select @data_end_tag=max(tag) from
( select @end_tag_ctc1 as tag
union select @end_tag_ctc2 as tag
union select @end_tag_cnc1 as tag union select @end_tag_cnc2 as tag) t
truncate table: 清空表數據,不刪除整個表結構 DROP TABLE: 不但會刪除表中所有數據,還會將整個表結構從數據庫中移除。如果想要重新向表中存儲數據的話,必須重建該數據表。