1.<iftest ="">—— 需要判断的时候，条件写在 test 中

<select id="selectDept" parameterType="int" resultType="com.X.crud.bean.Department"> 
    select * from tbl_dept where 1=1 
     <if test="deptId != null"> and dept_id = #{deptId,jdbcType=INTEGER} 
     </if> 
</select>

2.choose (when, otherwise) —— 需要选择一个条件的时候

<select id="getEmpList_choose" resultMap="empResultMap" parameterType="com.X.crud.bean.Employee"> 
    SELECT * FROM tbl_emp e 
    <where> 
       <choose> 
          <when test="empId !=null"> e.emp_id = #{emp_id, jdbcType=INTEGER} 
          </when> 
          <when test="empName != null and empName != ''"> AND e.emp_name LIKE CONCAT(CONCAT('%', #{emp_name, jdbcType=VARCHAR}),'%') 
          </when> 
          <when test="email != null "> AND e.email = #{email, jdbcType=VARCHAR} 
          </when> 
         <otherwise> 
         </otherwise> 
       </choose> 
     </where> 
</select>

3.trim (where, set)—需要去掉 where、and、逗号之类的符号

<update id="updateByPrimaryKeySelective" parameterType="com.X.crud.bean.Employee"> 
	update tbl_emp
    <set>
        <if test="empName != null"> emp_name = #{empName,jdbcType=VARCHAR}, </if>
        <if test="gender != null"> gender = #{gender,jdbcType=CHAR}, </if>
        <if test="email != null"> email = #{email,jdbcType=VARCHAR}, </if>
        <if test="dId != null"> d_id = #{dId,jdbcType=INTEGER}, </if>
    </set> where emp_id = #{empId,jdbcType=INTEGER} 
</update>

trim 用来指定或者去掉前缀或者后缀：

<insert id="insertSelective" parameterType="com.X.crud.bean.Employee"> insert into tbl_emp 
    <trim prefix="(" suffix=")" suffixOverrides=",">
        <if test="empId != null"> emp_id, </if>
        <if test="empName != null"> emp_name, </if>
        <if test="dId != null"> d_id, </if>
    </trim>
    <trim prefix="values (" suffix=")" suffixOverrides=",">
        <if test="empId != null"> #{empId,jdbcType=INTEGER}, </if>
        <if test="empName != null"> #{empName,jdbcType=VARCHAR}, </if>
        <if test="dId != null"> #{dId,jdbcType=INTEGER}, </if>
    </trim>
</insert>

4.foreach —— 需要遍历集合的时候：

<delete id="deleteByList" parameterType="java.util.List"> delete from tbl_emp where emp_id in 
    <foreach collection="list" item="item" open="(" separator="," close=")"> #{item.empId,jdbcType=VARCHAR} </foreach>
</delete>