Mybatis dynamic SQL
动态 SQL 是 MyBatis 的强大特性之一。如果你使用过 JDBC 或其它类似的框架，你应该能理解根据不同条件拼接 SQL 语句有多痛苦，例如拼接时要确保不能忘记添加必要的空格，还要注意去掉列表最后一个列名的逗号。利用动态 SQL，可以彻底摆脱这种痛苦
if & where & trim & set
if 、 where 主要用于简单的判断
<!-- 方式一 -->
<mapper namespace="com.shufang.mapper.EmployeeMapper">
<!-- public List<Employee> getEmpsByMultiFeature(Emmployee emp); -->
<select id="getEmpsByMultiFeature" resultType="Employee">
select empid,name,gender,age,deptid
from hr_emp
where 1 = 1
<if test="empId != null"> and empid = #{empId}</if>
<if test="name != null and name != ''"> and name = #{name} </if>
<if test="gender == 1 or gender == 2">and gender = #{gender} </if>
<if test="age != null and age < 100"> and age = #{age} </if>
</select>
</mapper>
<!--方式二-->
<!-- public List<Employee> getEmpsByMultiFeature(Emmployee emp); -->
<select id="getEmpsByMultiFeature" resultType="Employee">
select empid,name,gender,age,deptid
from hr_emp
<where>
<if test="empId != null"> empid = #{empId}</if>
<if test="name != null and name != ''"> and name = #{name} </if>
<if test="gender == 1 or gender == 2">and gender = #{gender} </if>
<if test="age != null and age < 100"> and age = #{age} </if>
</where>
</select>
<!--方式三-->
<!-- public List<Employee> getEmpsByMultiFeature(Emmployee emp); -->
<select id="getEmpsByMultiFeature" resultType="Employee">
select empid,name,gender,age,deptid
from hr_emp
<trim prefix="where" suffixOverrides="and|or">
<if test="empId != null"> empid = #{empId} and </if>
<if test="name != null and name != ''"> name = #{name} and</if>
<if test="gender == 1 or gender == 2"> gender = #{gender} or</if>
<if test="age != null and age < 100"> age = #{age} </if>
</trim>
</select>
public class TestSQL {
/**
* 1.<if test=""><if/>: 根据传入的参数条件，筛选不同的SQL语句进行拼接
* 2.<where> ... demo block such as <if></if> ...</where> 给SQL加上where 并且去掉where后面多余的 and
* 3.<trim prefix ="where" suffixOverrides = "and|or">... demo block such as <if></if> ...</trim> 给SQL添加前缀，并且去掉后缀多余的and 或者 or
* 4.<set></set>: 主要是解决update 在某些参数没有传入的时候，去掉多余的·,· 以及加上 set关键字
* 总结：-----> 实际工作中还是<trim></trim>用得最多
* @throws IOException
*/
@Test
public void testSQL() throws IOException {
SqlSessionFactory sqlSessionFactory = MybatisUtil.getSqlSession("mybatis-config.xml");
//TODO 这里就是测试的方法的调用
SqlSession sqlSession = sqlSessionFactory.openSession(true);
EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class);
Employee employee = new Employee();
// employee.setEmpId(1010);
employee.setName("SuperMan");
employee.setGender(2);
// employee.setAge(26);
List<Employee> emps = mapper.getEmpsByMultiFeature(employee);
for (Employee emp : emps) {
System.out.println(emp);
}
MybatisUtil.closeSession(sqlSession);
}
}
Choose & when & otherwise
when 和 otherwise久类似于java中的if else if else…的逻辑，可以根据传入的参数不同，查询不同的SQL,
只有一个条件的会执行
<!-- public List<Employee> getEmpsByOneFeature(Employee emp); -->
<select id="getEmpsByOneFeature" resultType="Employee">
select empid,name,gender,age,deptid
from hr_emp
<where>
<choose>
<when test="empId != null">
empid = #{empId}
</when>
<when test="name != null and name != ''">
name = #{name}
</when>
<when test="gender == 1 or gender == 2">
gender = #{gender}
</when>
<otherwise>
age = #{age}
</otherwise>
</choose>
</where>
</select>
foreach
相关参数解读
<foreach collection="ids" open="empid" close="" separator="," item="" index="">
</foreach>
collection: 代表传过来的参数集合或者数组
item： 集合中每个元素的别名
open：设置循环体的开始内容
close： 设置循环体的结束内容
index： 若遍历的是list，那么index代表下标，如果便利的是map，那么index代表的是key
separator：设置每一个循环之间元素的分隔符
如何实现批量删除，其实有2种方式
方式一
//通过一个集合传入id，实现批量删除
void deleteBatch(@Param("ids") String ids);
<delete id="deleteBatch">
delete from hr_emp where empid in (${ids})
</delete>
方式二
<!-- void deleteMoreByList(List<String> ids);-->
<delete id="deleteMoreByList">
delete from hr_emp where empid in
<foreach collection="ids" separator="," item="empid" open="(" close=")">
#{empid}
</foreach>
</delete>
方式三
<!-- void deleteByOr(@Param("ids") Array<String> ids);-->
<delete id="deleteByOr">
delete from hr_emp
<where>
<foreach collection="ids" item="empid" separator="or">
empid = #{empid}
</foreach>
</where>
</delete>
sql&include
<!--定义公用的SQL片段-->
<sql id="empColumns"> select empid,name,gender,age,deptid from hr_emp</sql>
<!--通过include标签引用SQL的id，进行SQL的拼接-->
<select id="getEmpsByMultiFeature" resultType="Employee">
<include refid="empColumns"/>
<trim prefix="where" suffixOverrides="and|or">
<if test="empId != null">empid = #{empId} and</if>
<if test="name != null and name != ''">name = #{name} and</if>
<if test="gender == 1 or gender == 2">gender = #{gender} or</if>
<if test="age != null and age <= 100">age = #{age}</if>
</trim>
</select>
mybatis批量操作
delete
delete from hr_emp where empid in ()
delete from hr_emp where empid = 1 or empid = 2....
select
select xxx from hr_emp where empid in ()
select xxx from hr_emp where empid = 1 or empid = 2 .......
update
-- 1、把每条数据修改成相同内容
update hr_emp set xx= ... where empid = 1 or empid = 2 .......
update hr_emp set xx= ... where empid in ()
-- 2、把每条数据修改成相同的数据
update hr_emp set xx= .... where empid = ?;
update hr_emp set xx= .... where empid = ?;
update hr_emp set xx= .... where empid = ?;
.....
insert
insert into hr_emp values (),(),(),().....
<!-- void insertBatch(List<Employee> emps);批量插入 -->
<insert id="insertBatch">
insert into hr_emp(name,gender,age) values
<foreach collection="list" item="emp" separator=",">
(#{emp.name},#{emp.gender},${emp.age})
</foreach>
</insert>
<!-- void updateBatch(@Param("emps) Employee[] emps); 批量修改
jdbc.username=root
jdbc.password=888888
#允许JDBC在预编译的过程中能执行多个SQL语句，只有update的时候才有可能会用到
jdbc.url=jdbc:mysql://localhost:3306/hello?allowMutilQueries=true
jdbc.driver=com.mysql.cj.jdbc.Driver
-->
<update id="updateBatch">
<foreach collection="emps" item="emp" >
update hr_emp set name = #{emp.name} ,gender = #{emp.gender} ,age = #{emp.age}
where empid = #{emp.empId};
</foreach>
</update>
