前言
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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：
prefix|suffix：将trim标签中内容前面或后面添加指定内容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
**/
@Test
public 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
**/
@Test
public 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
**/
@Test
public 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
**/
@Test
public 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