mybatis动态SQL语句

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>

Java
List<String> classList = new ArrayList<String>();
classList.add("20000002");
classList.add("20000003");
List<StudentEntity> studentList = studentMapper.getStudentListByClassIDs(classList);
for(StudentEntity entityTemp : studentList){
System.out.println(entityTemp.toString());
}

2 参数为Array实例的写法

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

Java
String[] ids = new String[2];
ids[0] = "20000002";
ids[1] = "20000003";
List<StudentEntity> studentList = studentMapper.getStudentListByClassIDs(ids);
for(StudentEntity entityTemp : studentList){
System.out.println(entityTemp.toString());
}