一起学习框架SSM之MyBatis(二)

程序员文章站 2022-12-04 23:06:52

MyBatis==MyBatis工具类==1.封装工具类==ORM映射==MyBatis自动ORM失效1.解决方案一：列的别名2.解决方案二：结果映射ResultMap==MyBatis处理关联关系-多表连接==1.一对一2.一对多3. 多对多4.关系总结==动态SQL==MyBatis工具类1.封装工具类目的是为了封装这些Mybatis的api，因为每次调用业务逻辑的时候都需要重复使用 InputStream resourceAsStream = Resources.getResourceAsS...

MyBatis工具类

1.封装工具类

目的是为了封装这些Mybatis的api，因为每次调用业务逻辑的时候都需要重复使用

  InputStream resourceAsStream = Resources.getResourceAsStream("Mybatis-config.xml");
//        2.创建SqlSessionFactory
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
//        3.通过SqlSessionFactory创建SqlSession
        SqlSession sqlSession = sqlSessionFactory.openSession();
        
        sqlSession.commit();
//
        sqlSession.rollback();

写一个工具类封装

一起学习框架SSM之MyBatis(二)

public class MyBatisUtils {
    private static SqlSessionFactory sqlSessionFactory;
//    创建ThreadLocal绑定当前线程中的SqlSession对象
    private static final ThreadLocal<SqlSession> t1=new ThreadLocal<SqlSession>();
//    加载配置文件加载一次就可以了，所以使用静态代码块
    static {
        try {
            InputStream resourceAsStream = Resources.getResourceAsStream("Mybatis-config.xml");
            sqlSessionFactory=new SqlSessionFactoryBuilder().build(resourceAsStream);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
//    创建SqlSession
    public static SqlSession openSession(){
        SqlSession sqlSession = t1.get();
        if (sqlSession==null){
             sqlSession = sqlSessionFactory.openSession();
             t1.set(sqlSession);
        }
        return sqlSession;
    }
//     事务提交
    public static void commit(){
        SqlSession sqlSession = openSession();
        sqlSession.commit();
        closeSession();
    }
//     事务回滚
    public static void rollback(){
        SqlSession sqlSession = openSession();
        sqlSession.rollback();
        closeSession();
    }

//    资源释放
    public static void closeSession(){
        SqlSession sqlSession = t1.get();
        sqlSession.close();
    }

//    通过SqlSession创建Dao接口的实现类对象
    public static <T> T getMapper(Class<T> mapper){
        SqlSession sqlSession = openSession();
        return sqlSession.getMapper(mapper);

    }
}

SqlSession类似JDBC中的Connection,线程唯一，全局不唯一的特点，所以需要调用TreadLocal来绑定线程唯一

//    创建ThreadLocal绑定当前线程中的SqlSession对象
    private static final ThreadLocal<SqlSession> t1=new ThreadLocal<SqlSession>();

//    创建SqlSession
    public static SqlSession openSession(){
        SqlSession sqlSession = t1.get();
        if (sqlSession==null){
             sqlSession = sqlSessionFactory.openSession();
             t1.set(sqlSession);
        }
        return sqlSession;
    }

ORM映射

MyBatis自动ORM失效

MyBatis只能自动维护数据库列名与持久化类的属性名相同时一一对应关系，二者不同时，无法自动ORM

1.解决方案一：列的别名

通过as来更改列的别名

    <select id="queryUserByNameById" resultType="User">
         select id,username,password,gender,regist_time as registtime
         from mybatis
         where id=#{param1} and username=#{param2}
    </select>

2.解决方案二：结果映射ResultMap

UserDao-Mapper.xml

  <resultMap id="rm" type="User"><!--唯一标识rm，返回结果为User-->
        <id column="id" property="id"/>
        <result column="username" property="username"/>
        <result column="password" property="password"/>
        <result column="gender" property="gender"/>
        <result column="regist_time" property="regist_time"/>
    </resultMap>

    <!--定位dao方法 返回的类型 写入sql语句-->
    <select id="queryUser" resultMap="rm">
        select id,username,password,gender,regist_time
        from mybatis
        where id=#{arg0}<!--第一个参数的值-->
    </select>

mybatistest

    UserDao mapper = MyBatisUtils.getMapper(UserDao.class);
        User user = mapper.queryUser(3);
        System.out.println(user);

