Mybatis的<where><foreach><set>等标签详解
Mybatis的<where><foreach><set>等标签详解
秦学强 发表于5个月前
Mybatis的<where><foreach><set>等标签详解
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sql语句where条件中,需要一些安全判断,例如按性别检索,如果传入的参数是空的,此时查询出的结果很可能是空的,也许我们需要参数为空 时,是查出全部的信息。这是我们可以使用动态sql,增加一个判断,当参数不符合要求的时候,我们可以不去判断此查询条件。
下文均采用mysql语法和函数(例如字符串链接函数CONCAT

3.1 if标签
一个很普通的查询:

Xml代码 

  1. <!-- 查询学生list,like姓名 -->    
  2. <select id="getStudentListLikeName" parameterType="StudentEntity" resultMap="studentResultMap">    
  3.     SELECT * from STUDENT_TBL ST      
  4. WHERE ST.STUDENT_NAME LIKE CONCAT(CONCAT('%', #{studentName}),'%')     
  5. </select>    
<!-- 查询学生list,like姓名 -->   
<select id="getStudentListLikeName" parameterType="StudentEntity" resultMap="studentResultMap">   
    SELECT * from STUDENT_TBL ST     
WHERE ST.STUDENT_NAME LIKE CONCAT(CONCAT('%', #{studentName}),'%')    
</select>


但是此时如果studentName是null或空字符串,此语句很可能报错或查询结果为空。此时我们使用if动态sql语句先进行判断,如果值为null或等于空字符串,我们就不进行此条件的判断。

修改为:

Xml代码 

  1. <!-- 查询学生list,like姓名 -->    
  2. <select id=" getStudentListLikeName " parameterType="StudentEntity" resultMap="studentResultMap">    
  3.     SELECT * from STUDENT_TBL ST     
  4.     <if test="studentName!=null and studentName!='' ">    
  5.         WHERE ST.STUDENT_NAME LIKE CONCAT(CONCAT('%', #{studentName}),'%')     
  6.     </if>    
  7. </select>    
<!-- 查询学生list,like姓名 -->   
<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>



此时,当studentName的值为null或’’的时候,我们并不进行where条件的判断,所以当studentName值为null或’’值,不附带这个条件,所以查询结果是全部。

由于参数是Java的实体类,所以我们可以把所有条件都附加上,使用时比较灵活, new一个这样的实体类,我们需要限制那个条件,只需要附上相应的值就会where这个条件,相反不去赋值就可以不在where中判断。


   代码中的where标签,请参考3.2.1.

Xml代码 

  1. <!-- 查询学生list,like姓名,=性别、=生日、=班级,使用where,参数entity类型 -->    
  2. <select id="getStudentListWhereEntity" parameterType="StudentEntity" resultMap="studentResultMap">    
  3.     SELECT * from STUDENT_TBL ST     
  4.     <where>    
  5.         <if test="studentName!=null and studentName!='' ">    
  6.             ST.STUDENT_NAME LIKE CONCAT(CONCAT('%', #{studentName}),'%')     
  7.         </if>    
  8.         <if test="studentSex!= null and studentSex!= '' ">    
  9.             AND ST.STUDENT_SEX = #{studentSex}     
  10.         </if>    
  11.         <if test="studentBirthday!=null">    
  12.             AND ST.STUDENT_BIRTHDAY = #{studentBirthday}     
  13.         </if>    
  14.         <if test="classEntity!=null and classEntity.classID !=null and classEntity.classID!='' ">    
  15.             AND ST.CLASS_ID = #{classEntity.classID}     
  16.         </if>    
  17.     </where>    
  18. </select>    
<!-- 查询学生list,like姓名,=性别、=生日、=班级,使用where,参数entity类型 -->   
<select id="getStudentListWhereEntity" 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>   
        <if test="studentBirthday!=null">   
            AND ST.STUDENT_BIRTHDAY = #{studentBirthday}    
        </if>   
        <if test="classEntity!=null and classEntity.classID !=null and classEntity.classID!='' ">   
            AND ST.CLASS_ID = #{classEntity.classID}    
        </if>   
    </where>   
</select>



