Table of contents
ctrl + alt + l format SQL statement
SQL that changes with user input or external conditions is called dynamic SQL
if
<if> is used to determine whether the condition is true. Use the test attribute to determine the condition. If true, splice SQL
where
The wehre element will only insert a where clause if the condition is met, and the leading AND or OR will be automatically removed.
If only the name is passed, the previous program will not be able to query successfully. The above problem can be solved through dynamic SQL.
EmpMapper.xml
<?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">
<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
@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);
}
}
The running results are as follows
set
<set> dynamically inserts the SET keyword at the beginning of the line and removes extra commas (used in update statements)
Change the username of employee with ID 18 to Tom111, name to Tom111, gender to 2, and other things remain unchanged.
Updating as before will make all other values null
Can be solved through dynamic SQL
EmpMapper.xml
<?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">
<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
@Mapper
public interface EmpMapper {
public void update(Emp emp);
}
This time the employee with ID 19 is updated, SpringbootMybatisCrudApplicationTests.java
@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);
}
}The running results are as follows. It is found that only four fields have been updated, and the remaining fields remain unchanged.
foreach
Implement batch deletion
collection collection name
The elements traversed from the item collection, items
separator The separator used for each traversal
open traverses the spliced fragments before starting
close The fragments spliced after the traversal is completed
EmpMapper.xml
<?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">
<delete id="deleteByIds">
delete from emp where id in
<foreach collection="ids" item="id" separator="," open="(" close=")">
#{id}
</foreach>
</delete>
</mapper>SpringbootMybatisCrudApplicationTests.java
@SpringBootTest
class SpringbootMybatisCrudApplicationTests {
@Autowired
private EmpMapper empMapper;
@Test
public void testDeleteByIds(){
List<Integer> ids = Arrays.asList(13,14,15);
empMapper.deleteByIds(ids);
}
}EmpMapper.java
@Mapper
public interface EmpMapper {
public void deleteByIds(List<Integer> ids);
}
The operation found that the deletion was successful.
sql&include
sql defines reusable fragments
include specifies the included sql fragment through the attribute refid
EmpMapper.xml
<?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">
<sql id="commonSelect">
select id, username, password, name, gender, image, job, entrydate, dept_id deptId, create_time createTime, update_time updateTime
from emp
</sql>
<select id="list" resultType="com.itheima.pojo.Emp">
<include refid="commonSelect"/>
<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>