oracle删除重复记录

1.1 查找表中多余的重复记录

--查询出所有有重复的数据
select DETAIL_ID,COMMENT_BODY,count(1)
from BBSCOMMENT
group by DETAIL_ID,COMMENT_BODY
having count(1)>1; --1955条
或
select rownum,DETAIL_ID,COMMENT_BODY from
(select DETAIL_ID,COMMENT_BODY,(count(1) over (partition by DETAIL_ID,COMMENT_BODY)) rk
from BBSCOMMENT)
where rk > 1;

1.2 显示了所有的非冗余的数据
--这一条命令显示了所有的非冗余的数据
select min(COMMENT_ID) as COMMENT_ID,DETAIL_ID,COMMENT_BODY
from BBSCOMMENT
group by DETAIL_ID,COMMENT_BODY; --21453条，之所以此值不等于表总记录数-1955，是因为1955条记录中，有的重复了不止一次。
1.3 如果记录数量少（千级别），可以把上面的语句做成子查询然后直接删除

--如果表数据量不是很大（1千条以内），可以把上面的语句做成子查询然后直接删除
delete from BBSCOMMENT where COMMENT_ID not in(
select min(COMMENT_ID)
from BBSCOMMENT
group by DETAIL_ID,COMMENT_BODY
); --782秒，在我这里，2万条记录，重复记录2千多（太慢了！！）

1.4 另一种删除方法

--这条语句也能够实现上述功能，但不好测试了，数据已经被我删除了
--删除条件一：有重复数据的记录；条件二：保留最小rowid的记录。
delete from BBSCOMMENT a
where
(a.DETAIL_ID,a.COMMENT_BODY) in(select DETAIL_ID,COMMENT_BODY from BBSCOMMENT group by DETAIL_ID,COMMENT_BODY having count(1) > 1)
and rowid not in (select min(rowid) from BBSCOMMENT group by DETAIL_ID,COMMENT_BODY having count(1)>1);
或
delete from BBSCOMMENT a
where rowid not in
(select min(row_id) from BBSCOMMENT group by DETAIL_ID,COMMENT_BODY);

注：rowid就是唯一标志记录物理位置的一个id。oracle数据库的表中的每一行数据都有一个唯一的标识符，或者称为rowid，在oracle内部通常就是使用它来访问数据的。rowid需要 10个字节的存储空间，并用18个字符来显示。该值表明了该行在oracle数据库中的物理具体位置。可以在一个查询中使用rowid来表明查询结果中包含该值。

1.5 大数据量还是用PL/SQL方便快捷

declare
--定义存储结构
type bbscomment_type is record
(
comment_id BBSCOMMENT.COMMENT_ID%type,
detail_id BBSCOMMENT.DETAIL_ID%type,
comment_body BBSCOMMENT.COMMENT_BODY%type
);
bbscomment_record bbscomment_type;

--可供比较的变量
v_comment_id BBSCOMMENT.COMMENT_ID%type;
v_detail_id BBSCOMMENT.DETAIL_ID%type;
v_comment_body BBSCOMMENT.COMMENT_BODY%type;

--其它变量
v_batch_size integer := 5000;
v_counter integer := 0;

cursor cur_dupl is
--取出所有有重复的记录
select COMMENT_ID, DETAIL_ID, COMMENT_BODY
from BBSCOMMENT
where(DETAIL_ID, COMMENT_BODY) in (
--这些记录有重复
select DETAIL_ID, COMMENT_BODY
from BBSCOMMENT
group by DETAIL_ID, COMMENT_BODY
having count(1) > 1)
order by DETAIL_ID, COMMENT_BODY;
begin
for bbscomment_record in cur_dupl loop
if v_detail_id is null or (bbscomment_record.detail_id != v_detail_id or nvl(bbscomment_record.comment_body, ' ') != nvl(v_comment_body, ' ')) then
--首次进入、换记录了，都重新赋值
v_detail_id := bbscomment_record.detail_id;
v_comment_body := bbscomment_record.comment_body;
else
--其它记录删除
delete from BBSCOMMENT where COMMENT_ID = bbscomment_record.comment_id;
v_counter := v_counter + 1;

        if mod(v_counter, v_batch_size) = 0 then
            --每多少条提交一次
            commit;
        end if;
    end if;
end loop;

if v_counter > 0 then
    --最后一次提交
    commit;
end if;

dbms_output.put_line(to_char(v_counter)||'条记录被删除！');

exception
when others then
dbms_output.put_line('sqlerrm-->' ||sqlerrm);
rollback;
end;