一、更新某列，自动递增
xhc_free_video： 表名
sort: 字段
set @rownum=0; update xhc_free_video SET sort = ( select @rownum := @rownum +1 as sort)
二、改变查询字段格式
cast(id as varchar)
三、三个表关联查询
SELECT t3.id, t3.NAME, t4.age FROM ( SELECT t1.id, t2.NAME FROM ( SELECT id FROM tb_user ) t1 LEFT JOIN tb_user_copy2 t2 ON t1.id = t2.id ) t3 LEFT JOIN tb_user_copy2_copy1 t4 ON t3.id = t4.id
