What is a Query in DBMS?

In the field of Database Management Systems (DBMS), a query serves as a fundamental tool for retrieving, manipulating, and managing data stored within a database. Queries act as the bridge between users and databases, enabling them to communicate with the system to extract specific information or perform various operations on the data. Understanding the nuances of queries and their associated terminologies is crucial for anyone working with databases, whether as a developer, analyst, or database administrator.

What is a Query in DBMS?

A query in a DBMS is a request made by a user or application to retrieve or manipulate data stored in a database. This request is typically formulated using a structured query language (SQL) or a query interface provided by the DBMS. The primary purpose of a query is to specify precisely what data is needed and how it should be retrieved or modified.

Terminologies Related to Queries

Database

A structured collection of data organized for efficient retrieval and storage. It serves as the repository for data accessed and manipulated through queries.

SQL (Structured Query Language)

A standardized programming language used to interact with relational databases. SQL provides a set of commands for querying, updating, and managing databases.

Table

A fundamental component of a relational database, representing a collection of related data organized into rows and columns. Each table in a database typically corresponds to a specific entity or concept.

Field/Column

A single piece of data stored within a table, representing a specific attribute or characteristic of the entities described by the table.

Record/Row

A complete set of data representing an individual instance or entity stored within a table. Each row contains values for each field/column defined in the table schema.

Primary Key

A unique identifier for each record in a table, ensuring that each row can be uniquely identified and accessed. Primary keys are used to establish relationships between tables and enforce data integrity.

Query Language

The language used to communicate with a database management system. This language allows users to perform operations such as data retrieval, manipulation, and schema definition.

Major Commands in SQL with Examples

To illustrate the major SQL commands, let's use a SQLite database file named `company.db`, which contains a table named `employees`. We'll demonstrate various SQL commands with real changes to this database.

Example Database Structure

Table: employees

SELECT Statement

The SELECT statement is used to retrieve data from one or more tables in a database.

Syntax

SELECT column1, column2, ...
FROM table_name
WHERE condition;

Example

SELECT * FROM employees WHERE department = 'IT';

This query selects all columns from the "employees" table where the department is 'IT'.

Output

3|Michael Lee|40|IT

INSERT Statement

The INSERT statement is used to add new records into a table.

Syntax

INSERT INTO table_name (column1, column2, ...)
VALUES (value1, value2, ...);

Example

INSERT INTO employees (name, age, department) 
VALUES ('Sarah Johnson', 28, 'Marketing');

This query inserts a new employee record into the "employees" table with specified values.

Output

To see, if the new data has been successfully inserted, you can execute the SELECT command, like this

SELECT * FROM employees;

Now, you'll get the entire table and you can see that the new data has been added to the database

1|John Doe|30|HR
2|Jane Smith|35|Finance
3|Michael Lee|40|IT
4|Sarah Johnson|28|Marketing

UPDATE Statement

The UPDATE statement is used to modify existing records in a table.

Syntax

UPDATE table_name
SET column1 = value1, column2 = value2, ...
WHERE condition;

Example

UPDATE employees
SET department = 'Operations'
WHERE name = 'Michael Lee';

This query updates the department of the employee named 'Michael Lee' to 'Operations'.

Output

Let's run the SELECT command to see the updated database

SELECT * FROM employees;

You can see that, the database has been updated and now Michael's department is set to Operations

1|John Doe|30|HR
2|Jane Smith|35|Finance
3|Michael Lee|40|Operations
4|Sarah Johnson|28|Marketing

DELETE Statement

The DELETE statement is used to remove existing records from a table.

Syntax

DELETE FROM table_name
WHERE condition;

Example

DELETE FROM employees
WHERE age > 35;

This query deletes records from the "employees" table where the age is greater than 35.

Output

Execute the SELECT command to check the updated database:

SELECT * FROM employees;

You can see that Michael has been removed from the database as he is the only one with an age over 35.

1|John Doe|30|HR
2|Jane Smith|35|Finance
4|Sarah Johnson|28|Marketing