yjh_study_command

1.show current user in oralce

ansower:show user

2.search  name of table  in current user model.

answer:SELECT table_name FROM user_tables;  remember: dictionary user_tables;

3.how to find user by talbe name.

eg.I knows the table name is job_history in oracle.

answer:SELECT owner, table_name FROM dba_tables WHERE table_name IN 'JOB_HISTORY';  

NOTE:  IN  replaced by LIKE.

4.how to use to_date() function in work

eg.SELECT to_date('20171026 22:42:50','yyyy-MM-dd hh24:mi:ss') FROM dual;   

NOTE:it can is 12  after in  the hh or if can't remeber to_date()  format use order by keyword  with desc.

5.if oracle database have two instance,you need login special instance.following the command.

eg.instance DBSQL   account sys password yangjinhong

sqlplus sys/yangjinhong@DBSQL AS SYSDBA