目录
ctrl + alt + l格式化SQL语句
?随着用户的输入或外部条件的变化而变化的SQL称为动态SQL
<if>用来判断条件是否成立,使用test属性进行条件判断,如果true,则拼接SQL?
wehre元素只会在有条件成立的情况下才插入where子句,而且会自动去除开头的AND或OR
如果存在只传递姓名的情况,之前的程序会无法成功查询,可以通过动态SQL解决上述问题?
EmpMapper.xml内容如下
<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.itheima.mapper.EmpMapper">
<!--resultType单条记录所封装的内容-->
<select id="list" resultType="com.itheima.pojo.Emp">
select *
from emp
<where>
<if test="name != null">
name like concat('%', #{name}, '%')
</if>
<if test="gender != null">
and gender = #{gender}
</if>
<if test="begin != null and end != null">
and entrydate between #{begin} and #{end}
</if>
</where>
order by update_time desc
</select>
</mapper>
?SpringbootMybatisCrudApplicationTests.java内容如下
package com.itheima;
import com.itheima.mapper.EmpMapper;
import com.itheima.pojo.Emp;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import java.util.List;
@SpringBootTest
class SpringbootMybatisCrudApplicationTests {
@Autowired
private EmpMapper empMapper;
@Test
public void testSelect(){
List<Emp> list = empMapper.list(null,(short)1,null,null);
System.out.println(list);
}
}
?运行结果如下?
<set>动态地在行首插入SET关键字,并会删掉额外的逗号(用在update语句中)?
将id为18的员工的username改为Tom111,name改为Tom111,gender改为2,其他不变
按照之前的方法进行更新会使其他值均变为null
可以通过动态SQL解决
EmpMapper.xml内容如下
<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.itheima.mapper.EmpMapper">
<!--resultType单条记录所封装的内容-->
<update id="update">
update emp
<set>
<if test="username != null">username=#{username},</if>
<if test="password != null">password=#{password},</if>
<if test="name != null">name=#{name},</if>
<if test="gender != null">gender=#{gender},</if>
<if test="image != null">image=#{image},</if>
<if test="job != null">job=#{job},</if>
<if test="entrydate != null">entrydate=#{entrydate},</if>
<if test="deptId != null">dept_id=#{deptId},</if>
<if test="updateTime != null">update_time=#{updateTime}</if>
</set>
where id=#{id}
</update>
</mapper>
EmpMapper.java内容如下?
package com.itheima.mapper;
import com.itheima.pojo.Emp;
import org.apache.ibatis.annotations.*;
@Mapper
public interface EmpMapper {
public void update(Emp emp);
}
此次更新id为19的员工,SpringbootMybatisCrudApplicationTests.java内容如下
package com.itheima;
import com.itheima.mapper.EmpMapper;
import com.itheima.pojo.Emp;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import java.time.LocalDateTime;
@SpringBootTest
class SpringbootMybatisCrudApplicationTests {
@Autowired
private EmpMapper empMapper;
@Test
public void testUpdate(){
Emp emp = new Emp();
emp.setId(19);
emp.setUsername("Tom2222");
emp.setName("Tom222");
emp.setGender((short)1);
emp.setUpdateTime(LocalDateTime.now());
empMapper.update(emp);
}
}
?运行结果如下,发现只更新了四个字段,其余字段不变