12.10易科

一 if标签

<select id=" getStudentListLikeName " parameterType="StudentEntity" resultMap="studentResultMap">
 SELECT * from STUDENT_TBL ST
 <if test="studentName!=null and studentName!='' ">
  WHERE ST.STUDENT_NAME LIKE CONCAT(CONCAT('%', #{studentName}),'%')
 </if>
</select>

二 where标签

<select id="getStudentListWhere" parameterType="StudentEntity" resultMap="studentResultMap">
 SELECT * from STUDENT_TBL ST
 <where>
  <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>
 </where>
</select>

如果它包含的标签中有返回值的话就插入一个where。此外如果标签返回的内容是以AND或OR开头的,则它会剔除掉。

三 set 标签

使用set+if标签修改后,如果某项为null则不进行更新,而是保持数据库原值

<update id="updateStudent" parameterType="StudentEntity">
 UPDATE STUDENT_TBL
 <set>
  <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>
 </set>
 WHERE STUDENT_TBL.STUDENT_ID = #{studentID};
</update>

四 trim标签

trim是更灵活的去处多余关键字的标签,他可以实践where和set的效果,where例子的等效trim语句

<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>

set例子的等效trim语句

<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>

五 choose (when, otherwise)

有时候并不想应用所有的条件,而只是想从多个选项中选择一个。MyBatis提供了choose 元素,按顺序判断when中的条件出否成立,如果有一个成立,则choose结束。当choose中所有when的条件都不满则时,则执行otherwise中的sql。类似于Java 的switch语句,choose为switch,when为case,otherwise则为default。

<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>

六 foreach

对于动态SQL 非常必须的,主是要迭代一个集合,通常是用于IN 条件。List实例将使用“list”做为键,数组实例以“array”做为键。

1 参数为list实例的写法

SqlMapper.xml

<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>

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