举出4种去重的方法：
第一种：distinct
SQL> create table t as select * from dba_objects;Table created.SQL> insert into t select * from t;87401 rows created.SQL> commit;Commit complete.SQL> create table tmp_t1 as select distinct * from t;Table created.SQL> drop table t;Table dropped. SQL> select count(*) from t; COUNT(*) ---------- 87401
第二种，用rowid
DELETE FROM t WHERE ROWID <> (SELECT MIN(ROWID) FROM t b WHERE b.object_id = t.object_id 5 AND b.object_name = t.object_name);174802 rows deleted.SQL> select count(*) from t; COUNT(*)---------- 87401
第三种,用rowid + group by 的方法:
SQL> insert into t select * from t where rownum<100;99 rows created.Elapsed: 00:00:00.21SQL> commit;Commit complete.Elapsed: 00:00:00.03DELETE FROM t WHERE ROWID NOT IN 2 3 (SELECT MIN(ROWID) FROM t GROUP BY object_id, object_name);99 rows deleted.Elapsed: 00:00:00.41
DELETE FROM t WHERE NOT EXISTS (SELECT 1 FROM (SELECT MIN(ROWID) rid FROM t GROUP BY object_id, object_name) b 5 WHERE b.rid = t.rowid);99 rows deleted.Elapsed: 00:00:01.32
---第四种， 用分析函数
DELETE FROM t WHERE ROWID IN (SELECT b.rd FROM (SELECT ROWID rd, row_number() over(PARTITION BY object_id, object_name ORDER BY object_id) rn FROM t) b 6 WHERE b.rn > 1);99 rows deleted.Elapsed: 00:00:00.50