Why are SQL queries needed?

SQL queries are used for interacting with relational databases. They allow:

• Manipulating data: Adding (INSERT), modifying (UPDATE), deleting (DELETE), and retrieving (SELECT) information from tables.
• Defining data structures: Creating (CREATE), altering (ALTER), and dropping (DROP) tables, indexes, views, and other database objects.
• Managing access: Setting permissions (GRANT, REVOKE) for users and roles.
• Controlling transactions: Ensuring data integrity with COMMIT, ROLLBACK.
• Analyzing data: Grouping, filtering, and sorting data to generate reports and analytics.

For QA, SQL queries are critical for verifying:

• Data correctness: Ensuring that information entered via UI/API is correctly stored in the database.
• Data integrity: Checking relationships between tables and absence of duplication or contradictions.
• Business logic: Verifying how complex queries (e.g., aggregate functions, JOINs) process data reflecting business rules.
• Performance: Assessing query execution times, especially for large data volumes.
• Report and export content: Comparing report data with source data in the database.

Example of a simple query:

SELECT
    column1, -- Selecting column 1
    column2  -- Selecting column 2
FROM
    table_name -- From which table to select
WHERE
    column1 = 'some_value'; -- Filter condition

Example for data verification:

SELECT
    user_id,
    email,
    registration_date
FROM
    users
WHERE
    email = 'testuser@example.com'; -- Checking if a user with this email is registered