Can you explain what an aggregate function is in the context of databases and how it is used in query execution?

An aggregate function in databases is a function that takes a set of values from one or more rows and returns a single summary value. It is used for calculating summary data, such as sum, average, maximum, or count of records.

Examples of aggregate functions:

• COUNT() — number of rows
• SUM() — sum of values
• AVG() — average value
• MAX() — maximum value
• MIN() — minimum value

In queries, aggregate functions are often used together with GROUP BY to group data by certain fields.

Example in Node.js using an SQL query:

const { Client } = require('pg');
const client = new Client();

async function getAverageSalary() {
  await client.connect();
  const res = await client.query('SELECT AVG(salary) AS avg_salary FROM employees');
  console.log('Average salary:', res.rows[0].avg_salary);
  await client.end();
}

getAverageSalary();

Here, AVG(salary) calculates the average salary of all employees.