oracle刪除重復記錄

1.1 查找表中多余的重復記錄

--查詢出所有有重復的數據
select DETAIL_ID,COMMENT_BODY,count(1)
from BBSCOMMENT
group by DETAIL_ID,COMMENT_BODY
having count(1)>1; --1955條
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 顯示了所有的非冗余的數據
--這一條命令顯示了所有的非冗余的數據
select min(COMMENT_ID) as COMMENT_ID,DETAIL_ID,COMMENT_BODY
from BBSCOMMENT
group by DETAIL_ID,COMMENT_BODY; --21453條，之所以此值不等于表總記錄數-1955，是因為1955條記錄中，有的重復了不止一次。
1.3 如果記錄數量少（千級別），可以把上面的語句做成子查詢然后直接刪除

--如果表數據量不是很大（1千條以內），可以把上面的語句做成子查詢然后直接刪除
delete from BBSCOMMENT where COMMENT_ID not in(
select min(COMMENT_ID)
from BBSCOMMENT
group by DETAIL_ID,COMMENT_BODY
); --782秒，在我這里，2萬條記錄，重復記錄2千多（太慢了?。。?/p>

1.4 另一種刪除方法

--這條語句也能夠實現上述功能，但不好測試了，數據已經被我刪除了
--刪除條件一：有重復數據的記錄；條件二：保留最小rowid的記錄。
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就是唯一標志記錄物理位置的一個id。oracle數據庫的表中的每一行數據都有一個唯一的標識符，或者稱為rowid，在oracle內部通常就是使用它來訪問數據的。rowid需要 10個字節的存儲空間，并用18個字符來顯示。該值表明了該行在oracle數據庫中的物理具體位置?？梢栽谝粋€查詢中使用rowid來表明查詢結果中包含該值。

1.5 大數據量還是用PL/SQL方便快捷

declare
--定義存儲結構
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;

--可供比較的變量
v_comment_id BBSCOMMENT.COMMENT_ID%type;
v_detail_id BBSCOMMENT.DETAIL_ID%type;
v_comment_body BBSCOMMENT.COMMENT_BODY%type;

--其它變量
v_batch_size integer := 5000;
v_counter integer := 0;

cursor cur_dupl is
--取出所有有重復的記錄
select COMMENT_ID, DETAIL_ID, COMMENT_BODY
from BBSCOMMENT
where(DETAIL_ID, COMMENT_BODY) in (
--這些記錄有重復
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
--首次進入、換記錄了，都重新賦值
v_detail_id := bbscomment_record.detail_id;
v_comment_body := bbscomment_record.comment_body;
else
--其它記錄刪除
delete from BBSCOMMENT where COMMENT_ID = bbscomment_record.comment_id;
v_counter := v_counter + 1;

        if mod(v_counter, v_batch_size) = 0 then
            --每多少條提交一次
            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)||'條記錄被刪除！');

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