MyBatis_07(动态SQL)

动态SQL:

Mybatis框架的动态SQL技术是一种根据特定条件动态拼装SQL语句的功能，
它存在的意义是："为了解决拼接SQL语句字符串时的痛点问题"。

一、If

• if标签可通过test属性（即传递过来的数据）的表达式进行判断，若表达式的结果为true，则标签中的内容会执行；反之标签中的内容不会执行

• 在where后面添加一个恒成立条件1=1

  • 这个恒成立条件并不会影响查询的结果

  • 这个1=1可以用来拼接and语句，例如：当empName为null时

    • 如果不加上恒成立条件，则SQL语句为select * from t_emp where and age = ? and sex = ? and email = ?，此时where会与and连用，SQL语句会报错

    • 如果加上一个恒成立条件，则SQL语句为select * from t_emp where 1= 1 and age = ? and sex = ? and email = ?，此时不报错

public interface DynamicMapper {
    /**
     * 动态sql，if语句
     * @param emp
     * @return
     */
    List<Emp> getEmpByCondition(Emp emp);
}

 <!--List<Emp> getEmpByCondition(Emp emp);-->
    <!-- and == && ，所以条件都要满足 -->
    <select id="getEmpByCondition" resultType="Emp">
        select * from t_emp where 1=1
        <if test="empName != null and empName !=''">
            and emp_name = #{empName}
        </if>
        <if test="age != null and age !=''">
            and age = #{age}
        </if>
        <if test="sex != null and sex !=''">
            and sex = #{sex}
        </if>
        <if test="email != null and email !=''">
            and email = #{email}
        </if>
    </select>

 @Test
    public void testGetEmpByCondition() throws IOException {
        InputStream is = Resources.getResourceAsStream("mybatis-config.xml");
        SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder();
        SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBuilder.build(is);
        SqlSession sqlSession = sqlSessionFactory.openSession(true);
        DynamicMapper mapper = sqlSession.getMapper(DynamicMapper.class);
        List<Emp> empByCondition = mapper.getEmpByCondition(new Emp(null,"王五",34,"女","423423"));
        System.err.println(empByCondition);
        sqlSession.close();
//        empName='王五', age=34, sex='女', email='423423'
    }

二、where

• where和if一般结合使用：

  • 若where标签中的if条件都不满足，则where标签没有任何功能，即不会添加where关键字

  • 若where标签中的if条件满足，则where标签会自动添加where关键字，并将条件最 "前方" 多余的and/or去掉

 /**
     * 动态sql，where
     * @param emp
     * @return
     */
    List<Emp> getEmpByCondition_where(Emp emp);

<!--List<Emp> getEmpByCondition_where(Emp emp);-->
    <select id="getEmpByCondition_where" resultType="Emp">
        select * from t_emp
        <where>
            <if test="empName != null and empName !=''">
                and emp_name = #{empName}
            </if>
            <if test="age != null and age !=''">
                and age = #{age}
            </if>
            <if test="sex != null and sex !=''">
                and sex = #{sex}
            </if>
            <if test="email != null and email !=''">
                and email = #{email}
            </if>
        </where>
    </select>

@Test //where
    public void testGetEmpByCondition_were() throws IOException {
        InputStream is = Resources.getResourceAsStream("mybatis-config.xml");
        SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder();
        SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBuilder.build(is);
        SqlSession sqlSession = sqlSessionFactory.openSession(true);
        DynamicMapper mapper = sqlSession.getMapper(DynamicMapper.class);
        List<Emp> empByCondition = mapper.getEmpByCondition_where(new Emp(null, "王五", 34, "女", "423423"));
        System.err.println(empByCondition);
        sqlSession.close();
//        empName='王五', age=34, sex='女', email='423423'
    }

错误示范：where标签不能去掉 "条件后" 多余的and/or

<!--这种用法是错误的，只能去掉条件前面的and/or，条件后面的不行-->
<if test="empName != null and empName !=''">
emp_name = #{empName} and
</if>
<if test="age != null and age !=''">
	age = #{age}
</if>

三、trim

• trim用于去掉或添加标签中的内容

• 常用属性

  • prefix：在trim标签中的内容的 {"前面添加"} - 某些内容

  • suffix：在trim标签中的内容的{"后面添加"} 某些内容

  • prefixOverrides：在trim标签中的内容的 {"前面去掉某"} 些内容

  • suffixOverrides：在trim标签中的内容的 {"后面去掉某"} 些内容

• 若trim中的标签都不满足条件，则trim标签没有任何效果，也就是只剩下select * from t_emp

 /**
     * 动态sql，trim
     * @param emp
     * @return
     */
    List<Emp> getEmpByCondition_trim(Emp emp);

 <!--List<Emp> getEmpByCondition_trim(Emp emp);-->
    <select id="getEmpByCondition_trim" resultType="Emp">
        select * from t_emp
        <trim prefix="where" suffixOverrides="and|or">
            <if test="empName != null and empName !=''">
                emp_name = #{empName} and
            </if>
            <if test="age != null and age !=''">
                age = #{age} and
            </if>
            <if test="sex != null and sex !=''">
                sex = #{sex} or
            </if>
            <if test="email != null and email !=''">
                email = #{email}
            </if>
        </trim>
    </select>

 @Test //trim
    public void testGetEmpByCondition_trim() throws IOException {
        InputStream is = Resources.getResourceAsStream("mybatis-config.xml");
        SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder();
        SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBuilder.build(is);
        SqlSession sqlSession = sqlSessionFactory.openSession(true);
        DynamicMapper mapper = sqlSession.getMapper(DynamicMapper.class);
        List<Emp> empByCondition = mapper.getEmpByCondition_trim(new Emp(null, "王五", 34, "女", "423423"));
        System.err.println(empByCondition);
        sqlSession.close();
//        empName='王五', age=34, sex='女', email='423423'
    }

