lesson 10 Calculations & data quality · intermediate

COUNT, SUM & AVG

Count rows and calculate totals and averages.

the idea

Aggregate functions collapse rows into a summary. COUNT(*) counts rows; COUNT(column) excludes NULL. SUM adds numeric values; AVG finds their mean.

tables in this lesson

The challenges use this data. Each column shows its type underneath; → marks a link to another table.

example

The idea in one query. Press Run to see what it returns; running examples never affects your progress.

SELECT COUNT(*) AS customer_count
FROM customers;

Watch for: COUNT(city) excludes Elena because her city is NULL.

build it step by step

Each step changes one thing. Run them in order and watch the result change.

1. 1

   Look at the rows being counted. Each customer is one row. An aggregate without GROUP BY collapses the whole table into one summary.

   step-1.sql Run

   sql›

   SELECT id, name
   FROM customers;

2. 2

   Collapse to a single count. COUNT(*) counts rows. SUM and AVG work the same way: many input rows become one summary value.

   step-2.sql Run

   sql›

   SELECT COUNT(*) AS customer_count
   FROM customers;