單列去重：
mysql:
drop table test;
create table test(id int(4));
insert into test values(1),(2),(3),(4),(1),(2);
select count(distinct id) from test;
oracle:
drop table test;
create table test(id number(4));
insert all
into test values(1)
into test values(2)
into test values(3)
into test values(4)
into test values(1)
into test values(2)
select * from dual;
select count(distinct id) from test;
多列去重：
select deptno, job from emp; -- 結果有重復的
select distinct deptno, job from emp; -- deptno 和 job 兩個字段的值都相同，那么去重