9. Dynamic SQL

9、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.

9.1、if

    //根据条件查询员工信息
    List<Emp>  getEmpByCondition(Emp emp);

    <!--
        动态SQL：
        1. if 通过testThe expression in the attribute determines whether the content in the tag will be splicedSQL中
    -->
    <!--List<Emp>  getEmpByCondition(Emp emp);-->
    <select id="getEmpByCondition" 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="gender != null and gender != ''">
            and gender = #{
    gender}
        </if>
    </select>

测试：

    @Test
    public void testGetEmpByCondition(){
    
        SqlSession sqlSession = sqlSessionUtil.getSqlSession();
        SQLMapper mapper = sqlSession.getMapper(SQLMapper.class);
        Emp emp = new Emp(null, "张三", 18, null);
// for (Emp emp1 : mapper.getEmpByCondition(emp)) {
    
// System.out.println(emp1);
// }

        List<Emp> empByCondition = mapper.getEmpByCondition(emp);
        empByCondition.forEach(System.out::println);
    }

9.2、where

---

上述存在bug：

        Emp emp = new Emp(null, "", 18, null);
       结果为： select * from t_emp where and age = ?

        Emp emp = new Emp(null, "", null, null);
        结果为：select * from t_emp where

解决办法一：

select * from t_emp where 1=1
Add the constant establishment condition

<select id="getEmpByCondition" resultType="emp">
        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="gender != null and gender != ''">
            and gender = #{
    gender}
        </if>
    </select>

解决办法二：

    <!--List<Emp>  getEmpByCondition(Emp emp);-->
    <select id="getEmpByCondition" 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="gender != null and gender != ''">
                and gender = #{
    gender}
            </if>
        </where>
    </select>

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去掉
注意：where标签不能去掉条件最后多余的and

9.3、trim

存在的bug：

<select id="getEmpByCondition" resultType="emp">
        select * from t_emp
        <where>
            <if test="empName != null and empName != ''">
                emp_name = #{
    empName} and
            </if>
            <if test="age != null and age != ''">
                 age = #{
    age} and
            </if>
            <if test="gender != null and gender != ''">
                 gender = #{
    gender}
            </if>
        </where>
    </select>

Emp emp = new Emp(null, "张三", 18, null);
结果为：select * from t_emp WHERE emp_name = ? and age = ? and

解决办法

    <select id="getEmpByCondition" resultType="emp">
        select * from t_emp
        <trim prefix="where" suffixOverrides="and">
            <if test="empName != null and empName != ''">
                emp_name = #{
    empName} and
            </if>
            <if test="age != null and age != ''">
                 age = #{
    age} and
            </if>
            <if test="gender != null and gender != ''">
                 gender = #{
    gender}
            </if>
        </trim>
    </select>

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
prefixOverrides：在trim标签中的内容的前面去掉某些内容
suffix：在trim标签中的内容的后面添加某些内容
suffixOverrides：在trim标签中的内容的后面去掉某些内容

---

9.4、choose、when、otherwise

choose、when、 otherwise相当于if…else if…else

    //根据choose来查询员工信息
    List<Emp>  getEmpByChoose(Emp emp);

 <!--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="gender != null and gender != ''">
            gender = #{
    gender}
        </when>
    </choose>
</where>
    </select>

测试

    @Test
    public void testGetEmpByChoose(){
    
        SqlSession sqlSession = sqlSessionUtil.getSqlSession();
        SQLMapper mapper = sqlSession.getMapper(SQLMapper.class);
        Emp emp = new Emp(null, "张三", 18, null);
        List<Emp> empByCondition = mapper.getEmpByChoose(emp);
        empByCondition.forEach(System.out::println);
    }

数据结果：

select * from t_emp WHERE emp_name = ?

---

9.5、foreach

批量添加

    //批量添加员工信息
    void insertMoreEmp(@Param("emps") List<Emp> emps);

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

测试：

    @Test
    public void testInsertMoreEmp(){
    
        SqlSession sqlSession = sqlSessionUtil.getSqlSession();
        SQLMapper mapper = sqlSession.getMapper(SQLMapper.class);
        Emp emp0 = new Emp(null, "小明0", 18, "男");
        Emp emp1 = new Emp(null, "小明1", 18, "男");
        Emp emp2 = new Emp(null, "小明2", 18, "男");
        List<Emp> emps = Arrays.asList(emp0, emp1, emp2);
        mapper.insertMoreEmp(emps);

    }

批量删除

    //批量删除员工
    void deleteMoreEmp(@Param("empIds") Integer[] empIds);

    <!--void deleteMoreEmp(@Param("empIds") Integer[] empIds);-->
    <delete id="deleteMoreEmp">
        delete from t_emp where emp_id in
        (
            <foreach collection="empIds" item="empId" separator=",">
                #{
    empId}
            </foreach>
            )
    </delete>

测试

    @Test
    public void testDeleteMoreEmp(){
    
        SqlSession sqlSession = sqlSessionUtil.getSqlSession();
        SQLMapper mapper = sqlSession.getMapper(SQLMapper.class);
        Integer[] integers = {
    11, 12, 13};
        mapper.deleteMoreEmp(integers);
    }

改进：

    <delete id="deleteMoreEmp">
        delete from t_emp where emp_id in
            <foreach collection="empIds" item="empId" separator="," open="(" close=")">
                #{
    empId}
            </foreach>
    </delete>

改进2：

    <delete id="deleteMoreEmp">
        delete from t_emp where emp_id in
            <foreach collection="empIds" item="empId" separator="or" >
                emp_id = #{
    empId}
            </foreach>
    </delete>

---

9.6、SQL片段

sql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

<sql id="empColumns">
        emp_id,emp_name,age,gender,dept_id
    </sql>

select <include refid="empColumns"></include> from t_emp

---

总结：

动态SQL：
1. if 通过testThe expression in the attribute determines whether the content in the tag will be splicedSQL中
2.where
若where标签中有条件成立,会自动生成where关键字
会自动将whereRedundant before the content in the tagand去掉,But after the contentand不能去掉
若whereNone of the conditions in the label are true,则where没有任何功能
3.trim
prefix、suffix：Add the specified content before and after the content in the label
prefixOverrides、suffixOverrides：Remove the specified content before and after the content in the tag
4.choose、when、otherwise
相当于java中的if····else if···else
when至少有一个,otherwisedo one more
5.foreach
collection:设置需要循环的数组或者集合
item：Use a string to represent each item in the array or collection
separator：设置每次循环的数据之间的分隔符
open：循环的所有内容以什么开始
close：循环的所有内容以什么结束
6.sql片段
可以记录一段SQL,在需要使用的<include>
<sql id=“empColumns”>
emp_id,emp_name,age,gender,dept_id
</sql>
select <include refid=“empColumns”></include> from t_emp