MyBatis处理关联关系-多表连接

1.一对一

建立表结构

CREATE TABLE `t_passengers` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(50) DEFAULT NULL,
  `sex` varchar(1) DEFAULT NULL,
  `birthday` date DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1003 DEFAULT CHARSET=utf8;

CREATE TABLE `t_passports` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `nationality` varchar(50) DEFAULT NULL,
  `expire` date DEFAULT NULL,
  `passenger_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `passenger_id` (`passenger_id`),
  CONSTRAINT `t_passports_ibfk_1` FOREIGN KEY (`passenger_id`) REFERENCES `t_passengers` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=100003 DEFAULT CHARSET=utf8;

一起学习框架SSM之MyBatis(二)

1.1单向查询

旅客实体类

public class Passenger {
    private Integer id;
    private String name;
    private Boolean sex;
    private Date birthday;

    //存储旅客的护照信息 关联属性
    private Passport passport;

    public Passenger(){}
    public Passenger(Integer id, String name, Boolean sex, Date birthday) {
        this.id = id;
        this.name = name;
        this.sex = sex;
        this.birthday = birthday;
    }

    public Passport getPassport() {
        return passport;
    }

    public void setPassport(Passport passport) {
        this.passport = passport;
    }

    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 Boolean getSex() {
        return sex;
    }

    public void setSex(Boolean sex) {
        this.sex = sex;
    }

    public Date getBirthday() {
        return birthday;
    }

    public void setBirthday(Date birthday) {
        this.birthday = birthday;
    }

    @Override
    public String toString() {
        return "Passenger{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", sex=" + sex +
                ", birthday=" + birthday +
                ", passport=" + passport +
                '}';
    }
}

护照实体类

public class Passport {
    private Integer id;
    private String nationality;
    private Date expire;
    private Integer passengers_id;

    //存储旅客的信息
    private Passenger passenger;

    public Passport(){}
    public Passport(Integer id, String nationality, Date expire, Integer passengers_id) {
        this.id = id;
        this.nationality = nationality;
        this.expire = expire;
        this.passengers_id = passengers_id;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getNationality() {
        return nationality;
    }

    public void setNationality(String nationality) {
        this.nationality = nationality;
    }

    public Date getExpire() {
        return expire;
    }

    public void setExpire(Date expire) {
        this.expire = expire;
    }

    public Integer getPassengers_id() {
        return passengers_id;
    }

    public void setPassengers_id(Integer passengers_id) {
        this.passengers_id = passengers_id;
    }
}

PassengerDao

public interface PassengerDao {
//    通过旅客的id，查询旅客信息以及护照信息 关联查询
    Passenger queryPassengerById(@Param("id") Integer id);

  
}

PassengerDao-Mapper.xml

<?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="cn.ozl.dao.PassengerDao">

    <resultMap id="passengermap" type="Passenger">
        <id column="id" property="id"/>
        <result column="name" property="name"/>
        <result column="sex" property="sex"/>
        <result column="birthday" property="birthday"/>

        <!--描述passid，nationality，expire和passport映射规则-->
        <association property="passport" javaType="Passport">
            <id column="passid" property="id"/>
            <result column="nationality" property="nationality"/>
            <result column="expire" property="expire"/>
        </association>

    </resultMap>

    <!--查询  旅客以及护照信息-->
    <select id="queryPassengerById" resultMap="passengermap">
        select t_passengers.id,t_passengers.name,t_passengers.sex,t_passengers.birthday,t_passports.id passid,t_passports.nationality,t_passports.expire
        from t_passengers join t_passports on t_passengers.id = t_passports.passenger_id
        where t_passengers.id=#{id}

    </select>

    <select id="queryPassengerByname" resultType="Passenger">
        select id,name,sex,birthday
        from t_passengers
        where name=#{name}
    </select>
</mapper>

测试

public class mybatistest {
    public static void main(String[] args) throws IOException {
        PassengerDao mapper = MyBatisUtils.getMapper(PassengerDao.class);
        Passenger passenger = mapper.queryPassengerById(1001);
        System.out.println(passenger);
        System.out.println(passenger.getPassport());

    }

1.2双向查询

在单向查询的基础上再加上以下代码

PassportDao

public interface PassportDao {
    Passport queryPassportById(@Param("id") Integer id);
}

