mybatis 学习五 动态SQL语句

3.1 selectKey 标签

在insert语句中，在Oracle经常使用序列、在MySQL中使用函数来自动生成插入表的主键,而且需要方法能返回这个生成主键。使用myBatis的selectKey标签可以实现这个效果。

下面例子，使用mysql数据库自定义函数nextval('student')，用来生成一个key，并把他设置到传入的实体类中的studentId属性上。所以在执行完此方法后，边可以通过这个实体类获取生成的key。

　　

<!-- 插入学生 自动主键-->
<insert id="createStudentAutoKey" parameterType="liming.student.manager.data.model.StudentEntity" keyProperty="studentId">
    <selectKey keyProperty="studentId" resultType="String" order="BEFORE">
        select nextval('student')
    </selectKey>
    INSERT INTO STUDENT_TBL(STUDENT_ID,
                            STUDENT_NAME,
                            STUDENT_SEX,
                            STUDENT_BIRTHDAY,
                            STUDENT_PHOTO,
                            CLASS_ID,
                            PLACE_ID)
    VALUES (#{studentId},
            #{studentName},
            #{studentSex},
            #{studentBirthday},
            #{studentPhoto, javaType=byte[], jdbcType=BLOB, typeHandler=org.apache.ibatis.type.BlobTypeHandler},
            #{classId},
            #{placeId})
</insert>  

StudentEntity entity = new StudentEntity();
entity.setStudentName("黎明你好");
entity.setStudentSex(1);
entity.setStudentBirthday(DateUtil.parse("1985-05-28"));
entity.setClassId("20000001");
entity.setPlaceId("70000001");
this.dynamicSqlMapper.createStudentAutoKey(entity);
System.out.println("新增学生ID: " + entity.getStudentId());  

selectKey语句属性配置细节：

属性	描述	取值
keyProperty	selectKey 语句生成结果需要设置的属性。
resultType	生成结果类型，MyBatis 允许使用基本的数据类型，包括String 、int类型。
order	1：BEFORE，会先选择主键，然后设置keyProperty，再执行insert语句； 2：AFTER，就先运行insert 语句再运行selectKey 语句。	BEFORE AFTER
statementType	MyBatis 支持STATEMENT，PREPARED和CALLABLE 的语句形式， 对应Statement ，PreparedStatement 和CallableStatement 响应	STATEMENT PREPARED CALLABLE

3.2 if标签

if标签可用在许多类型的sql语句中，我们以查询为例。首先看一个很普通的查询：

<!-- 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或studentSex为null，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，增加灵活性。

参数为实体类StudentEntity。将实体类中所有的属性均进行判断，如果不为空则执行判断条件。

<!-- 2 if(判断参数) - 将实体类不为空的属性作为where条件 -->
<select id="getStudentList_if" resultMap="resultMap_studentEntity" parameterType="liming.student.manager.data.model.StudentEntity">
    SELECT ST.STUDENT_ID,
           ST.STUDENT_NAME,
           ST.STUDENT_SEX,
           ST.STUDENT_BIRTHDAY,
           ST.STUDENT_PHOTO,
           ST.CLASS_ID,
           ST.PLACE_ID
      FROM STUDENT_TBL ST
     WHERE
    <if test="studentName !=null ">
        ST.STUDENT_NAME LIKE CONCAT(CONCAT('%', #{studentName, jdbcType=VARCHAR}),'%')
    </if>
    <if test="studentSex != null and studentSex != '' ">
        AND ST.STUDENT_SEX = #{studentSex, jdbcType=INTEGER}
    </if>
    <if test="studentBirthday != null ">
        AND ST.STUDENT_BIRTHDAY = #{studentBirthday, jdbcType=DATE}
    </if>
    <if test="classId != null and classId!= '' ">
        AND ST.CLASS_ID = #{classId, jdbcType=VARCHAR}
    </if>
    <if test="classEntity != null and classEntity.classId !=null and classEntity.classId !=' ' ">
        AND ST.CLASS_ID = #{classEntity.classId, jdbcType=VARCHAR}
    </if>
    <if test="placeId != null and placeId != '' ">
        AND ST.PLACE_ID = #{placeId, jdbcType=VARCHAR}
    </if>
    <if test="placeEntity != null and placeEntity.placeId != null and placeEntity.placeId != '' ">
        AND ST.PLACE_ID = #{placeEntity.placeId, jdbcType=VARCHAR}
    </if>
    <if test="studentId != null and studentId != '' ">
        AND ST.STUDENT_ID = #{studentId, jdbcType=VARCHAR}
    </if>
</select>  

