What are the main functions most frequently used when working with databases in SQL?

When working with databases in SQL, the most commonly used functions are:

• Aggregate functions:

  • COUNT() — counts the number of rows.
  • SUM() — sums the values of a column.
  • AVG() — calculates the average value.
  • MIN() and MAX() — find the minimum and maximum values.

• String functions:

  • CONCAT() — concatenates strings.
  • UPPER() / LOWER() — converts case.
  • SUBSTRING() — extracts a substring.

• Date and time:

  • NOW() — current date and time.
  • DATE() — extracts the date from a datetime.

• Logical and conditional functions:

  • CASE WHEN — conditional logic.

Example of using an aggregate function:

SELECT department, COUNT(*) AS employee_count
FROM employees
GROUP BY department;

This query counts the number of employees in each department.