Mybatis--动态sql

XML映射文件（简单的SQL用注解，复杂的用xml）

规范：
XML映射文件的名称和Mapper接口名称一样（同包同名）注意：不能直接用.创建文件夹,用/分层
xml映射文件的namespace属性为mapper接口全限定名一致
xml映射文件的sql语句和mapper接口的方法名一致，并保持返回类型一致

xml约束

<?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">

示例

<?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.example.demo.mapper.EmpMapper">
    <select id="select" resultType="com.example.demo.pojo.Emp">
--         resultType 单条记录封装的类型
        select * from emp where username like concat('%',#{name},'%') and gender = #{gender} order by create_time desc
    </select>
</mapper>

namespace

MybatisX 插件

快速定位 方便

ctrl+ALT+L SQL语句格式化（好用）

Mybatis 动态SQL

动态SQL ：if 标签

示例1

查找的时候没输入条件则全部查找

select * from emp where username like concat('%',#{name},'%') and gender = #{gender} order by create_time desc

where标签可以自动去除sql中的 and where

<?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.example.demo.mapper.EmpMapper">
    <select id="select" resultType="com.example.demo.pojo.Emp">
--         resultType 单条记录封装的类型
        select * from emp
        <where>
            <if test="name != null and name!= ''">
                username like concat('%',#{name},'%')
            </if>
            <if test="gender != null">
                and gender = #{gender}
            </if>
        </where>
        order by create_time desc
    </select>
</mapper>

示例2

更新时候如果没传值会变成null -->有值更新无值不更新
set标签 能去掉sql里面的,

update emp set username =#{username},image = #{image},update_time =#{updateTime} where id =#{id}

  <update id="update">
        update emp
        <set>
            <if test="username != null">
                username=#{username},
            </if>
            <if test="image != null">
                image = #{image},
            </if>
            <if test="updateTime != null">
                update_time =#{updateTime}
            </if>
        </set>
        where id = #{id}
    </update>

动态SQL ：foreach 标签

批量删除

delete from emp where id in(9,10,11);

 <delete id="deleteByIds">
        delete
        from emp
        where id in
        <!--
            collection 遍历的集合
            item 遍历出的元素
            separator 分隔符
            open 遍历开始片接前的SQL片段
            close 遍历开始片接后的SQL片段
        -->
        <foreach collection="ids" item="id" separator="," open="(" close=")">
            #{id}
        </foreach>
    </delete>

重复的sql片段

<sql id="xxx">
</sql>
<include refid="xxx">
</include>