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。

public void select_test_2_1() {
    StudentEntity entity = new StudentEntity();
    entity.setStudentName("");
    entity.setStudentSex(1);
    entity.setStudentBirthday(DateUtil.parse("1985-05-28"));
    entity.setClassId("20000001");
    //entity.setPlaceId("70000001");
    List<StudentEntity> list = this.dynamicSqlMapper.getStudentList_if(entity);
    for (StudentEntity e : list) {
        System.out.println(e.toString());
    }
}  

3.3 if + where 的条件判断

当where中的条件使用的if标签较多时，这样的组合可能会导致错误。我们以在3.1中的查询语句为例子，当java代码按如下方法调用时：

@Test
public void select_test_2_1() {
    StudentEntity entity = new StudentEntity();
    entity.setStudentName(null);
    entity.setStudentSex(1);
    List<StudentEntity> list = this.dynamicSqlMapper.getStudentList_if(entity);
    for (StudentEntity e : list) {
        System.out.println(e.toString());
    }
}  

如果上面例子，参数studentName为null，将不会进行STUDENT_NAME列的判断，则会直接导“WHERE AND”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。

<!-- 3 select - where/if(判断参数) - 将实体类不为空的属性作为where条件 -->
<select id="getStudentList_whereIf" resultMap="resultMap_studentEntity" parameterType="liming.student.manager.data.model.StudentEntity">
    SELECT ST.STUDENT_ID,
           ST.STUDENT_NAME,
           ST.STUDENT_SEX,
           ST.STUDENT_BIRTHDAY,
           ST.STUDENT_PHOTO,
           ST.CLASS_ID,
           ST.PLACE_ID
      FROM STUDENT_TBL ST
    <where>
        <if test="studentName !=null ">
            ST.STUDENT_NAME LIKE CONCAT(CONCAT('%', #{studentName, jdbcType=VARCHAR}),'%')
        </if>
        <if test="studentSex != null and studentSex != '' ">
            AND ST.STUDENT_SEX = #{studentSex, jdbcType=INTEGER}
        </if>
        <if test="studentBirthday != null ">
            AND ST.STUDENT_BIRTHDAY = #{studentBirthday, jdbcType=DATE}
        </if>
        <if test="classId != null and classId!= '' ">
            AND ST.CLASS_ID = #{classId, jdbcType=VARCHAR}
        </if>
        <if test="classEntity != null and classEntity.classId !=null and classEntity.classId !=' ' ">
            AND ST.CLASS_ID = #{classEntity.classId, jdbcType=VARCHAR}
        </if>
        <if test="placeId != null and placeId != '' ">
            AND ST.PLACE_ID = #{placeId, jdbcType=VARCHAR}
        </if>
        <if test="placeEntity != null and placeEntity.placeId != null and placeEntity.placeId != '' ">
            AND ST.PLACE_ID = #{placeEntity.placeId, jdbcType=VARCHAR}
        </if>
        <if test="studentId != null and studentId != '' ">
            AND ST.STUDENT_ID = #{studentId, jdbcType=VARCHAR}
        </if>
    </where>
</select>  

