mybatis的标签

<where>与<if>结合使用，那么and将不受影响

更新语句时，防止逗号的多余<if>结合<set>使用

<trim>更灵活的去除多余的关键字，可实现where和set的功能

<select id="getStudentListWhere" parameterType="StudentEntity" resultMap="studentResultMap">     
    SELECT * from STUDENT_TBL ST      
    <trim prefix="WHERE" prefixOverrides="AND|OR">     
        <if test="studentName!=null and studentName!='' ">     
            ST.STUDENT_NAME LIKE CONCAT(CONCAT('%', #{studentName}),'%')      
        </if>     
        <if test="studentSex!= null and studentSex!= '' ">     
            AND ST.STUDENT_SEX = #{studentSex}      
        </if>     
    </trim>     
</select>     

update id="updateStudent" parameterType="StudentEntity">     
    UPDATE STUDENT_TBL      
    <trim prefix="SET" suffixOverrides=",">     
        <if test="studentName!=null and studentName!='' ">     
            STUDENT_TBL.STUDENT_NAME = #{studentName},      
        </if>     
        <if test="studentSex!=null and studentSex!='' ">     
            STUDENT_TBL.STUDENT_SEX = #{studentSex},      
        </if>     
        <if test="studentBirthday!=null ">     
            STUDENT_TBL.STUDENT_BIRTHDAY = #{studentBirthday},      
        </if>     
        <if test="classEntity!=null and classEntity.classID!=null and classEntity.classID!='' ">     
            STUDENT_TBL.CLASS_ID = #{classEntity.classID}      
        </if>     
    </trim>     
    WHERE STUDENT_TBL.STUDENT_ID = #{studentID};      
</update>     

 choose (when, otherwise)

!-- 查询学生list，like姓名、或=性别、或=生日、或=班级，使用choose -->     
<select id="getStudentListChooseEntity" parameterType="StudentEntity" resultMap="studentResultMap">     
    SELECT * from STUDENT_TBL ST      
    <where>     
        <choose>     
            <when test="studentName!=null and studentName!='' ">     
                    ST.STUDENT_NAME LIKE CONCAT(CONCAT('%', #{studentName}),'%')      
            </when>     
            <when test="studentSex!= null and studentSex!= '' ">     
                    AND ST.STUDENT_SEX = #{studentSex}      
            </when>     
            <when test="studentBirthday!=null">     
                AND ST.STUDENT_BIRTHDAY = #{studentBirthday}      
            </when>     
            <when test="classEntity!=null and classEntity.classID !=null and classEntity.classID!='' ">     
                AND ST.CLASS_ID = #{classEntity.classID}      
            </when>     
            <otherwise>     
                      
            </otherwise>     
        </choose>     
    </where>     
</select>    

 if是与(and)的关系，而choose是或（or）的关系，从多个选项中选择一个。

foreach：

<select id="getStudentListByClassIDs" resultMap="studentResultMap">     
    SELECT * FROM STUDENT_TBL ST      
     WHERE ST.CLASS_ID IN       
     <foreach collection="list" item="classList"  open="(" separator="," close=")">     
        #{classList}      
     </foreach>         
</select>