Understanding Basic SQL Queries in DBMS

Table of Contents

Data Manipulation Language ( DML ) is the language used to maintain the database in a DMBS. The SQL (Structured Query Language) is the most popular language to manipulate databases.

DML Commands

The DML commands perform following tasks on a database.

INSERT command

The INSERT command helps insert data into the relation. For example, you can insert values in the ‘MANAGER’ relation as follows.

Figure 1: Inserting Data into Manager Relation

The schema for Manager relation is given in the previous post about DDL commands.

SELECT command

The SELECT command helps query the database and retrieve the tuples from one or more relations.

There are two types of query – Selection(σ) and Projection(π).

Selection(σ)

This unary operator selects all the tuple in a relation that meet specific condition using normal conditional operators and logical operator such as AND, OR and NOT.

For example

, will return all the tuples that have an age greater than 30.

Figure 2: Select Statement On Manager Relation

Projection(π)

This unary operator selects columns, but you can add selection with those retrieved columns.

Rename operator(ρ)

The rename operator will rename the existing field of a relation to a different specified name. Alter is a DDL command that modifies the relation name.

Figure 4: RENAME operator on MANAGER relation

UPDATE command

The UPDATE command modifies the value for one or more tuple in the relation. In the following example, we have an updated salary of a manager whose salary is less than 20000.