出发是一对多的关系，从B表出发到A表也是一对多的关系
SQL语句：
SELECT r.RID,r.RNAME,r.RDESC,u.id,u.username,u.sex,u.birthday,u.address FROM role r LEFT JOIN user_role ur
ON r.RID=ur.RID LEFT JOIN USER u ON u.id=ur.UID
- pojo对象
public class Role {
private Integer rid;
private String rname;
private String rdesc;
private List<User> userList;
public Integer getRid() {
return rid;
}
public void setRid(Integer rid) {
this.rid = rid;
}
public String getRname() {
return rname;
}
public void setRname(String rname) {
this.rname = rname;
}
public String getRdesc() {
return rdesc;
}
public void setRdesc(String rdesc) {
this.rdesc = rdesc;
}
public List<User> getUserList() {
return userList;
}
public void setUserList(List<User> userList) {
this.userList = userList;
}
@Override
public String toString() {
return "Role{" +
"rid=" + rid +
", rname='" + rname + '\'' +
", rdesc='" + rdesc + '\'' +
", userList=" + userList +
'}';
}
}
- 手动映射roleMapper.xml
<mapper namespace="com.itheima.mapper.RoleMapper">
<select id="queryRoleUser" resultMap="roleUserResultMap">
SELECT r.RID,r.RNAME,r.RDESC,u.id,u.username,u.sex,u.birthday,u.address FROM role r LEFT JOIN user_role ur ON r.RID=ur.RID LEFT JOIN USER u ON u.id=ur.UID
</select>
<resultMap id="roleUserResultMap" type="role">
<id column="rid" property="rid"></id>
<result column="rname" property="rname"></result>
<result column="rdesc" property="rdesc"></result>
<result column="" property=""></result>
<collection property="userList" ofType="user">
<id column="id" property="id"></id>
<result column="username" property="username"></result>
<result column="sex" property="sex"></result>
<result column="birthday" property="birthday"></result>
<result column="address" property="address"></result>
</collection>
</resultMap>
</mapper>
@Test
public void queryRoleUser(){
SqlSession sqlSession = sqlSessionFactory.openSession();
RoleMapper mapper = sqlSession.getMapper(RoleMapper.class);
List<Role> roleList = mapper.queryRoleUser();
if(roleList != null && roleList.size() > 0){
for (Role role : roleList){
System.out.println(role);
}
}
sqlSession.close();
}
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