SELECT (@i :=@i + 1)和(SELECT @i := 0)在sql里什么含义

今天接着别人的代码写，发现xml里这样一段sql，里面有不常用的函数：

1.  
   <select id="getStringAuditBpm" resultMap="CqProjectAuditBpmVo">
2.  
   SELECT
3.  
   c.process_node_code,
4.  
   c.process_node_name,
5.  
   ( @i := @i 1 ) AS serial_number
6.  
   FROM
7.  
   ext_act_flow_data a,
8.  
   ext_act_process b,
9.  
   ext_act_process_node c,
10.  
    ( SELECT @i := 0 ) AS itable
11.  
    WHERE
12.  
    a.form_data_id = #{id}
13.  
    AND a.process_key = b.process_key
14.  
    AND b.id = c.process_id
15.  
    ORDER BY c.id ASC;
16.  
    </select>

我查了一下其中 SELECT (@i :=@i 1) 和 (SELECT @i := 0) 的用法

SELECT (@i :=@i 1)  是为了生成自增的序列号字段

SELECT @i := 0 是为了将i进行初始化每次查询的序列号都会从1开始进行排序生成序列号

案例：
       1、

SELECT  (@i :=@i 1) AS i,         d.pkid,d.createTime,d.updateTime,d.date,d.employeeNum         ,d.serviceNum,d.serviceGodNum,d.serviceProjectNum         ,d.totalAmount,d.avgProject,d.avgAmount,d.empForNum

,d.storeId,d.isValid,s.name as storeName

FROM daily d,store s,(SELECT @i := 0) AS it        

WHERE d.isValid=1 and s.pkid=d.storeId    

2、

 SELECT
            c.process_node_code,
            c.process_node_name,
            ( @i := @i 1 ) AS serial_number
        FROM
            ext_act_flow_data a,
            ext_act_process b,
            ext_act_process_node c,
            ( SELECT @i := 0 ) AS itable
        WHERE
            a.form_data_id = #{id}
          AND a.process_key = b.process_key
          AND b.id = c.process_id
        ORDER BY c.id ASC;

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