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Interview Questions
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How Databases Work - Oracle / Mysql / Mssql

Almost all of the most useful sites on the web use databases to organise their content, and they often use them to allow users to register and leave comments too. Any time you do something that a website seems to ‘remember’ the next time, the chances are that a database is involved. Every day, new webmasters become database administrators without even understanding the first thing about databases. When you use a database on the web today, you’re not just using any database: you’re using ones that rely on concepts built up over decades of database development and proven effective. Here are some of those concepts.


A common sight in databases is a table that contains the start and end values of a range - usually dates. One or both of the dates are typically part of the primary key; sometimes they are the entire key. Some examples:
History tables
History tables record the changes made to a row over time. Instead of updating a row and losing the prior values of each non-key attribute, a new row in inserted. Each row is differentiated by two columns that record the date the row was created (START_DATE) and the date it was superceded (END_DATE).

The Oracle FAQ

The Oracle FAQ puts a wealth of information at the fingertips of Oracle professionals all over the world. Use our directories and powerful search facilities to quickly locate all the information you need. We provide not only answers to frequently asked questions, but also Message Boards, Sample Code, oracle Books, Links, Job Listings, Tools, USENET Archives, Mailing List Archives, and many more features to enrich your Oracle journey.

Logging Into Oracle

If you are using SQL*Plus at work or at college then your Oracle DBA will have created you an Oracle userid or more correctly called, schema. There are two types of 'Users' the DBA could have created for you.
The first is called an OPS$ user. This 'users' name is the same as the Operating System userid that you may have i.e. your Unix login is s725710 then your OPS$ user would be OPS$s725710. This user is authenticated against the Operating system login. You don't have an extra password to remember.

The second user type the DBA could have created for you is a conventional user. In Oracle Corporation texts they like to use the user scott and password tiger as their example user. I will user the user fred and the password barney. If you are logging into an OPS$ user account then all you need to login to SQL*Plus is

Oracle DBA's

The most common SQL statement executed on an Oracle database is a query or SELECT statement. SELECT statements are used to pull or extract data (information) from the Oracle database. This data is stored in TABLES. The concept of an Oracle table is the same as an excel spreadsheet table, it also contains rows and columns.. When SQL*Plus is invoked (run) it first displays the version of SQL*Plus used, the date, the version of the Oracle database used, the version of PL/SQL used and any server options available on the database. In set theory NULL represents the value of an empty dataset. There are times when we may wish to set a default value instead of the NULL value, in this instance we use the function nvl().

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