查询,姓名中有‘李’,男,生日在‘1985-05-28’,班级在‘20000002’的学生。

Java代码 

  1. StudentEntity entity = new StudentEntity();     
  2. entity.setStudentName("李");     
  3. entity.setStudentSex("男");     
  4. entity.setStudentBirthday(StringUtil.parse("1985-05-28"));     
  5. entity.setClassEntity(classMapper.getClassByID("20000002"));     
  6. List<StudentEntity> studentList = studentMapper.getStudentListWhereEntity(entity);     
  7. for( StudentEntity entityTemp : studentList){     
  8.     System.out.println(entityTemp.toString());     
  9. }    
StudentEntity entity = new StudentEntity();    
entity.setStudentName("李");    
entity.setStudentSex("男");    
entity.setStudentBirthday(StringUtil.parse("1985-05-28"));    
entity.setClassEntity(classMapper.getClassByID("20000002"));    
List<StudentEntity> studentList = studentMapper.getStudentListWhereEntity(entity);    
for( StudentEntity entityTemp : studentList){    
    System.out.println(entityTemp.toString());    
}



3.2 where、set、trim标签

3.2.1 where
当if标签较多时,这样的组合可能会导致错误。例如,like姓名,等于指定性别等:

Xml代码 

  1. <!-- 查询学生list,like姓名,=性别 -->    
  2. <select id="getStudentListWhere" parameterType="StudentEntity" resultMap="studentResultMap">    
  3.     SELECT * from STUDENT_TBL ST     
  4.         WHERE     
  5.         <if test="studentName!=null and studentName!='' ">    
  6.             ST.STUDENT_NAME LIKE CONCAT(CONCAT('%', #{studentName}),'%')     
  7.         </if>    
  8.         <if test="studentSex!= null and studentSex!= '' ">    
  9.             AND ST.STUDENT_SEX = #{studentSex}     
  10.         </if>    
  11. </select>    
<!-- 查询学生list,like姓名,=性别 -->   
<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>   
</select>



如果上面例子,参数studentName为null或’’,则或导致此sql组合成“WHERE AND”之类的关键字多余的错误SQL。
这时我们可以使用where动态语句来解决。这个“where”标签会知道如果它包含的标签中有返回值的话,它就插入一个‘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。
上面例子修改为:

Xml代码 

  1. <!-- 查询学生list,like姓名,=性别 -->    
  2. <select id="getStudentListWhere" parameterType="StudentEntity" resultMap="studentResultMap">    
  3.     SELECT * from STUDENT_TBL ST     
  4.     <where>    
  5.         <if test="studentName!=null and studentName!='' ">    
  6.             ST.STUDENT_NAME LIKE CONCAT(CONCAT('%', #{studentName}),'%')     
  7.         </if>    
  8.         <if test="studentSex!= null and studentSex!= '' ">    
  9.             AND ST.STUDENT_SEX = #{studentSex}     
  10.         </if>    
  11.     </where>    
  12. </select>    
<!-- 查询学生list,like姓名,=性别 -->   
<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>


3.2.2 set
当在update语句中使用if标签时,如果前面的if没有执行,则或导致逗号多余错误。使用set标签可以将动态的配置SET 关键字,和剔除追加到条件末尾的任何不相关的逗号。
没有使用if标签时,如果有一个参数为null,都会导致错误,如下示例:

Xml代码 

  1. <!-- 更新学生信息 -->    
  2. <update id="updateStudent" parameterType="StudentEntity">    
  3.     UPDATE STUDENT_TBL     
  4.        SET STUDENT_TBL.STUDENT_NAME = #{studentName},     
  5.            STUDENT_TBL.STUDENT_SEX = #{studentSex},     
  6.            STUDENT_TBL.STUDENT_BIRTHDAY = #{studentBirthday},     
  7.            STUDENT_TBL.CLASS_ID = #{classEntity.classID}     
  8.      WHERE STUDENT_TBL.STUDENT_ID = #{studentID};     
  9. </update>    
<!-- 更新学生信息 -->   
<update id="updateStudent" parameterType="StudentEntity">   
    UPDATE STUDENT_TBL    
       SET STUDENT_TBL.STUDENT_NAME = #{studentName},    
           STUDENT_TBL.STUDENT_SEX = #{studentSex},    
           STUDENT_TBL.STUDENT_BIRTHDAY = #{studentBirthday},    
           STUDENT_TBL.CLASS_ID = #{classEntity.classID}    
     WHERE STUDENT_TBL.STUDENT_ID = #{studentID};    
</update>


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。如下示例:

Xml代码 

  1. <!-- 更新学生信息 -->    
  2. <update id="updateStudent" parameterType="StudentEntity">    
  3.     UPDATE STUDENT_TBL     
  4.     <set>    
  5.         <if test="studentName!=null and studentName!='' ">    
  6.             STUDENT_TBL.STUDENT_NAME = #{studentName},     
  7.         </if>    
  8.         <if test="studentSex!=null and studentSex!='' ">    
  9.             STUDENT_TBL.STUDENT_SEX = #{studentSex},     
  10.         </if>    
  11.         <if test="studentBirthday!=null ">    
  12.             STUDENT_TBL.STUDENT_BIRTHDAY = #{studentBirthday},     
  13.         </if>    
  14.         <if test="classEntity!=null and classEntity.classID!=null and classEntity.classID!='' ">    
  15.             STUDENT_TBL.CLASS_ID = #{classEntity.classID}     
  16.         </if>    
  17.     </set>    
  18.     WHERE STUDENT_TBL.STUDENT_ID = #{studentID};     
  19. </update>    
<!-- 更新学生信息 -->   
<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>


3.2.3 trim
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:

Xml代码 

  1. <!-- 查询学生list,like姓名,=性别 -->    
  2. <select id="getStudentListWhere" parameterType="StudentEntity" resultMap="studentResultMap">    
  3.     SELECT * from STUDENT_TBL ST     
  4.     <trim prefix="WHERE" prefixOverrides="AND|OR">    
  5.         <if test="studentName!=null and studentName!='' ">    
  6.             ST.STUDENT_NAME LIKE CONCAT(CONCAT('%', #{studentName}),'%')     
  7.         </if>    
  8.         <if test="studentSex!= null and studentSex!= '' ">    
  9.             AND ST.STUDENT_SEX = #{studentSex}     
  10.         </if>    
  11.     </trim>    
  12. </select>    
<!-- 查询学生list,like姓名,=性别 -->   
<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:

Xml代码 

  1. <!-- 更新学生信息 -->    
  2. <update id="updateStudent" parameterType="StudentEntity">    
  3.     UPDATE STUDENT_TBL     
  4.     <trim prefix="SET" suffixOverrides=",">    
  5.         <if test="studentName!=null and studentName!='' ">    
  6.             STUDENT_TBL.STUDENT_NAME = #{studentName},     
  7.         </if>    
  8.         <if test="studentSex!=null and studentSex!='' ">    
  9.             STUDENT_TBL.STUDENT_SEX = #{studentSex},     
  10.         </if>    
  11.         <if test="studentBirthday!=null ">    
  12.             STUDENT_TBL.STUDENT_BIRTHDAY = #{studentBirthday},     
  13.         </if>    
  14.         <if test="classEntity!=null and classEntity.classID!=null and classEntity.classID!='' ">    
  15.             STUDENT_TBL.CLASS_ID = #{classEntity.classID}     
  16.         </if>    
  17.     </trim>    
  18.     WHERE STUDENT_TBL.STUDENT_ID = #{studentID};     
  19. </update>    
<!-- 更新学生信息 -->   
<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>


3.3 choose (when, otherwise)
         有时候我们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。
         if是与(and)的关系,而choose是或(or)的关系。


