动态SQL语句
有些时候,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)。
源代码http://limingnihao.javaeye.com/admin/blogs/782190
3.1 if标签
一个很普通的查询:
Xml代码
<!-- 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>
<!-- 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
<!-- 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>
<!-- 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
<!-- 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
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());
}
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
<!-- 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>
<!-- 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
<!-- 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>
<!-- 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
<!-- 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>
<!-- 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
<!-- 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>
<!-- 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
<!-- 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>
<!-- 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
<!-- 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>
<!-- 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
<!-- 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>
<!-- 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
<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>
<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:
Java代码
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:
Java代码
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:
Xml代码
<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>
<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:
Java代码
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:
Java代码
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());
}