Aggregate functions

Question 1

Count

Answer 1

SELECT COUNT(*) FROM customers;

SELECT COUNT(last_name) FROM customers;

βαζουμε και where clause
SELECT COUNT(*) FROM customers
WHERE last_name = 'Smith';

 SELECT COUNT(*) FROM customers
WHERE first_name  IS NULL;

Question 2

Sum

Answer 2

SELECT SUM(no_seats) FROM rooms; (δεν μπαινει αστερακι γιατι πρεπει να προσθεσει αριθμους)

SELECT SUM(no_seats) FROM rooms
WHERE id > 1;

Question 3

Max and Min

Answer 3

SELECT MAX(length_min) FROM films;
SELECT MIN(length_min) FROM films;

Question 4

AVG

Answer 4

SELECT AVG(length_min) FROM films;

SELECT AVG(length_min) FROM films
WHERE length_min > 120;