Mybatis详解动态SQL以及单表多表查询的应用-木庄网络博客

本文整理自网络，侵删。

单表查询操作

参数占位符#{}和${}

#{}：相当于JDBC里面替换占位符的操作方式（#{}->“”）.相当于预编译处理（预编译处理可以防止SQL注入问题）
${}：相当于直接替换(desc这种关键字)，但这种不能预防SQL注入

select * from userinfo where username='${name}'

${} VS #{}

${}是直接替换，#{}是预执行；
${} 会存在SQL 注入问题，#{}不存在SQL注入问题

SQL 注入

UserInfo userInfo = userMapper.login("admin","' or 1='1");

mysql> select * from userinfo where username = 'admin' and password ='' or 1='1';
+----+----------+----------+-------+---------------------+---------------------+-------+
| id | username | password | photo | createtime | updatetime | state |
+----+----------+----------+-------+---------------------+---------------------+-------+
| 1 | admin | admin | | 2021-12-06 17:10:48 | 2021-12-06 17:10:48 | 1 |
+----+----------+----------+-------+---------------------+---------------------+-------+
1 row in set (0.00 sec)

like模糊查询

用concat进行字符串拼接

   <select id="findListByName" resultMap="BaseMap">
        select * from userinfo where username like concat('%',#{name},'%')
    </select>

多表查询操作

一对一多表查询

一对一的多表查询：需要设置resultMap中有个association标签，property对应实体类的属性名，resultMap是关联属性的字典映射(必须要设置)，columnPrefix是设置前缀，当多表查询中有相同的字段的话，就会报错

<?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.ArticleInfoMapper">
    <resultMap id="BaseMap" type="com.example.demo.model.ArticleInfo">
        <!--主键-->
        <id property="id" column="id"></id>
        <!--普通属性-->
        <result property="updatetime" column="updatetime"></result>
        <result property="title" column="title"></result>
        <result property="content" column="content"></result>
        <result property="createtime" column="createtime"></result>
        <result property="rcount" column="rcount"></result>
        <!--自定义对象属性-->
        <association property="user"
                     resultMap="com.example.demo.mapper.UserMapper.BaseMap"
                     columnPrefix="u_">
        </association>
    </resultMap>
    <select id="getAll" resultType="com.example.demo.model.ArticleInfo">
        select a.*,u.id from articleinfo as a left join userinfo as u on a.uid = u.id;
    </select>
    <select id="getAll2" resultMap="BaseMap">
        select a.*,u.id as u_id ,u.username as u_username,u.password as u_password from articleinfo as a left join userinfo as u on a.uid = u.id;
    </select>
</mapper>

一对多多表查询

collection标签，用法同association

 <resultMap id="BaseMapper2" type="com.example.demo.model.UserInfo">
        <!--映射主键的）（表中主键和程序实体类中的主键）-->
        <id column="id" property="id"></id>
        <!--普通列的映射-->
        <result column="username" property="name"></result>
        <result column="password" property="password"></result>
        <result column="photo" property="photo"></result>
        <result column="createtime" property="createtime"></result>
        <result column="updatetime" property="updatetime"></result>
        <!--外部关联-->
        <collection property="artlist" resultMap="com.example.demo.mapper.ArticleInfoMapper.BaseMap"
                    columnPrefix="a_"></collection>
    </resultMap>
 <select id="getAll3" resultMap="BaseMapper2">
        select u.*,a.id a_id,a.title a_title from userinfo u left join articleinfo a on u.id=a.uid
 </select>

动态SQL使用

if标签

注册分为必填和选填，如果在添加用户的时候有不确定的字段传入，就需要使用动态标签if来判断

//p是传递过来的参数名，并不是表的字段名
 <insert id="add3">
        insert into userinfo(username,password,
        <if test="p!=null">
         photo,
        </if>
         state)
        values(#{username},#{password},
        <if test="p!=null">
            #{p},
        </if>
       #{state})
 </insert>

trim标签

trim标签的属性

prefix：表示整个语句块，以prefix的值作为前缀
suffix：表示整个语句块，以suffix的值作为后缀
prefixOverrides：去掉最前面的符合条件的字符
suffixOverrides：去掉最后面的符合条件的字符

 <insert id="add4">
        insert into userinfo
        <trim prefix="(" suffix=")" suffixOverrides=",">
            <if test="username!=null">
                username,
            </if>
            <if test="password!=null">
                password,
            </if>
            <if test="p!=null">
                photo,
            </if>
            <if test="state!=null">
                state,
            </if>
        </trim>
        values
        <trim prefix="(" suffix=")" suffixOverrides=",">
            <if test="username!=null">
                #{username},
            </if>
            <if test="password!=null">
                #{password},
            </if>
            <if test="p!=null">
                #{p},
            </if>
            <if test="state!=null">
                #{state},
            </if>
        </trim>
    </insert>