PassportDao-Mapper.xml

<?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="cn.ozl.dao.PassportDao">

    <resultMap id="passportmap" type="Passport">
        <id column="id" property="id"/>
        <result column="nationality" property="nationality"/>
        <result column="expire" property="expire"/>
        <result column="passenger_id" property="passenger_id"/>

        <!--描述Passenger的映射规则-->
        <association property="passenger" javaType="Passenger">
            <id column="passengerid" property="id"/>
            <result column="name" property="name"/>
            <result column="sex" property="sex"/>
            <result column="birthday" property="birthday"/>
        </association>

    </resultMap>

    <!--查询  护照信息以及旅客-->
    <select id="queryPassportById" resultMap="passportmap">
        select t_passports.id,t_passports.nationality,t_passports.expire,t_passengers.id passengerid,t_passengers.name,t_passengers.sex,t_passengers.birthday
        from t_passports join t_passengers on t_passengers.id = t_passports.passenger_id
        where t_passports.id=#{id}

    </select>

<!--    <select id="queryPassengerByname" resultType="Passenger">-->
<!--        select id,name,sex,birthday-->
<!--        from t_passengers-->
<!--        where name=#{name}-->
<!--    </select>-->
</mapper>

Passport

public class Passport {
    private Integer id;
    private String nationality;
    private Date expire;
    private Integer passengers_id;

    //存储旅客的信息
    private Passenger passenger;

    public Passport(){}
    public Passport(Integer id, String nationality, Date expire, Integer passengers_id) {
        this.id = id;
        this.nationality = nationality;
        this.expire = expire;
        this.passengers_id = passengers_id;
    }

    public Passenger getPassenger() {
        return passenger;
    }

    public void setPassenger(Passenger passenger) {
        this.passenger = passenger;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getNationality() {
        return nationality;
    }

    public void setNationality(String nationality) {
        this.nationality = nationality;
    }

    public Date getExpire() {
        return expire;
    }

    public void setExpire(Date expire) {
        this.expire = expire;
    }

    public Integer getPassengers_id() {
        return passengers_id;
    }

    public void setPassengers_id(Integer passengers_id) {
        this.passengers_id = passengers_id;
    }

    @Override
    public String toString() {
        return "Passport{" +
                "id=" + id +
                ", nationality='" + nationality + '\'' +
                ", expire=" + expire +
                ", passengers_id=" + passengers_id +
                ", passenger=" + passenger +
                '}';
    }
}

mybatistest

public class mybatistest {
    public static void main(String[] args) throws IOException {
        PassportDao mapper = MyBatisUtils.getMapper(PassportDao.class);
        Passport passport = mapper.queryPassportById(1);
        System.out.println(passport);
        System.out.println(passport.getPassenger());

    }
}

一起学习框架SSM之MyBatis(二)

2.一对多

建立表结构

create table t_departments(
    id int primary key auto_increment,
    name varchar(50),
    location varchar(100)
)default charset =utf8;

create table t_employees(
    id int primary key auto_increment,
    name varchar(50),
    salary double,
    dept_id int,
    foreign key (dept_id) references t_departments(id)
)default charset =utf8;

insert into t_departments values(1,"教学部","北京"),(2,"研发部","上海");
insert into t_employees values (1,"ozl01",12000.5,1),(2,"ozl02",12888.8,1),(3,"李四",10000.9,1),(4,"张三",8000,2);

一起学习框架SSM之MyBatis(二)

雇员实体类

public class Employee {
    private Integer id;
    private String name;
    private double salary;
    //    员工的部门信息
    private Department department;

    public Employee() {
    }

    public Employee(Integer id, String name, double salary) {
        this.id = id;
        this.name = name;
        this.salary = salary;
    }

    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 double getSalary() {
        return salary;
    }

    public void setSalary(double salary) {
        this.salary = salary;
    }

    public Department getDepartment() {
        return department;
    }

    public void setDepartment(Department department) {
        this.department = department;
    }

    @Override
    public String toString() {
        return "Employee{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", salary=" + salary +
                '}';
    }
}

部门实体类


public class Department {
    private Integer id;
    private String name;
    private String location;

