COUNT

Syntax

COUNT(column_name)
COUNT(*)

What it does

COUNT(*) counts all rows, including those with NULL values. COUNT(column_name) counts only rows where the expression is not NULL.

Example

SELECT
  customer_id,
  COUNT(*) AS total_orders,
  COUNT(cancelled_at) AS cancelled_orders
FROM orders
GROUP BY customer_id;

Common gotchas

• COUNT(*) is different from COUNT(column) when the column can be NULL
• a filter in the WHERE clause changes which rows are counted
• a LEFT JOIN with COUNT(*) can be misleading unless you are counting the joined rows intentionally

Safety checklist

• decide whether NULLs should count or be excluded
• confirm the correct grain before grouping
• check whether the join semantics are producing the row set you expect