MyBatis 动态sql

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。

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

    public interface UserDao {
    
        //复杂条件查询
        public List<User> findByUser(User user);
    }
    
  • mapper映射文件

    
    DOCTYPE mapper
            PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
            "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
    <mapper namespace="com.by.dao.UserDao">
        
        <select id="findByUser" resultType="User">
            select * from user where 1=1
            <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>
        select>
    mapper>
    
  • 测试

        @Test
        public void testFindAll(){
            UserDao userDao = sqlSession.getMapper(UserDao.class);
            User user = new User();
            user.setSex("男");
            user.setAddress("香港");
            List<User> userList = userDao.findByUser(user);
            for(User u : userList){
                System.out.println(u);
            }
        }
    

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

    
    DOCTYPE mapper
            PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
            "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
    <mapper namespace="com.by.dao.UserDao">
        <select id="findByUser" resultType="User">
            select * from user
            
            <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>
    

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

    public void updateByUser(User user);
    
        <update id="updateByUser" parameterType="user">
            update user
            <set>
                <if test="username!=null and username != '' ">
                    username=#{username},
                if>
                <if test="birthday!=null">
                    birthday=#{birthday},
                if>
                <if test="sex!=null and sex != '' ">
                    sex=#{sex},
                if>
                <if test="address!=null and address != '' ">
                    address=#{address},
                if>
            set>
            where id=#{id}
        update>
    
  • 测试

        @Test
        public void testUpdateByUser(){
            UserDao userDao = sqlSession.getMapper(UserDao.class);
            User user = new User();
            user.setId(50);
            user.setBirthday(new Date());
            user.setAddress("加拿大");
            userDao.updateByUser(user);
        }
    

5.trim标签

trim标签可以代替where标签、set标签

  • mapper

        //修改
        public void updateByUser2(User user);
    
    <update id="updateByUser2" parameterType="User">
            update user
            
            <trim prefix="SET" suffixOverrides=",">
                <if test="birthday!=null">
                    birthday=#{birthday},
                if>
                <if test="sex!=null and username != '' ">
                    sex=#{sex},
                if>
                <if test="address!=null and username != '' ">
                    address=#{address},
                if>
            trim>
            where id=#{id}
        update>
    

6.foreach标签

foreach标签的常见使用场景是集合进行遍历

  • mapper

        //批量删除
        public void deleteUserByIds(@Param("ids") List<Integer> ids);
    	//批量添加
        public void insertUsers(@Param("userList") List<User> userList);
    
        <delete id="deleteUserByIds" parameterType="list">
            delete from user where id in
            
            <foreach collection="ids" item="id" open="(" close=")" separator=",">
                #{id}
            foreach>
        delete>
    
    	<insert id="insertUsers" parameterType="list">
            INSERT INTO user(username,password,birthday,sex,address)
            VALUES
            <foreach collection ="userList" item="user" separator =",">
                (#{user.username},#{user.password},#{user.birthday},
                							#{user.sex},#{user.address})
            foreach>
        insert>
    
  • 测试

        @Test
        public void testDeleteUserByIds(){
            UserDao userDao = sqlSession.getMapper(UserDao.class);
            List<Integer> ids = new ArrayList();
            ids.add(50);
            ids.add(64);
            ids.add(67);
            userDao.deleteUserByIds(ids);
        }
    
    	@Test
        public void testInsertUsers(){
            UserDao userDao = sqlSession.getMapper(UserDao.class);
            long start = System.currentTimeMillis();
            List<User> userList = new ArrayList<>();
            for(int i = 0 ;i < 10000; i++) {
                User user = new User();
                user.setUsername("刘德华");
                user.setPassword("111");
                user.setBirthday(new Date());
                user.setSex("男");
                user.setAddress("香港");
                //userDao.insertUser(user);
                userList.add(user);
            }
            userDao.insertUsers(userList);
            long end = System.currentTimeMillis();
            System.out.println("一万条数据总耗时:" + (end-start) + "ms" );
            sqlSession.commit();
        }
    

7.sql标签

sql元素标签用来定义可重复使用的SQL代码片段,使用时只需要用include元素标签引用即可

  • mapper

        //复杂条件查询
        public List<User> findByUser3(User user);
    
        
        <sql id="query_user_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>
        sql>
        <select id="findByUser3" resultType="User">
            select * from user
            <where>
                <include refid="query_user_where">include>
            where>
        select>
    
  • 测试

        @Test
        public void testFindAll3(){
            UserDao userDao = sqlSession.getMapper(UserDao.class);
            User user = new User();
            user.setAddress("香港");
            user.setUsername("刘德华");
            List<User> userList = userDao.findByUser3(user);
            for(User u : userList){
                System.out.println(u);
            }
        }
    

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