//    部门的所有员工信息
    private List<Employee> employees;

    public Department(){}
    public Department(Integer id, String name, String location) {
        this.id = id;
        this.name = name;
        this.location = location;
    }

    public List<Employee> getEmployees() {
        return employees;
    }

    public void setEmployees(List<Employee> employees) {
        this.employees = employees;
    }

    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 getLocation() {
        return location;
    }

    public void setLocation(String location) {
        this.location = location;
    }

    @Override
    public String toString() {
        return "Department{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", location='" + location + '\'' +
                ", employees=" + employees +
                '}';
    }
}

DepartmentDao

public interface DepartmentDao {
    Department queryDepartmentById(@Param("id") Integer id);
}

DepartmentDao-Mapper.xml

<?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="cn.ozl.dao.DepartmentDao">
    <resultMap id="dept_emp" type="Department">
        <id column="id" property="id"/>
        <result column="name" property="name"/>
        <result column="location" property="location"/>
        
<!--        关系属性employee-->
       <collection property="employees" ofType="Employee">
           <id column="emp_id" property="id"/>
           <result column="emp_name" property="name"/>
           <result column="salary" property="salary"/>
       </collection>
    </resultMap>
    <select id="queryDepartmentById" resultMap="dept_emp">
        select t_departments.id,t_departments.name,t_departments.location,t_employees.id  emp_id,t_employees.name emp_name,t_employees.salary
        from t_departments join t_employees on t_departments.id = t_employees.dept_id
        where t_departments.id=#{id}
    </select>
</mapper>

测试类

public class mybatistest {
    public static void main(String[] args) throws IOException {
        DepartmentDao mapper = MyBatisUtils.getMapper(DepartmentDao.class);
        Department department = mapper.queryDepartmentById(1);
        System.out.println(department);
        List<Employee> employees = department.getEmployees();
        for (Employee employee : employees) {
            System.out.println(employees);
        }


    }

结果
一起学习框架SSM之MyBatis(二)

2.1 多对一

EmployeeDao

public interface EmployeeDao {
    Employee queryEmployeeById(@Param("id") Integer id);
}

EmployeeDao-Mapper.xml

<?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="cn.ozl.dao.EmployeeDao">
    <resultMap id="emp_det" type="Employee">
        <id column="id" property="id"/>
        <result column="name" property="name"/>
        <result column="salary" property="salary"/>
        
<!--        关系属性部门department-->
        <association property="department" javaType="Department">
            <id column="det_id" property="id"/>
            <result column="det_name" property="name"/>
            <result column="location" property="location"/>
        </association>
    </resultMap>
    <select id="queryEmployeeById" resultMap="emp_det">
        select t_employees.id,t_employees.name,t_employees.salary,t_departments.id  det_id,t_departments.name det_name,t_departments.location
        from t_employees join t_departments on t_employees.dept_id=t_departments.id
        where t_employees.id=#{id}
    </select>
</mapper>

注册Mapper

<mapper resource="cn/ozl/dao/EmployeeDao-Mapper.xml"/>

测试类

public static void main(String[] args) throws IOException {
//        DepartmentDao mapper = MyBatisUtils.getMapper(DepartmentDao.class);
//        Department department = mapper.queryDepartmentById(1);
//        System.out.println(department);
//        List<Employee> employees = department.getEmployees();
//        for (Employee employee : employees) {
//            System.out.println(employees);
//        }
        EmployeeDao mapper = MyBatisUtils.getMapper(EmployeeDao.class);
        Employee employee = mapper.queryEmployeeById(2);
        System.out.println(employee);
        System.out.println(employee.getDepartment());

    }

结果
一起学习框架SSM之MyBatis(二)

3. 多对多``

建立表结构

create table t_students(
    id int primary key auto_increment,
    name varchar(50),
    sex varchar(1)
)default charset =utf8;

create table t_subjects(
    id int primary key auto_increment,
    name varchar(50),
    grade int
)default charset =utf8;

create table t_stu_sub(
    student_id int,
    subject_id int,
    foreign key (student_id) references t_students(id),
    foreign key (subject_id) references t_subjects(id),
    primary key (student_id,subject_id)
)default charset =utf8;

insert into t_students values (1,"張三",'m'),(2,"李四",'f');
insert into t_subjects values (1001,"java",1),(1002,"php",2);
insert into t_stu_sub values (1,1001),(1,1002),(2,1001),(2,1002);

