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Mybatis框架的动态SQL技术是一种根据特定条件动态拼装SQL语句的功能，它存在的意义是为了解决拼接SQL语句字符串时的痛点问题。

动态SQL：

⭕ if：根据标签中test属性所对应的表达式决定标签中的内容是否需要拼接到SQL中

⭕ where：

● 当where标签中有内容时，会自动生成where关键字，并且将内容前多余的and或or去掉
● 当where标签中没有内容时，此时where标签没有任何效果
注意：where标签不能将其中内容后面多余的and或or去掉

⭕ trim：
● 若标签中有内容时：

1. prefix|suffix：将trim标签中内容前面或后面添加指定内容
2. suffixOverrides|prefixOverrides：将trim标签中内容前面或后面去掉指定内容

● 若标签中没有内容时，trim标签也没有任何效果

⭕ choose、when、otherwise
● 相当于if...else if...else
● when至少要有一个，otherwise最多只能有一个

⭕ foreach

● collection:设置需要循环的数组或集合
● item:表示数组或集合中的每一个数据
● separator:循环体之间的分割符
● open:foreach标签所循环的所有内容的开始符
● close:foreach标签所循环的所有内容的结束符

⭕ sql 标签
● 设置SQL片段：<sql id="empColumns">eid,emp_name,age,sex,email</sql>
● 引用SQL片段：<include refid="empColumns"></include>

一、if

if标签可通过test属性的表达式进行判断，若表达式的结果为true，则标签中的内容会执行；反之标签中的内容不会执行

DynamicSQLMapper

/*** @description:多条件查询* @author: Hey* @date: 2022/7/4 13:13* @param: [emp]* @return: java.util.List<com.ir.mybatis.pojo.Emp>**/List<Emp> getEmpByCondition(Emp emp);

DynamicSQLMapper.xml

<select id="getEmpByConditionOne" resultType="Emp"><!--"1=1"有两个作用：①不影响查询结果②更好的拼接后面的条件-->select * from t_emp where 1=1<if test="empName != null and empName != ''">emp_name = #{empName}</if><if test="age != null and age != ''">and age = #{age}</if><if test="sex != null and sex != ''">and sex = #{sex}</if><if test="email != null and email != ''">and email = #{email}</if></select>

DynamicSQLMapperTest

/*** @description:测试if标签* @author: Hey * @date: 2022/7/4 13:25* @param: []* @return: void**/@Testpublic void testGetEmpByCondition(){SqlSession sqlSession = SqlSessionUtils.getSqlSession();DynamicSQLMapper mapper = sqlSession.getMapper(DynamicSQLMapper.class);List<Emp> list = mapper.getEmpByCondition(new Emp(null, "", null, "", null));System.out.println(list);}

二、where

DynamicSQLMapper.xml

<select id="getEmpByConditionTwo" resultType="Emp">select * from t_emp<where><if test="empName != null and empName != ''">emp_name = #{empName}</if><if test="age != null and age != ''">and age = #{age}</if><if test="sex != null and sex != ''">or sex = #{sex}</if><if test="email != null and email != ''">and email = #{email}</if></where></select>

DynamicSQLMapperTest

/*** @description:测试where标签* @author: Hey * @date: 2022/7/4 13:25* @param: []* @return: void**/@Testpublic void testGetEmpByCondition(){SqlSession sqlSession = SqlSessionUtils.getSqlSession();DynamicSQLMapper mapper = sqlSession.getMapper(DynamicSQLMapper.class);List<Emp> list = mapper.getEmpByCondition(new Emp(null, "", null, "", null));System.out.println(list);}

三、trim

DynamicSQLMapper.xml

 <select id="getEmpByCondition" resultType="Emp">select <include refid="empColumns"></include> from t_emp<trim prefix="where" suffixOverrides="and|or"><if test="empName != null and empName != ''">emp_name = #{empName} and</if><if test="age != null and age != ''">age = #{age} or</if><if test="sex != null and sex != ''">sex = #{sex} and</if><if test="email != null and email != ''">email = #{email}</if></trim></select>

