Data deletion specification

[lezhang0912]

File: /zh/basic-principles.md
MySQL [demo]> select min(t.serialno) as start_key, max(t.serialno) as end_key, count() as page_size from ( select , row_number () over (order by serialno) as row_num from tmp_loan ) t group by floor((t.row_num - 1) / 50000) order by start_key;

数据删除规范，

( select , row_number () over (order by serialno) as row_num from tmp_loan ) t 中的逗号是什么含义？