SQL Query
An SQL Query is a command written in the SQL (Structured Query Language) used to communicate with a database: to retrieve, add, modify, or delete data. The query specifies exactly what to do with tables and records.
Core Idea
An SQL query is a structured statement that the database “reads” and executes.
It allows you to:
- Select specific rows from a table.
- Filter data based on conditions.
- Calculate sums, averages, counts.
- Combine data from multiple tables.
- Change or delete records.
Examples of Basic SQL Queries
- Data Selection (SELECT)
Get all users from the users table:
sql
SELECT * FROM users;
- Filtering (WHERE)
Find orders with an amount greater than 10,000 ₽:
sql
SELECT * FROM orders
WHERE amount > 10000;
- Inserting Data (INSERT)
Add a new user:
sql
INSERT INTO users (name, email)
VALUES (‘Anna’, ‘anna@example.com’);
- Updating Data (UPDATE)
Update a user’s email:
sql
UPDATE users
SET email = ‘new@example.com’
WHERE id = 1;
- Deleting Data (DELETE)
Delete an order:
sql
DELETE FROM orders
WHERE id = 10;
Structure of a Typical SELECT Query
Most often, an “SQL query” refers to data retrieval.
The classic structure is:
sql
SELECT fields
FROM table
WHERE condition
GROUP BY grouping
HAVING group_condition
ORDER BY sorting
LIMIT limit;
Not all clauses are required — they are used as needed.
Where SQL Queries are Used
- In analytics (reports, dashboards).
- In web applications and CRMs.
- In e-commerce (product search, filters, orders).
- In BI systems and Big Data processing.
In Short
An SQL Query is the primary tool for interacting with a database. The speed of a service, the accuracy of reports, and data quality depend on how well it is written.