四、choose、when、otherwise

DeptMapper

/*** @description:测试choose、when、otherwise* @author: Hey * @date: 2022/7/4 16:50* @param: [emp]* @return: java.util.List<com.ir.mybatis.pojo.Emp>**/List<Emp> getEmpByChoose(Emp emp);

DynamicSQLMapper.xml:

  <!--List<Emp> getEmpByChoose(Emp emp);--><select id="getEmpByChoose" resultType="Emp">select * from t_emp<where><choose><when test="empName != null and empName != ''">emp_name = #{empName}</when><when test="age != null and age != ''">age = #{age}</when><when test="sex != null and sex != ''">sex = #{sex}</when><when test="email != null and email != ''">email = #{email}</when><otherwise>did = 1</otherwise></choose></where></select><sql id="empColumns">eid,emp_name,age,sex,email</sql><!--List<Emp> getEmpByCondition(Emp emp);-->

五、foreach

1. 示例一

DynamicSQLMapper

   /*** @description:通过数组实现批量删除* @author: Hey* @date: 2022/7/4 17:03* @param: [eids]* @return: int**/int deleteMoreByArray(@Param("eids") Integer[] eids);

DynamicSQLMapper.xml

<!--int deleteMoreByArray(@Param("eids") Integer[] eids);--><delete id="deleteMoreByArray">delete from t_emp where<foreach collection="eids" item="eid" separator="or">eid = #{eid}</foreach><!--delete from t_emp where eid in<foreach collection="eids" item="eid" separator="," open="(" close=")">#{eid}</foreach>--></delete>

DynamicSQLMapperTest

 /*** @description:通过数组实现批量删除* @author: Hey* @date: 2022/7/4 17:11* @param: []* @return: void**/@Testpublic void testDeleteMoreByArray(){SqlSession sqlSession = SqlSessionUtils.getSqlSession();DynamicSQLMapper mapper = sqlSession.getMapper(DynamicSQLMapper.class);int result = mapper.deleteMoreByArray(new Integer[]{6, 7, 8});System.out.println(result);}

2. 示例二

DynamicSQLMapper

/*** @description:通过list集合实现批量添加* @author: Hey* @date: 2022/7/4 17:23* @param: [emps]* @return: int**/int insertMoreByList(@Param("emps") List<Emp> emps);

DynamicSQLMapper.xml

 <!--int insertMoreByList(@Param("emps") List<Emp> emps);--><insert id="insertMoreByList">insert into t_emp values<foreach collection="emps" item="emp" separator=",">(null,#{emp.empName},#{emp.age},#{emp.sex},#{emp.email},null)</foreach></insert>

DynamicSQLMapperTest

 /*** @description:通过list集合实现批量添加* @author: Hey* @date: 2022/7/4 17:24* @param: []* @return: void**/@Testpublic void testInsertMoreByList(){SqlSession sqlSession = SqlSessionUtils.getSqlSession();DynamicSQLMapper mapper = sqlSession.getMapper(DynamicSQLMapper.class);Emp emp1 = new Emp(null,"a1",23,"男","123@qq.com");Emp emp2 = new Emp(null,"a2",23,"男","123@qq.com");Emp emp3 = new Emp(null,"a3",23,"男","123@qq.com");List<Emp> emps = Arrays.asList(emp1, emp2, emp3);System.out.println(mapper.insertMoreByList(emps));}

属性：
collection：设置要循环的数组或集合
item：表示集合或数组中的每一个数据
separator：设置循环体之间的分隔符
open：设置foreach标签中的内容的开始符
close：设置foreach标签中的内容的结束符

六、SQL片段

<sql id="empColumns">eid,ename,age,sex,did
</sql>
select <include refid="empColumns"></include> from t_emp