四、choose、when、otherwise

• "choose、when、otherwise" 相当于 "if...else if..else"

• when至少要有一个，otherwise至多只有一个

/**
     * 动态sql，choose、when、otherwise
     * @param emp
     * @return
     */
    List<Emp> getEmpByCondition_choose(Emp emp);

<!--    List<Emp> getEmpByCondition_choose(Emp emp);-->
    <select id="getEmpByCondition_choose" resultType="Emp">
        select * from t_emp
        <where>
            <choose>
                <!--当一个when执行了一个，剩下就不会执行。因为他们是if...else if..else-->
                <when test="empName != null and empName != ''">
                    emp_name = #{empName}
                </when>
                <when test="age != null and age != ''">
                    age = #{age}
                </when>
                <when test="sex != null and sex != ''">
                    sex = #{sex}
                </when>
                <when test="email != null and email != ''">
                    email = #{email}
                </when>
                <otherwise>
                    did = 1
                </otherwise>
            </choose>
        </where>
    </select>

 @Test //choose
    public void testGetEmpByCondition_choose() throws IOException {
        InputStream is = Resources.getResourceAsStream("mybatis-config.xml");
        SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder();
        SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBuilder.build(is);
        SqlSession sqlSession = sqlSessionFactory.openSession(true);
        DynamicMapper mapper = sqlSession.getMapper(DynamicMapper.class);
        List<Emp> empByCondition = mapper.getEmpByCondition_choose(new Emp(null, "王五", 34, "女", "423423"));
        System.err.println(empByCondition);
        sqlSession.close();
//        empName='王五', age=34, sex='女', email='423423'
    }

解析：

五、foreach

• 属性：

  • collection：设置要循环的数组或集合

  • item：表示集合或数组中的每一个数据

  • separator：设置循环体之间的分隔符，分隔符前后默认有一个空格，如,

  • open：设置foreach标签中的内容的开始符

  • close：设置foreach标签中的内容的结束符

• 5.1-批量删除：

/**
     * 动态sql,"foreach-实现批量删除"
     * @param eids
     * @return
     */
    Integer deleteMoreByArray(@Param("eids") Integer[] eids);

// 删除的SQL：DELETE  FROM  t_emp where eid IN (9,10)

  <!--Integer deleteMoreByArray(@Param("eids") Integer[] eids);-->
    <!--SQL：DELETE  FROM  t_emp where eid IN (9,10)-->
    <delete id="deleteMoreByArray">
        delete from t_emp where eid in
        <foreach collection="eids" item="eid" separator="," open="(" close=")">
            #{eid}
        </foreach>
    </delete>

@Test //foreach-批量删除
    public void testDeleteMoreByArray() throws IOException {
        InputStream is = Resources.getResourceAsStream("mybatis-config.xml");
        SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder();
        SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBuilder.build(is);
        SqlSession sqlSession = sqlSessionFactory.openSession(true);
        DynamicMapper mapper = sqlSession.getMapper(DynamicMapper.class);
        Integer integer = mapper.deleteMoreByArray(new Integer[]{7, 8});
        System.err.println("受影响的行数：" + integer);
        sqlSession.close();
    }

• 5.2-批量增加：

/**
     * 动态sql,"foreach-实现批量增加"
     * @param emps
     * @return
     */
    Integer insertMoreByList(@Param("emps") List<Emp> emps);

//批量添加的SQL语句：insert into t_emp values (null,"a",23,"男","321412@111",NULL),(null,"a1",23,"男","321412@111",NULL) 

<!--    Integer insertMoreByList(@Param("emps") List<Emp> emps);-->
    <insert id="insertMoreByList">
        insert into t_emp values
        <foreach collection="emps" item="emp" separator=",">
            (null,#{emp.empName},#{emp.age},#{emp.sex},#{emp.email},null)
        </foreach>
    </insert>

@Test
    public void insertMoreByList() throws IOException {
        InputStream is = Resources.getResourceAsStream("mybatis-config.xml");
        SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder();
        SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBuilder.build(is);
        SqlSession sqlSession = sqlSessionFactory.openSession(true);
        DynamicMapper mapper = sqlSession.getMapper(DynamicMapper.class);
        Emp emp1 = new Emp(null, "a", 13, "人妖", "2412343@qq.com");
        Emp emp2 = new Emp(null, "b", 1, "男", "123@321.com");
        Emp emp3 = new Emp(null, "c", 1, "男", "123@321.com");
        List<Emp> emps = Arrays.asList(emp1, emp2, emp3);
        int result = mapper.insertMoreByList(emps);
        System.out.println(result);
    }

六、SQL片段：

• ql片段，可以记录一段公共sql片段，在使用的地方通过include标签进行引入

• 声明sql片段：标签

<sql id="empColumns">eid,emp_name,age,sex,email</sql>

• 引用sql片段：标签

<sql id="empColumns">eid,emp_name,age,sex,email</sql><!--sql片段-->

    <!--List<Emp> getEmpByCondition(Emp emp);-->
    <!-- and == && ，所以条件都要满足 -->
    <select id="getEmpByCondition_if" resultType="Emp">
        select <include refid="empColumns"></include> from t_emp where 1=1
        <if test="empName != null and empName !=''">
            and emp_name = #{empName}
        </if>
        <if test="age != null and age !=''">
            and age = #{age}
        </if>
        <if test="sex != null and sex !=''">
            and sex = #{sex}
        </if>
        <if test="email != null and email !=''">
            and email = #{email}
        </if>
    </select>