一起学习框架SSM之MyBatis(二)

Student和Subject实体类

public class Student2 {
    private Integer id;
    private String name;
    private Boolean sex;

    private List<Subject> subjects;
    public Student2(){}
    public Student2(Integer id, String name, Boolean sex) {
        this.id = id;
        this.name = name;
        this.sex = sex;
    }

    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 Boolean getSex() {
        return sex;
    }

    public void setSex(Boolean sex) {
        this.sex = sex;
    }

    public List<Subject> getSubjects() {
        return subjects;
    }

    public void setSubjects(List<Subject> subjects) {
        this.subjects = subjects;
    }

    @Override
    public String toString() {
        return "Student2{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", sex=" + sex +
                '}';
    }

}


public class Subject {
    private Integer id;
    private String name;
    private Integer grade;

    private List<Student2> student2s;

    public Subject(){}
    public Subject(Integer id, String name, Integer grade) {
        this.id = id;
        this.name = name;
        this.grade = grade;
    }

    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 Integer getGrade() {
        return grade;
    }

    public void setGrade(Integer grade) {
        this.grade = grade;
    }

    public List<Student2> getStudent2s() {
        return student2s;
    }

    public void setStudent2s(List<Student2> student2s) {
        this.student2s = student2s;
    }

    @Override
    public String toString() {
        return "Subject{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", grade=" + grade +
                '}';
    }
}

实体类的Dao文件和映射文件

public interface Student2Dao {
    Student2 queryStudent2ById(@Param("id") Integer id);
}

public interface SubjectDao {
    Subject querySubjectById(@Param("id") Integer id);
}

在这里插入代码片<?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="cn.ozl.dao.Student2Dao"><!--定位哪个Dao的接口进行描述-->
    <resultMap id="stu_sub" type="Student2">
        <id column="id" property="id"/>
        <result column="name" property="name"/>
        <result column="sex" property="sex"/>

        <collection property="subjects" ofType="Subject">
            <id column="sub_id" property="id"/>
            <result column="sub_name" property="name"/>
            <result column="grade" property="grade"/>
        </collection>
    </resultMap>


