Learn SQL: SUM
Adding up with SUM
"And what did that bring in?" Lien pushes a shoebox full of till receipts aside. "The accountant wants a total." That can be done faster.
SUM adds up values
COUNT counts rows, SUM adds up the values in a column. This is the total revenue of the summer.
Just like COUNT, SUM returns one row, and just like COUNT(column), it skips NULL.
SELECT SUM(amount) AS revenue FROM rentals| revenue |
|---|
| 1701 |
Count or add up?
Mind the difference: COUNT(days) tells you how many rentals there are, SUM(days) how many days were rented out in total.
And a pitfall: if not a single row is left after WHERE, SUM doesn't return 0 but NULL. After all, there was nothing to add up.
SELECT COUNT(days) AS rentals, SUM(days) AS rental_days FROM rentals| rentals | rental_days |
|---|---|
| 30 | 67 |
Try it yourself
What was the revenue in August?
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