3.4 if + set 的更新语句

当update语句中没有使用if标签时，如果有一个参数为null，都会导致错误。

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。

使用if+set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：

<!-- 4 if/set(判断参数) - 将实体类不为空的属性更新 -->
<update id="updateStudent_if_set" parameterType="liming.student.manager.data.model.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="studentPhoto != null ">
            STUDENT_TBL.STUDENT_PHOTO = #{studentPhoto, javaType=byte[], jdbcType=BLOB, typeHandler=org.apache.ibatis.type.BlobTypeHandler},
        </if>
        <if test="classId != '' ">
            STUDENT_TBL.CLASS_ID = #{classId}
        </if>
        <if test="placeId != '' ">
            STUDENT_TBL.PLACE_ID = #{placeId}
        </if>
    </set>
    WHERE STUDENT_TBL.STUDENT_ID = #{studentId};
</update>  

3.5 if + trim代替where/set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。

3.5.1 trim代替where

<!-- 5.1 if/trim代替where(判断参数) - 将实体类不为空的属性作为where条件 -->
<select id="getStudentList_if_trim" resultMap="resultMap_studentEntity">
    SELECT ST.STUDENT_ID,
           ST.STUDENT_NAME,
           ST.STUDENT_SEX,
           ST.STUDENT_BIRTHDAY,
           ST.STUDENT_PHOTO,
           ST.CLASS_ID,
           ST.PLACE_ID
      FROM STUDENT_TBL ST
    <trim prefix="WHERE" prefixOverrides="AND|OR">
        <if test="studentName !=null ">
            ST.STUDENT_NAME LIKE CONCAT(CONCAT('%', #{studentName, jdbcType=VARCHAR}),'%')
        </if>
        <if test="studentSex != null and studentSex != '' ">
            AND ST.STUDENT_SEX = #{studentSex, jdbcType=INTEGER}
        </if>
        <if test="studentBirthday != null ">
            AND ST.STUDENT_BIRTHDAY = #{studentBirthday, jdbcType=DATE}
        </if>
        <if test="classId != null and classId!= '' ">
            AND ST.CLASS_ID = #{classId, jdbcType=VARCHAR}
        </if>
        <if test="classEntity != null and classEntity.classId !=null and classEntity.classId !=' ' ">
            AND ST.CLASS_ID = #{classEntity.classId, jdbcType=VARCHAR}
        </if>
        <if test="placeId != null and placeId != '' ">
            AND ST.PLACE_ID = #{placeId, jdbcType=VARCHAR}
        </if>
        <if test="placeEntity != null and placeEntity.placeId != null and placeEntity.placeId != '' ">
            AND ST.PLACE_ID = #{placeEntity.placeId, jdbcType=VARCHAR}
        </if>
        <if test="studentId != null and studentId != '' ">
            AND ST.STUDENT_ID = #{studentId, jdbcType=VARCHAR}
        </if>
    </trim>
</select>  

3.5.2 trim代替set

<!-- 5.2 if/trim代替set(判断参数) - 将实体类不为空的属性更新 -->
<update id="updateStudent_if_trim" parameterType="liming.student.manager.data.model.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="studentPhoto != null ">
            STUDENT_TBL.STUDENT_PHOTO = #{studentPhoto, javaType=byte[], jdbcType=BLOB, typeHandler=org.apache.ibatis.type.BlobTypeHandler},
        </if>
        <if test="classId != '' ">
            STUDENT_TBL.CLASS_ID = #{classId},
        </if>
        <if test="placeId != '' ">
            STUDENT_TBL.PLACE_ID = #{placeId}
        </if>
    </trim>
    WHERE STUDENT_TBL.STUDENT_ID = #{studentId}
</update>  

3.6 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。而使用if标签时，只要test中的表达式为true，就会执行if标签中的条件

choose标签是按顺序判断其内部when标签中的test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。

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。choose会从上到下选择一个when标签的test为true的sql执行。安全考虑，我们使用where将choose包起来，放置关键字多于错误。

