Programming and Web Design

A subquery is a query inside another query or it is a nested query which is embedded within the WHERE clause. Subqueries can be used with the SELECT, INSERT, UPDATE, and DELETE statements along with the operators like =, <, >, >=, <=, IN, BETWEEN etc.

Primary Key: Primary Key is something which will uniquely identifies each record in a database table.

What is an array? An array is a data structure that stores one or more similar type of values in a single value. There are three kinds of array.

Sorting: While retrieving information from database, you can sort information or data as per your requirement. Like in some case, if you want to show the latest entered value in the table then you can show data in descending order of primary key. Similarly you can also show data displayed according to the name in ascending or decending order. Use " ORDER BY " Keyword.

As the name implies a constant is a value used in a PL/SQL Block that remains unchanged throughout the program. A constant is a user-defined literal value. You can declare a constant and use it instead of actual value. For example: If you want to write a program which will increase the salary of the employees by 25%, you can declare a constant and use it throughout the program. Next time when you want to increase the salary again you can change the value of the constant which will be easier than changing the actual value throughout the program. The General Syntax to declare a constant is:

The user input should always validated so that we can get correct and reliable data.  For example, if you want user to input their age but if user inputs any letters then the system should not accept is as an input.

We should validate a form for null values, otherwise user can click submit button without filling necessary information. In that case, our database will have many null values. So below is a simple validation using javascript for validating a form.