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create table myt(name varchar2,create_time date)
insert into myt values('john',to_date(sysdate,'DD-MON-YY'));
insert into myt values('tom',to_date(sysdate,'DD-MON-YY'));
insert into myt values('lili',to_date(sysdate,'DD-MON-YY'));
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SQL> select * from myt;
NAME CREATE_TIME
---------- -----------
john 2010-5-19
tom 2010-5-19
lili 2010-5-19
SQL> select 1 from myt;
1
----------
1
1
1
SQL> select 0 from myt;
0
----------
0
0
0
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Èçselect * from T1 where exists(select 1 from T2 where T1.a=T2.a) ;
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SQL> select 1/0 from dual;
select 1/0 from dual
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SQL> select * from myt where exists(select 1/0 from dual);
NAME CREATE_TIME
---------- -----------
john 2010-5-19
tom 2010-5-19
lili &nbs
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1.forcing log switches
sql> alter system switch logfile;
2.forcing checkpoints
sql> alter system checkpoint;
3.adding online redo log groups
sql> alter database add logfile [group 4]
sql> ('/disk3/log4a.rdo','/disk4/log4b.rdo') size 1m;
4.adding online re ......