Counting & Summarizing (Aggregates)

One answer from many rows

SELECT COUNT(*) FROM employees;

SELECT
  COUNT(*)            AS people,
  SUM(salary)         AS payroll,
  ROUND(AVG(salary))  AS average,
  MIN(hired)          AS first_hire
FROM employees;

AS gives a result column a readable name. Get in the habit; reports and apps read columns by name.

Combine with WHERE

WHERE filters first, then the aggregate runs on what's left:

SELECT COUNT(*) AS lagos_customers
FROM customers WHERE city = 'Lagos';

COUNT(*) vs COUNT(column)

COUNT(*) counts rows. COUNT(column) counts rows where that column is not NULL. COUNT(DISTINCT column) counts unique values:

SELECT COUNT(DISTINCT city) AS cities FROM customers;