（九）Mybatis的入门教程—— 动态sql之trim、set、choose标签

trim（where(封装查询条件)，set(封装修改条件)）标签：自定义字符串的截取规则

接口

public interface EmployeeMapperDynamicSQL{
    public List<Employee> getEmpsByConditionTrim(Employee employee);
}

映射文件

trim标签体中是整个字符串拼串后的结果。

prefix=" "；该属性表示前缀，给拼串后的整个字符串加一个前缀。（可写可不写）

prefixOverrides=" "；该属性表示前缀覆盖，去掉整个字符串前面多余的字符。（可写可不写）

suffix=" "；该属性表示后缀，给拼串后的整个字符串加一个后缀。（可写可不写）

suffixOverrides=" "； 该属性表示后缀覆盖，去掉整个字符串后面多余的字符。（可写可不写）

关于where标签

<select id="getEmpsByConditionTrim" resultType="com.mybatis.bean.Employee">
    select * from tbl_employee 
    <trim prefix="where" suffixOverrides="and"> 
        <if test="id!=null">
            id=#{id} and
        </if>
 
        <if test="lastName!=null &amp;&amp; lastName!=''">
            last_name like #{lastName} and 
        </if>
 
        <if test="email!=null and email.trim()!=&quot;&quot;">
            email=#{email} and 
        </if>
 
        <if test="gender==0 or gender==1">
            gender=#{gender}
        </if>
    </trim>
</select>

测试

@Test
public void testDynamicSql(){
    SqlSessionFactory sqlSessionFactory = getSqlSessionFactory();
    SqlSession openSession = sqlSessionFactory.openSession();
        try{
            EmployeeMapperDynamicSQL mapper =                                                 
                                  openSession.getMapper(EmployeeMapperDynamicSQL.class);
            Employee employee = new Employee(null, "%e%", null, null);
            List<Employee> emps = mapper.getEmpsByConditionIf(employee);
            for(Employee emp : emps){
                System.out.println(emp);
                }
            List<Employee> emps2 = mapper.getEmpsByConditionIf(employee);
             for(Employee emp : emps2){
                System.out.println(emp);
                    }
            }finally{    
    
            }
}

就算对于后面多余的and，该标签也能处理好，结果就正常了

trim标签用的不太多，可以了解一下就行。

---

关于set标签

接口

public interface EmployeeMapperDynamicSQL{
    public void updateEmp(Employee employee);
}

映射文件

<update id="updateEmp">
    update tbl_employee
    set 
    <if test="lastName!=null">
        last_name=#{lastName},
    </if>
    <if test="email!=enull">
        email=#{email},
    </if>
    <if test="gender!=null">
        gender=#{gender}
    </if>
    where id=#{id}
</update>

测试

@Test
public void testDynamicSql(){
    SqlSessionFactory sqlSessionFactory = getSqlSessionFactory();
    SqlSession openSession = sqlSessionFactory.openSession();
        try{
            EmployeeMapperDynamicSQL mapper =                                                 
                                  openSession.getMapper(EmployeeMapperDynamicSQL.class);
            Employee employee = new Employee(1, "Admin", null, null);
            mapper.updateEmp(employee);
            openSession.commit();
         }finally{    
                openSession.close();
            }
}

结果出现报错，

但是修改映射文件为以下样式，则就不用担心这些多出来的逗号之类的问题了。

<update id="updateEmp">
    update tbl_employee
    <set> 
        <if test="lastName!=null">
            last_name=#{lastName},
        </if>
        <if test="email!=enull">
            email=#{email},
        </if>
        <if test="gender!=null">
            gender=#{gender}
        </if>
    </set>
    where id=#{id}
</update>

结果正常，数据库也更新成功

或者用trim标签也能完成相同操作（但更新操作，一般还是推荐用set标签）

<update id="updateEmp">
    update tbl_employee
    <trim prefix="set" suffixOverrides=","> 
        <if test="lastName!=null">
            last_name=#{lastName},
        </if>
        <if test="email!=enull">
            email=#{email},
        </if>
        <if test="gender!=null">
            gender=#{gender}
        </if>
    </trim>
    where id=#{id}
</update>

---

choose（when, otherwise）标签：分支选择，相当于带了break的switch-case。

场景：如果查询员工信息的时候带了id就用id查询，如果传入的是last_name，就用last_name查询；只会进入其中一个分支。

接口

public interface EmployeeMapperDynamicSQL{
    public List<Employee> getEmpsByConditionChoose(Employee employee);
}

映射文件

<select id="getEmpsByConditionChoose" resultType="com.mybatis.bean.Employee">
    select * from tbl_employee 
      <where>
           <choose>
                <when test="id!=null">
                    id=#{id}
                </when>

                <when test="lastName!=null">
                    last_name like {lastName}
                </when>

                <when test="email">
                    email=#{email}
                </when>

                <otherwise>
                    gender=0
                </otherwise>
           </choose>
    </where>
</select>

测试

@Test
public void testDynamicSql(){
    SqlSessionFactory sqlSessionFactory = getSqlSessionFactory();
    SqlSession openSession = sqlSessionFactory.openSession();
        try{
            EmployeeMapperDynamicSQL mapper =                                                 
                                  openSession.getMapper(EmployeeMapperDynamicSQL.class);
            Employee employee = new Employee(null, "%e%", null, null);
            List<Employee> list = mapper.getEmpsByConditionChoose(employee);
            for(Employee emp : list){
                System.out.println(emp);
             }

            }finally{    
    
            }
}

结果

如果传入lastName的同时，id也传入1，则查询不会进入last_name，仅仅是进入id分支。