转载：db2,oracle,mysql常用命令比较

一、常用知识点
1、查看可以登陆的数据库：
oracle:查看tnsname.ora 或者 echo $ORACLE_SID
db2:
mysql:show databases --查看当前数据库：select database()

2、查看用户表，视图，表索引，表列，
oracle:
select table_name from user_tables;
select view_name from user_views;
select constraint_name,constraint_type from user_constraints where table_name='';
select column_name from all_tab_columns where table_name='';
db2:
list tables or select tabname from syscat.tables;
select view_name from syscat.views;
describe indexes for table table_name;
select tabname from syscat.columns where tabname='';
mysql:information_schema
select table_name from information_schema.tables where table_schema='USER'; or show tables;
select table_name from information_schema.views where table_schema='USER'; or show table status where comment='view';
select constraint_name,constraint_type from information_schedma.table_constraints where table_name='';
select index_name,table_name from information_schema.statistics where table_name='';
select column_name from information_schedma.columns where table_name='' and table_schema='USER';
3、查看表空间
oracle:select name from v$tablespace
db2: list tablespaces
mysql:
4、查看表结构
oracle:describe table_name
db2:describe table table_name
mysql:describe table_name
5、取前n行数据
oracle:select * from table_name where rownum<n
db2:select * from table_name fetch first n rows only
mysql:select * from table_name limit n
6、load数据
oracle: sqlldr username/passwd control=ctr.ctl data=data.txt
db2:load from data.txt of del insert into table_name
mysql:load data local infile 'e:Mysqlmysql.txt' into table test lines terminated by 'rn'
7、运用sql语句
oracle:@file.sql
db2:db2 -tvf file.sql
mysql:
8、查看用户
oracle:select user_name from all_users;
db2:
mysql:select schema_name from schemata;
9、insert date类型
oracle:insert into table_name (time_id) values (to_date('2009-09-01','YYYY-MM-DD'));
db2:insert into table_name (time_id) values ('2009-09-01');
mysql:insert into table_name (time_id) values ('2009-09-01');
10、char to int
oracle:select to_number('300')+200 from dual;
db2:select cast(char_id as integer)+200 from table_name;
mysql:
11、把表的数据导出到文本
oracle:spool data.txt ->select * from table_name
db2:db2" export to "/test.sql" of del select * from test"
mysql:
12、查看当前数据库
oracle:show parameter db_name
db2:list active databases
mysql:show databases;
13、查看数据库目录
oracle:echo $ORACLE_HOME
db2:list database derictory
mysql:
14、查看数据库系统参数信息
oracle:show parameter or v$parameter
db2:get dbm cfg
mysql:
15、修改表结构
oracle:alter table table_name add column_name number
db2:alter table table_name alter column column_name set data type varchar(12)
mysql:
16、导出建表语句
oracle: select dbms_metadata.get_ddl('TABLE',u.table_name) from user_tables u;
db2:db2look -d database_name -e -z schedma [-t table_name] -o outfile.sql
mysql:show create table table_nameG (--查看视图：show create view view_name)
17、如何恢复已提交的数据

18、模式，OWNER的区别
模式 ：一组对象的集合，如表、视图、和序列号等。
oracle:
SQL> SHOW USER
USER is "WGWH"
SQL> create table etl.wgwh ( id number);
Table created.
查看wgwh所属的OWNER：
SQL> select owner from all_tables where table_name = 'WGWH';
OWNER
------------------------------
ETL
备注：oracle模式由数据库的用户所拥有并且与用户具有相同的名字。其实，在实际使用中模式和用户是一回事。
oracle查看当前模式与切换模式，相当简单，只需要查看用户或者切换用户。
db2:
db2 => select tabschema,owner from syscat.tables where tabname='WGWH' with ur

TABSCHEMA OWNER
-------------------------------------------------------------------- -----------------------------------------------------------------------------------
ETL ETL
WGWH ETL
备注：db2的OWNER区别于oracle的OWNER，即oracle同一个OWNER下，不能存在相同的object，但db2可以，同时增加模式以区分。
查看当前模式：
select current schema from sysibm.sysdummy1
select current schema from sysibm.dual
或
select current sqlid from sysibm.sysdummy1
select current sqlid from sysibm.dual
或
values current schema
values current sqlid　
切换模式:
set current schema schedma_name
mysql:
19、显示数据库版本
oracle:select * from product_component_version
db2: db2level 显示db2的版本号
mysql:select version()
20、查看应用程序的端口号：
oracle--em,isqlplus:$ORACLE_HOME/../../install/protlist.ini
db2:
mysql:
21、应用程序服务：
oracle:启动EM--emctl start dbconsole
db2:
mysql:
22、重新配置应用程序
oracle：配置em-- emca -config dbcontrol db
db2:
mysql:
23、查看当前时间
oracle:select sysdate from dual
db2:
mysql:select now()

24、connect database

oracle:sqlplus user/passwd@dbname

db2:db2 connect to dbname user user_name using passwd

mysql:use database