mybatis动态sql设置
?? ? ? ?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）。
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());????
} ?
