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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。

动态SQL：

⭕ if：根据标签中test属性所对应的表达式决定标签中的内容是否需要拼接到SQL中

⭕ where：

● 当where标签中有内容时，会自动生成where关键字，并且将内容前多余的and或or去掉
● 当where标签中没有内容时，此时where标签没有任何效果
注意：where标签不能将其中内容后面多余的and或or去掉

⭕ trim：
● 若标签中有内容时：

1. prefix|suffix：将trim标签中内容前面或后面添加指定内容
2. suffixOverrides|prefixOverrides：将trim标签中内容前面或后面去掉指定内容

● 若标签中没有内容时，trim标签也没有任何效果

⭕ choose、when、otherwise
● 相当于if...else if...else
● when至少要有一个，otherwise最多只能有一个

⭕ foreach

● collection:设置需要循环的数组或集合
● item:表示数组或集合中的每一个数据
● separator:循环体之间的分割符
● open:foreach标签所循环的所有内容的开始符
● close:foreach标签所循环的所有内容的结束符

⭕ sql 标签
● 设置SQL片段：<sql id="empColumns">eid,emp_name,age,sex,email</sql>
● 引用SQL片段：<include refid="empColumns"></include>

一、if

if标签可通过test属性的表达式进行判断，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

DynamicSQLMapper

/**
     * @description:多条件查询
     * @author: Hey
     * @date: 2022/7/4 13:13
     * @param: [emp]
     * @return: java.util.List<com.ir.mybatis.pojo.Emp>
     **/
    List<Emp> getEmpByCondition(Emp emp);

DynamicSQLMapper.xml

<select id="getEmpByConditionOne" resultType="Emp">
        <!--"1=1"有两个作用：①不影响查询结果②更好的拼接后面的条件-->
        select * from t_emp where 1=1
        <if test="empName != null and empName != ''">
            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>

DynamicSQLMapperTest

/**
     * @description:测试if标签
     * @author: Hey 
     * @date: 2022/7/4 13:25
     * @param: []
     * @return: void
     **/
    @Test
    public void testGetEmpByCondition(){
        SqlSession sqlSession = SqlSessionUtils.getSqlSession();
        DynamicSQLMapper mapper = sqlSession.getMapper(DynamicSQLMapper.class);
        List<Emp> list = mapper.getEmpByCondition(new Emp(null, "", null, "", null));
        System.out.println(list);
    }

二、where

DynamicSQLMapper.xml

<select id="getEmpByConditionTwo" resultType="Emp">
        select * from t_emp
        <where>
            <if test="empName != null and empName != ''">
                emp_name = #{empName}
            </if>
            <if test="age != null and age != ''">
                and age = #{age}
            </if>
            <if test="sex != null and sex != ''">
                or sex = #{sex}
            </if>
            <if test="email != null and email != ''">
                and email = #{email}
            </if>
        </where>
    </select>

DynamicSQLMapperTest

/**
     * @description:测试where标签
     * @author: Hey 
     * @date: 2022/7/4 13:25
     * @param: []
     * @return: void
     **/
    @Test
    public void testGetEmpByCondition(){
        SqlSession sqlSession = SqlSessionUtils.getSqlSession();
        DynamicSQLMapper mapper = sqlSession.getMapper(DynamicSQLMapper.class);
        List<Emp> list = mapper.getEmpByCondition(new Emp(null, "", null, "", null));
        System.out.println(list);
    }

三、trim

DynamicSQLMapper.xml

 <select id="getEmpByCondition" resultType="Emp">
        select <include refid="empColumns"></include> 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} or
            </if>
            <if test="sex != null and sex != ''">
                sex = #{sex} and
            </if>
            <if test="email != null and email != ''">
                email = #{email}
            </if>
        </trim>
    </select>

四、choose、when、otherwise

DeptMapper

/**
         * @description:测试choose、when、otherwise
         * @author: Hey 
         * @date: 2022/7/4 16:50
         * @param: [emp]
         * @return: java.util.List<com.ir.mybatis.pojo.Emp>
         **/
        List<Emp> getEmpByChoose(Emp emp);

DynamicSQLMapper.xml:

  <!--List<Emp> getEmpByChoose(Emp emp);-->
    <select id="getEmpByChoose" resultType="Emp">
        select * from t_emp
        <where>
            <choose>
                <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>

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

    <!--List<Emp> getEmpByCondition(Emp emp);-->

五、foreach

1. 示例一

DynamicSQLMapper

   /**
         * @description:通过数组实现批量删除
         * @author: Hey
         * @date: 2022/7/4 17:03
         * @param: [eids]
         * @return: int
         **/
        int deleteMoreByArray(@Param("eids") Integer[] eids);

DynamicSQLMapper.xml

<!--int deleteMoreByArray(@Param("eids") Integer[] eids);-->
    <delete id="deleteMoreByArray">
        delete from t_emp where
        <foreach collection="eids" item="eid" separator="or">
            eid = #{eid}
        </foreach>
        <!--
            delete from t_emp where eid in
            <foreach collection="eids" item="eid" separator="," open="(" close=")">
                #{eid}
            </foreach>
        -->
    </delete>

DynamicSQLMapperTest

 /**
     * @description:通过数组实现批量删除
     * @author: Hey
     * @date: 2022/7/4 17:11
     * @param: []
     * @return: void
     **/
    @Test
    public void testDeleteMoreByArray(){
        SqlSession sqlSession = SqlSessionUtils.getSqlSession();
        DynamicSQLMapper mapper = sqlSession.getMapper(DynamicSQLMapper.class);
        int result = mapper.deleteMoreByArray(new Integer[]{6, 7, 8});
        System.out.println(result);
    }

2. 示例二

DynamicSQLMapper

/**
         * @description:通过list集合实现批量添加
         * @author: Hey
         * @date: 2022/7/4 17:23
         * @param: [emps]
         * @return: int
         **/
        int insertMoreByList(@Param("emps") List<Emp> emps);

DynamicSQLMapper.xml

 <!--int 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>

DynamicSQLMapperTest

 /**
     * @description:通过list集合实现批量添加
     * @author: Hey
     * @date: 2022/7/4 17:24
     * @param: []
     * @return: void
     **/
    @Test
    public void testInsertMoreByList(){
        SqlSession sqlSession = SqlSessionUtils.getSqlSession();
        DynamicSQLMapper mapper = sqlSession.getMapper(DynamicSQLMapper.class);
        Emp emp1 = new Emp(null,"a1",23,"男","123@qq.com");
        Emp emp2 = new Emp(null,"a2",23,"男","123@qq.com");
        Emp emp3 = new Emp(null,"a3",23,"男","123@qq.com");
        List<Emp> emps = Arrays.asList(emp1, emp2, emp3);
        System.out.println(mapper.insertMoreByList(emps));
    }

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
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

六、SQL片段

<sql id="empColumns">
 eid,ename,age,sex,did
</sql>
select <include refid="empColumns"></include> from t_emp