--去重查詢方法一:根據(jù)id

select * from sxe where id in(select min(id) from sxe group by username) order by id asc;
--去重查詢方法二：根據(jù)rownum
select * from (select s.*,rownum rn from sxe s ) x where x.rn in (select min(rownum) from sxe group by username) order by id asc;
--去重查詢方法三：根據(jù)rowid
select * from (select s.*,rowid rid from sxe s) x where x.rid in (select min(rowid) from sxe group by username) order by id asc;
select s.*,rowid from sxe s where rowid in (select min(rowid) from sxe group by username) order by id asc;


--去重刪除方法一：根據(jù)ID
delete from sxe where id not in (select min(id) from sxe group by username);
--去重刪除方法二：根據(jù)rownum
--delete from (select s.*,rownum from sxe s) x where rownum not in (select min(rownum) from sxe group by username);
--去重刪除方法三：根據(jù)rowid

delete from sxe where rowid not in (select min(rowid) from sxe group by username);

?

Disctinct關(guān)鍵詞多列問題：

?

關(guān)于數(shù)據(jù)遷移，如何處理大數(shù)據(jù)量重復(fù)問題？針對Oracle

create table table1 selet * from table2; --按照table2的結(jié)構(gòu)創(chuàng)建table1，并將table2的數(shù)據(jù)導(dǎo)入table1;

create table table1 select * from table2 where 1 = 2; --按照table2的結(jié)構(gòu)創(chuàng)建table1，但不導(dǎo)入數(shù)據(jù);

開發(fā)過程中，如果涉及的數(shù)據(jù)量小的情況下刪除可以用簡單的sql執(zhí)行。但是數(shù)據(jù)量很大的遷移，百萬千萬級的數(shù)據(jù)量，性能是瓶頸的發(fā)生點(diǎn)；

因?yàn)椴樵冃枰獣r(shí)間，執(zhí)行刪除需要時(shí)間，刪除完畢執(zhí)行事務(wù)需要時(shí)間，因此性能基本上為零，弄不好數(shù)據(jù)庫假死，甚至電腦假死。

所我的大數(shù)據(jù)遷移經(jīng)驗(yàn)就是：

create table temp_table select * from table2 where id not in (select min(id) from table1 group by coln) order by coln asc;

drop table table2;

rename temp_table to table2;

?

關(guān)于數(shù)據(jù)庫表結(jié)構(gòu)編輯,針對Oracle：

增加列：

alter table table1 add column_name column_type;

修改列大小：

alter table table1 modify column_name new_column_type;

修改列名稱：

alter table table1 rename column cln1 to cln2;

刪除列：

alter table tabl1 drop column cln1;

轉(zhuǎn)載于:https://www.cnblogs.com/lcngu/p/4976287.html

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