Trappola

COUNT(*) vs COUNT(colonna)

COUNT(column) salta i NULL; COUNT(*) conta le righe.

Per contare le righe, usa COUNT(*). Usa COUNT(column) solo quando intendi "le righe in cui questa colonna è valorizzata", e COUNT(DISTINCT column) per i valori distinti.

SELECT COUNT(*) AS all_rows,
       COUNT(city) AS with_city,
       COUNT(DISTINCT city) AS cities
FROM customers;

all_rows	with_city	cities
400	382	27

all_rows

with_city

cities

400

382

Provalo in Randori

Esercitati:COUNT(*) or COUNT(city)?