Oracle查询

--查询DQL
select column
  from table_name
 where search_condition
 group by expression
having search_condition
 order by column asc / desc
          
1、消除重复信息（完全重复） distinct
select distinct column from table_name;
(1) 作用于单列
  select distinct column from table_name;
(2) 作用于多列
  select distince column1, column2 from table_name;
(3) 统计count函数是不能统计多个字段的，下面语句不能执行
  select count(distinct name, id) from table_name;
若要使用，请使用嵌套查询，如下：
  select count(*) from (select distinct name, id form table_name);
(4) distinct 必须放在开头