sql 几句

update table_name set colname="";
delete column all the content;

alter table table_name drop column column_name #delete the field in the table# 
alter table table_name add column column_name #add a field in the table#


#modify field_name????????FAQ
alert table change oldcolnm newcolnm coltype;
change is MYSQL systax....
ORACLE is modify.
但安全的方法应是先添加一个列，把旧列的数据复制到新列，完成其他的检查，确认无误后丢弃旧列#

     select 
             m.user_id,m.group_id, m.tenant_id, m.description, g.group_name
       from 
            m_user_group m left join m_group g
         on 
             m.group_id=g.group_id and m.tenant_id=g.tenant_id
      where  
             m.user_id=#user_id# and g.tenant_id=#tenant_id#