         例如下面例子,同样把所有可以限制的条件都写上,方面使用。选择条件顺序,when标签的从上到下的书写顺序:

Xml代码 

  1. <!-- 查询学生list,like姓名、或=性别、或=生日、或=班级,使用choose -->    
  2. <select id="getStudentListChooseEntity" parameterType="StudentEntity" resultMap="studentResultMap">    
  3.     SELECT * from STUDENT_TBL ST     
  4.     <where>    
  5.         <choose>    
  6.             <when test="studentName!=null and studentName!='' ">    
  7.                     ST.STUDENT_NAME LIKE CONCAT(CONCAT('%', #{studentName}),'%')     
  8.             </when>    
  9.             <when test="studentSex!= null and studentSex!= '' ">    
  10.                     AND ST.STUDENT_SEX = #{studentSex}     
  11.             </when>    
  12.             <when test="studentBirthday!=null">    
  13.                 AND ST.STUDENT_BIRTHDAY = #{studentBirthday}     
  14.             </when>    
  15.             <when test="classEntity!=null and classEntity.classID !=null and classEntity.classID!='' ">    
  16.                 AND ST.CLASS_ID = #{classEntity.classID}     
  17.             </when>    
  18.             <otherwise>    
  19.                      
  20.             </otherwise>    
  21.         </choose>    
  22.     </where>    
  23. </select>    
<!-- 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>


3.4 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。

3.4.1参数为list实例的写法:
SQL写法:

Xml代码 

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


接口的方法声明:

  1. public List<StudentEntity> getStudentListByClassIDs(List<String> classList);    
  2. public List<StudentEntity> getStudentListByClassIDs(List<String> classList); 测试代码,查询学生中,在2000000220000003这两个班级的学生: 
public List<StudentEntity> getStudentListByClassIDs(List<String> classList);   
public List<StudentEntity> getStudentListByClassIDs(List<String> classList); 测试代码,查询学生中,在20000002、20000003这两个班级的学生:

  1. List<String> classList = new ArrayList<String>();     
  2. classList.add("20000002");     
  3. classList.add("20000003");     
  4.     
  5. List<StudentEntity> studentList = studentMapper.getStudentListByClassIDs(classList);     
  6. for( StudentEntity entityTemp : studentList){     
  7.     System.out.println(entityTemp.toString());     
  8. }    
  9. List<String> classList = new ArrayList<String>(); 
  10. classList.add("20000002"); 
  11. classList.add("20000003"); 
  12. List<StudentEntity> studentList = studentMapper.getStudentListByClassIDs(classList); 
  13. for( StudentEntity entityTemp : studentList){ 
  14. System.out.println(entityTemp.toString()); 
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());    
}   
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());
}


3.4.2参数为Array实例的写法:
SQL语句:

  1. <select id="getStudentListByClassIDs" resultMap="studentResultMap">    
  2.     SELECT * FROM STUDENT_TBL ST     
  3.      WHERE ST.CLASS_ID IN      
  4.      <foreach collection="array" item="ids"  open="(" separator="," close=")">    
  5.         #{ids}     
  6.      </foreach>    
  7. </select>    
<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>


接口的方法声明:

  1. public List<StudentEntity> getStudentListByClassIDs(String[] ids);    
  2. public List<StudentEntity> getStudentListByClassIDs(String[] ids);测试代码,查询学生中,在2000000220000003这两个班级的学生: 
public List<StudentEntity> getStudentListByClassIDs(String[] ids);   
public List<StudentEntity> getStudentListByClassIDs(String[] ids);测试代码,查询学生中,在20000002、20000003这两个班级的学生:

  1. String[] ids = new String[2];     
  2. ids[0] = "20000002";     
  3. ids[1] = "20000003";     
  4. List<StudentEntity> studentList = studentMapper.getStudentListByClassIDs(ids);     
  5. for( StudentEntity entityTemp : studentList){     
  6.     System.out.println(entityTemp.toString());     
  7. }  
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());    
}


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