mybatis总结

1.mybatis执行流程：

 2.依赖

        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>8.0.16</version>
        </dependency>
        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.5.2</version>
        </dependency>

        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.13</version>
        </dependency>

        <dependency>
            <groupId>log4j</groupId>
            <artifactId>log4j</artifactId>
            <version>1.2.17</version>
        </dependency>

        <!-- https://mvnrepository.com/artifact/org.projectlombok/lombok -->
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <version>1.18.12</version>
            <scope>provided</scope>
        </dependency>

3. mybatis多对一查询：

示例：多个学生对应一个老师。

实体类示例：

@Data
public class Teacher {
    private int id;
    private String name;

}

@Data
public class Student {
    private int id;
    private String name;
    private Teacher teacher;
}

工具类

public class MybatisUtils {
    private static SqlSessionFactory sqlSessionFactory;
    static {
        try {
            InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
             sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static SqlSession getSqlSession(){
       return sqlSessionFactory.openSession();
    }
}

xml示例：

<?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.sf.dao.StudentMapper">

    <!--需求：获取所有学生及对应老师的信息-->
    <!--子查询-->
    <select id="getStudent" resultMap="studentTeacher">
        select * from student
    </select>
    <!--association关联属性，property属性名，column数据库列名，javaType属性类型。-->
    <resultMap id="studentTeacher" type="student">
        <association property="teacher" column="tid" javaType="teacher" select="getTeacher"/>
    </resultMap>
    
    <select id="getTeacher" resultType="teacher">
        select * from teacher where id = #{tid}
    </select>

    <!--按结果嵌套查询-->
    <select id="getStudent2" resultMap="studentTeacher2">
        SELECT s.id sid,s.name sname,t.name tname,t.id tid
        FROM student s,teacher t
        WHERE s.tid = t.id
    </select>

    <resultMap id="studentTeacher2" type="student">
       <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <association property="teacher" javaType="teacher">
            <result property="name" column="tname"/>
            <result property="id" column="tid"/>
        </association>
    </resultMap>
</mapper>

测试：

    @Test
    public void testStudent2(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
        List<Student> student = mapper.getStudent2();
        for (Student student1 : student) {
            System.out.println(student1);
        }
    }

4.一对多查询：

示例：一个老师拥有多个学生

实体类示例：

@Data
public class Student {
    private int id;
    private String name;
    private int tid;
}

@Data
public class Teacher {
    private int id;
    private String name;
    private List<Student> students;
}

xml示例

<?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.sf.dao.TeacherMapper">

    <!--需求：查询老师信息，并查出其所拥有的学生信息-->
    <!--方法一：按结果嵌套处理-->
    <select id="getTeacher2" resultMap="teacherStudent">
        select t.id tid,t.name tname,s.id sid,s.name sname
        from teacher t,student s
        where t.id = s.tid and t.id = #{tid}
    </select>

    <resultMap id="teacherStudent" type="teacher">
        <result property="id" column="tid"/>
        <result property="name" column="tname"/>
        <!--使用collection标签，ofType 指映射到集合中的泛型类型-->
        <collection property="students" ofType="student">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
            <result property="tid" column="tid"/>
        </collection>
    </resultMap>

    <!--方法二：按查询嵌套处理-->
    <select id="getTeacher3" resultMap="teacherStudent3">
        select * from teacher where id = #{tid}
    </select>

    <resultMap id="teacherStudent3" type="teacher">
        <!--javaType指实体类中属性的类型，ofType 指映射到集合中的泛型类型-->
        <collection property="students" javaType="ArrayList" ofType="student" select="getStudentByTeacherId" column="id"/>
    </resultMap>

    <select id="getStudentByTeacherId" resultType="student">
        select * from student where tid = #{tid}
    </select>

</mapper>

测试：

    @Test
    public void testTeacher3(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
        Teacher teacher = mapper.getTeacher3(1);
        System.out.println(teacher);
    }

5.mybatis动态sql：

指根据不同的查询条件，生成不同的sql语句。

实体类示例

@Data
public class Blog implements Serializable {
    private String id;
    private String title;
    private String author;
    private Date createTime;
    private int views;
}

xml示例

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.sf.mapper.BlogMapper">
    <cache eviction="LRU"/>
    <!--插入数据-->
    <insert id="addBook" parameterType="Blog">
        insert into blog(id,title,author,create_time,views)
        values (#{id},#{title},#{author},#{createTime},#{views});
    </insert>

    <!--sql片段，抽取共有部分，简化代码-->
    <sql id="if-title-author">
        <if test="title !=null">
            title =#{title}
        </if>
        <if test="author !=null">
            and author = #{author}
        </if>
    </sql>

    <!--使用sql片段-->
    <select id="queryBlogIf" parameterType="map" resultType="blog">
        select * from blog
        <where>
            <include refid="if-title-author"></include>
        </where>
    </select>

    <!--foreach对集合进行遍历-->
    <select id="queryBlogForeach" parameterType="list" resultType="blog">
        select * from blog
        <where>
            id in
            <foreach collection="list" item="arrayList" open="(" close=")" separator="," >
                #{arrayList}
            </foreach>
        </where>
    </select>

    <!--choose类似于java中switch语句，满足一项即返回-->
    <select id="queryBlogChoose" resultType="blog" parameterType="map">
        select * from blog
        <where>
            <choose>
                <when test="title !=null">
                    and title = #{title}
                </when>
                <when test="author !=null">
                    and author = #{author}
                </when>
                <otherwise>
                    and views = #{views}
                </otherwise>
            </choose>
        </where>
    </select>

    <!--set会删掉额外的逗号，用于更新-->
    <update id="updateBlog" parameterType="map">
        update blog
        <set>
            <if test="title !=null">
                title =#{title},
            </if>
            <if test="author!=null">
                author = #{author}
            </if>
        </set>
        where id = #{id}
    </update>
</mapper>