    <select id="queryStudent2ById" resultMap="stu_sub">
        select t_students.id,t_students.name,t_students.sex,t_subjects.id sub_id,t_subjects.name sub_name,t_subjects.grade
        from t_students join t_stu_sub tss on t_students.id = tss.student_id
        join t_subjects  on tss.subject_id = t_subjects.id
        where student_id=#{id}
    </select>
</mapper>


```java
<?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="cn.ozl.dao.SubjectDao"><!--定位哪个Dao的接口进行描述-->
    <resultMap id="sub_stu" type="Subject">
        <id column="id" property="id"/>
        <result column="name" property="name"/>
        <result column="grade" property="grade"/>

        <collection property="student2s"  ofType="Student2">
            <id column="stu_id" property="id"/>
            <result column="stu_name" property="name"/>
            <result column="sex" property="sex"/>
        </collection>
    </resultMap>
    <select id="querySubjectById" resultMap="sub_stu">
        select t_subjects.id,t_subjects.name,t_subjects.grade,t_students.id stu_id,t_students.name stu_name,t_students.sex
        from t_subjects join t_stu_sub tss on t_subjects.id = tss.subject_id
        join t_students  on t_students.id = tss.student_id
        where subject_id=#{id}
    </select>
</mapper>

注册映射文件

     <mapper resource="cn/ozl/dao/SubjectDao-Mapper.xml"/>
     <mapper resource="cn/ozl/dao/Student2Dao-Mapper.xml"/>

测试

  		SubjectDao mapper = MyBatisUtils.getMapper(SubjectDao.class);
        Subject subject = mapper.querySubjectById(1001);
        System.out.println(subject);
        List<Student2> student2s = subject.getStudent2s();
        for (Student2 student2 : student2s) {
            System.out.println(student2);
        }
        Student2Dao mapper1 = MyBatisUtils.getMapper(Student2Dao.class);
        Student2 student2 = mapper1.queryStudent2ById(1);
        System.out.println(student2);
        List<Subject> subjects = student2.getSubjects();
        for (Subject subject1 : subjects) {
            System.out.println(subject1);
        }

结果

动态SQL

Mybatis支持映射文件中在基础Sql上添加一些逻辑操作，并动态拼接完成完整的sql之后在执行，以达到sql的复用，简化编程效果

1.sql标签

重复的sql语句可以用sql标签复用


//      sql片段
    <sql id="usersql">
//      复用sql代码
        select id,username,password,gender,regist_time
        from mybatis
    </sql>
    <!--定位dao方法 返回的类型 写入sql语句-->
    <select id="queryUser" resultMap="rm">
        <!--引用sql片段-->
        <include refid="usersql"></include>
        where id=#{arg0}<!--第一个参数的值-->
    </select>

2.if标签

//      sql片段
    <sql id="usersql">
//      复用sql代码
        select id,username,password,gender,regist_time
        from mybatis
    </sql>
    <!--定位dao方法 返回的类型 写入sql语句-->
    <select id="queryUser" resultMap="rm">
        <!--引用sql片段-->
        <include refid="usersql"></include>
        where
        <!--通过id或者username参数查询方法可以合并通过if标签-->
        <if test="id!=null">
            id=#{id}
        </if>
        <if test="username!=null">
            username=#{username}
        </if>
    </select>

3.where标签

 <select id="queryUser1" resultMap="rm">
        <include refid="usersql"></include>
        <!--where标签
            1.补充where关键字
            2.识别where子句中如果 以or,and开头，会将or,and去除
        -->
        <where>
            <if test="username!=null">
                username=#{username}
            </if>
            <if test="gender">
                or gender=#{gender}
            </if>
        </where>
    </select>

4.set标签

 update mybatis
        <!--set标签
            1.补充set关键字
            2.自动将set子句的最后逗号去除
        -->
        <set>
            <if test="username!=null">
                username=#{username}
            </if>
            <if test="password!=null">
                paswword=#{password}
            </if>
        </set>
        where id=#{id}
    </update>

5.trim标签

trim可以替代set和where的作用

 <!--prefix="where"补充where关键字
            prefixOverrides="or|and"以or and开头会被覆盖
        -->
        <trim prefix="where" prefixOverrides="or|and">
            <if test="username!=null">
                username=#{username}
            </if>
            <if test="gender">
                or gender=#{gender}
            </if>
        </trim>

 <!--prefix="set"补充set关键字
        suffixOverrides=","末尾以，结尾去除-->
       <trim prefix="set" suffixOverrides=",">
           <if test="username!=null">
               username=#{username}
           </if>
           <if test="password!=null">
               paswword=#{password}
           </if>
       </trim>

6.foreach标签

   
    <delete id="deleteManyUser" parameterType="java.util.List">
        <!--delete from mybatis where id in (x,x,x,x)-->
        delete from mybatis where id in
        <foreach collection="list" open="(" close=")" item="ids9" separator=",">
            #{ids9}
        </foreach>
    </delete>

  <insert id="insertManyUser" parameterType="java.util.List">
        <!--insert into mybatis values (null,x,x),(null,x,x)-->
        insert into mybatis values 
        <foreach collection="list" open="" close="" item="users9" separator=",">
            (null,#{users9.username},#{users9.password})
        </foreach>
    </insert>

本文地址：https://blog.csdn.net/ozl520/article/details/108853254

一起学习框架SSM之MyBatis(二)

MyBatis

MyBatis工具类

1.封装工具类

ORM映射

MyBatis自动ORM失效

1.解决方案一：列的别名

2.解决方案二：结果映射ResultMap

MyBatis处理关联关系-多表连接

1.一对一

1.1单向查询

1.2双向查询

2.一对多

2.1 多对一

3. 多对多``

动态SQL

1.sql标签

2.if标签

3.where标签

4.set标签

5.trim标签

6.foreach标签

SSM框架学习：使用MyBatis Generator自动创建代码

Laravel框架学习笔记（二）项目实战之模型（Models）

一起学习框架SSM之MyBatis(二)

Mybatis框架学习之增删改查

JAVA框架学习之mybatis(一个文章吃透mybatis)

【Mybatis学习】利用SpringBoot搭建SSM框架

SSM框架学习之路-Mybatis（七）分页

SSM框架学习之路-Mybatis（四）配置解析

SSM框架学习之路-Mybatis（八）使用注解开发

Spring mybatis 之-ssm框架环境搭建（方案一）