<!-- 6 choose(判断参数) - 按顺序将实体类第一个不为空的属性作为where条件 -->
<select id="getStudentList_choose" resultMap="resultMap_studentEntity" parameterType="liming.student.manager.data.model.StudentEntity">
    SELECT ST.STUDENT_ID,
           ST.STUDENT_NAME,
           ST.STUDENT_SEX,
           ST.STUDENT_BIRTHDAY,
           ST.STUDENT_PHOTO,
           ST.CLASS_ID,
           ST.PLACE_ID
      FROM STUDENT_TBL ST
    <where>
        <choose>
            <when test="studentName !=null ">
                ST.STUDENT_NAME LIKE CONCAT(CONCAT('%', #{studentName, jdbcType=VARCHAR}),'%')
            </when >
            <when test="studentSex != null and studentSex != '' ">
                AND ST.STUDENT_SEX = #{studentSex, jdbcType=INTEGER}
            </when >
            <when test="studentBirthday != null ">
                AND ST.STUDENT_BIRTHDAY = #{studentBirthday, jdbcType=DATE}
            </when >
            <when test="classId != null and classId!= '' ">
                AND ST.CLASS_ID = #{classId, jdbcType=VARCHAR}
            </when >
            <when test="classEntity != null and classEntity.classId !=null and classEntity.classId !=' ' ">
                AND ST.CLASS_ID = #{classEntity.classId, jdbcType=VARCHAR}
            </when >
            <when test="placeId != null and placeId != '' ">
                AND ST.PLACE_ID = #{placeId, jdbcType=VARCHAR}
            </when >
            <when test="placeEntity != null and placeEntity.placeId != null and placeEntity.placeId != '' ">
                AND ST.PLACE_ID = #{placeEntity.placeId, jdbcType=VARCHAR}
            </when >
            <when test="studentId != null and studentId != '' ">
                AND ST.STUDENT_ID = #{studentId, jdbcType=VARCHAR}
            </when >
            <otherwise>
            </otherwise>
        </choose>
    </where>
</select>  

3.7 foreach

接口的方法声明：

public List<StudentEntity> getStudentListByClassIds_foreach_array(String[] classIds);  

动态SQL语句：

<!— 7.1 foreach(循环array参数) - 作为where中in的条件 -->
<select id="getStudentListByClassIds_foreach_array" resultMap="resultMap_studentEntity">
    SELECT ST.STUDENT_ID,
           ST.STUDENT_NAME,
           ST.STUDENT_SEX,
           ST.STUDENT_BIRTHDAY,
           ST.STUDENT_PHOTO,
           ST.CLASS_ID,
           ST.PLACE_ID
      FROM STUDENT_TBL ST
      WHERE ST.CLASS_ID IN
     <foreach collection="array" item="classIds"  open="(" separator="," close=")">
        #{classIds}
     </foreach>
</select>  

@Test
public void test7_foreach() {
    String[] classIds = { "20000001", "20000002" };
    List<StudentEntity> list = this.dynamicSqlMapper.getStudentListByClassIds_foreach_array(classIds);
    for (StudentEntity e : list) {
        System.out.println(e.toString());
    }  

3.7.2参数为list示例的写法

接口的方法声明：

public List<StudentEntity> getStudentListByClassIds_foreach_list(List<String> classIdList);  

动态SQL语句：

<!-- 7.2 foreach(循环List<String>参数) - 作为where中in的条件 -->
<select id="getStudentListByClassIds_foreach_list" resultMap="resultMap_studentEntity">
    SELECT ST.STUDENT_ID,
           ST.STUDENT_NAME,
           ST.STUDENT_SEX,
           ST.STUDENT_BIRTHDAY,
           ST.STUDENT_PHOTO,
           ST.CLASS_ID,
           ST.PLACE_ID
      FROM STUDENT_TBL ST
      WHERE ST.CLASS_ID IN
     <foreach collection="list" item="classIdList"  open="(" separator="," close=")">
        #{classIdList}
     </foreach>
</select>  

@Test
public void test7_2_foreach() {
    ArrayList<String> classIdList = new ArrayList<String>();
    classIdList.add("20000001");
    classIdList.add("20000002");
    List<StudentEntity> list = this.dynamicSqlMapper.getStudentListByClassIds_foreach_list(classIdList);
    for (StudentEntity e : list) {
        System.out.println(e.toString());
    }
}