数据管理员面试7个顶级问题
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5. How do you return the top-N results of a query in Oracle? Why doesn't the obvious method work?
Most people think of using the ROWNUM pseudocolumn with ORDER BY. Unfortunately the ROWNUM is determined before the ORDER BY so you don't get the results you want. The answer is to use a subquery to do the ORDER BY first. For example to return the top five employees by salary: SELECT * FROM (SELECT * FROM employees ORDER BY salary) WHERE ROWNUM < 5;
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6. Can Oracle's Data Guard be used on Standard Edition, and if so how? How can you test that the standby database is in sync?
Oracle's Data Guard technology is a layer of software and automation built on top of the standby database facility. In Oracle Standard Edition it is possible to be a standby database, and update it manually. Roughly, put your production database in archivelog mode. Create a hotbackup of the database and move it to the standby machine. Then create a standby controlfile on the production machine, and ship that file, along with all the archived redolog files to the standby server. Once you have all these files assembled, place them in their proper locations, recover the standby database, and you're ready to roll. From this point on, you must manually ship, and manually apply those archived redologs to stay in sync with production.
To test your standby database, make a change to a table on the production server, and commit the change. Then manually switch a logfile so those changes are archived. Manually ship the newest archived redolog file, and manually apply it on the standby database. Then open your standby database in read-only mode, and select from your changed table to verify those changes are available. Once you're done, shutdown your standby and startup again in standby mode.
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7. What is a database link? What is the difference between a public and a private database link? What is a fixed user database link?
A database link allows you to make a connection with a remote database, Oracle or not, and query tables from it, even incorporating those accesses with joins to local tables.
A private database link only works for, and is accessible to the user/schema that owns it. Any user in the database can access a global one. A fixed user link specifies that you will connect to the remote db as one and only one user that is defined in the link. Alternatively, a current user database link will connect as the current user you are logged in as.
As you prepare for your DBA Interview, or prepare to give one, we hope these questions provide some new ideas and directions for your study. Keep in mind that there are a lot of directions an interview can go. As a DBA, emphasize what you know, even if it is not the direct answer to the question, and as an interviewee, allow the interview to go in creative directions.
In the end, what is important is potential or aptitude, not specific memorized answers. So listen for problem-solving ability, and thinking outside the box, and you will surely find or be the candidate for the job.