常用sql语句-蒲公英云

常用sql语句

1.限制返回记录的行数（4-10行）

select rownum,c1 from t144 where rownum<10
minus
select rownum,c1 from t144 where rownum<5

2.批量插入

insert into tablename(select * from othertable)

或

insert into (column1,column2,,,,,,,)tablename(select column1,column2,,,,,,,from othertable)

3,分组函数(结果中出现的列要在group by 中出现)

select count(age), username, , ,… from table group by age,usename,,,,…

4.查看数据库中的存在的表

select table_name from dba_tables (include all users)

select * from tab(include view)

5.高效update，一次修改多个列

update smsmonitor sms
set (sms.smsmonitor_sendflag, sms.smsmonitor_maxnum) = (select s.smsmonitor_sendflag,
s.smsmonitor_maxnum
from smsmonitor s
where s.smsmonitor_id =
‘i74668’)
where sms.smsmonitor_id = ‘i74669’

6，高效删除表中重复记录

DELETE FROM EMP E

WHERE E.ROWID > (SELECT MIN(X.ROWID)

FROM EMP X

WHERE X.EMP_NO = E.EMP_NO);

7.如何定位重要(消耗资源多)的SQL
select sql_text
from v$sql
where disk_reads > 1000 or (executions > 0 and buffer_gets/executions > 30000);

8.如何跟踪某个session的SQL
exec dbms_system.set_sql_trace_in_session(sid,serial#,&sql_trace);

select sid,serial# from v$session where sid = (select sid from v$mystat where rownum = 1);
exec dbms_system.set_ev(&sid,&serial#,&event_10046,&level_12,’’);

8，mysql中字段操作

增加字段
alter table name add code char(200) default 0
删除字段
ALTER TABLE table_NAME DROP COLUMN column_NAME
修改字段类型
ALTER TABLE table_name ALTER COLUMN column_name new_data_type

添加主键字段

alter table tb add 列名 int primary key default 0