Mybatis标签语法大全

Mybatis标签语法大全 ，更多用法请参照：https://baomidou.gitee.io/mybatis-plus-doc/#/quick-start

1. <sql>

   • 在配置文件中定义可复用的 SQL 片段，便于在其他查询语句中引用：

     <sql id="userColumns">id, username, password</sql>
     ...
     <select id="selectAllUsers" resultType="com.example.model.User">
       SELECT <include refid="userColumns"/> FROM users
     </select>
     

2. <select/>, <insert/>, <update/>, <delete/>

   • 这些标签在映射文件中分别用来定义 SQL 查询、插入、更新和删除操作，以及它们的执行逻辑和结果映射：

     <select id="getUserById" parameterType="int" resultType="com.example.model.User">
       SELECT * FROM users WHERE id = #{id}
     </select>
     
     <insert id="insertUser" parameterType="com.example.model.User">
       INSERT INTO users(username, password)
       VALUES(#{username}, #{password})
     </insert>
     

3. <if>, <choose>, <when>, <otherwise>, <where>, <set>, <foreach>

   • 动态 SQL 标签使得 SQL 语句可以根据参数动态构建：

     <!-- 使用 if 条件判断 -->
     <select id="selectUsersWithConditions" parameterType="map">
       SELECT * FROM users as users
       <where>
         <if test="param.id != null">AND users.id = #{param.id}</if>
           <if test="param.userName !=null and param.userName != ''">
            AND users.user_name LIKE CONCAT('%',#{param.userName},'%')
        </if>
     

   and users.id in (#{id})

 ```xml
    <!-- 使用 foreach 循环遍历集合参数 -->
    <insert id="batchInsertUsers">
      INSERT INTO users(username, email)
      VALUES
      <foreach item="user" index="index" collection="list" open="(" separator="),(" close=")">
        #{user.username}, #{user.email}
      </foreach>
    </insert>
    ```
```xml

<!-- 使用 choose when 判断 -->
<if test="param.regType !=null and param.regType != ''">
   <choose>
   <when test="param.regType == 'weixin'">
       and users.reg_type = 'weixin'
   </when>
   <when test="param.regType == 'app'">
      and users.reg_type ='app'
   </when>
    <otherwise>
       and users.reg_type = 'other'
   </otherwise>
   </choose>
</if>

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