SQL (Structured Query Language) Interview Preparation Guide

Download PDF

SQL is an English like language consisting of commands to store, retrieve, maintain & regulate access to your database.

SQL*Plus is an application that recognizes & executes SQL commands & specialized SQL*Plus commands that can customize reports, provide help & edit facility & maintain system variables.

NVL : Null value function converts a null value to a non-null value for the purpose of evaluating an expression. Numeric Functions accept numeric I/P & return numeric values. They are MOD, SQRT, ROUND, TRUNC & POWER.

Character Functions are INITCAP, UPPER, LOWER, SUBSTR & LENGTH. Additional functions are GREATEST & LEAST. Group Functions returns results based upon groups of rows rather than one result per row, use group functions. They are AVG, COUNT, MAX, MIN & SUM.

SET command changes the system variables affecting the report environment.

TTITLE & BTITLE are commands to control report headings & footers.

BREAK command clarify reports by suppressing repeated values, skipping lines & allowing for controlled break points.

JOIN is the form of SELECT command that combines info from two or more tables.
Types of Joins are Simple (Equijoin & Non-Equijoin), Outer & Self join.
Equijoin returns rows from two or more tables joined together based upon a equality condition in the WHERE clause.
Non-Equijoin returns rows from two or more tables based upon a relationship other than the equality condition in the WHERE clause.
Outer Join combines two or more tables returning those rows from one table that have no direct match in the other table.
Self Join joins a table to itself as though it were two separate tables.

command control computations on subsets created by the BREAK command.

SQL*Loader is a product for moving data in external files into tables in an Oracle database. To load data from external files into an Oracle database, two types of input must be provided to SQL*Loader : the data itself and the control file. The control file describes the data to be loaded. It describes the Names and format of the data files, Specifications for loading data and the Data to be loaded (optional). Invoking the loader sqlload username/password controlfilename <options>.

Consistency : Assures users that the data they are changing or viewing is not changed until the are thro' with it.

Set Transaction is to establish properties for the current transaction.

Posting is an event that writes Inserts, Updates & Deletes in the forms to the database but not committing these transactions to the database.

Where clause, Group By clause, Having clause, Order By clause & Select.

Synonyms is the alias name for table, views, sequences & procedures and are created for reasons of Security and Convenience.
Two levels are Public - created by DBA & accessible to all the users. Private - Accessible to creator only. Advantages are referencing without specifying the owner and Flexibility to customize a more meaningful naming convention.

Multiple columns can be returned from a Nested Subquery.

Minus is the product of two tables listing only the non-matching rows.

Union is the product of two or more tables.

Transaction is defined as all changes made to the database between successive commits.

Locking are mechanisms intended to prevent destructive interaction between users accessing data. Locks are used to achieve.

Savepoint is a point within a particular transaction to which you may rollback without rolling back the entire transaction.

Commit is an event that attempts to make data in the database identical to the data in the form. It involves writing or posting data to the database and committing data to the database. Forms check the validity of the data in fields and records during a commit. Validity check are uniqueness, consistency and db restrictions.

Sequences are used for generating sequence numbers without any overhead of locking. Drawback is that after generating a sequence number if the transaction is rolled back, then that sequence number is lost.

Correlated Subquery is a subquery that is evaluated once for each row processed by the parent statement. Parent statement can be Select, Update or Delete. Use CRSQ to answer multipart questions whose answer depends on the value in each row processed by parent statement.

Intersect is the product of two tables listing only the matching rows.