How to Include Zero Results in SQL COUNT Aggregates Using Outer Joins?

Handling Zero Counts in SQL COUNT Aggregates with Outer Joins

SQL's COUNT aggregate function typically counts non-null values, omitting entities with zero results. To include these zero-count entities, use an outer join. Outer joins, unlike inner joins, retain all rows from one or both tables, even when there's no match in the other table.

Let's illustrate with two tables: person and appointment. appointment links to person via person_id. To count appointments per person, including those with zero appointments, use a LEFT JOIN:

SELECT
  p.person_id,
  COUNT(a.person_id) AS number_of_appointments
FROM
  person p
LEFT JOIN
  appointment a
ON
  p.person_id = a.person_id
GROUP BY
  p.person_id;

This LEFT JOIN ensures all rows from the person table (aliased as p) are included. If a person has no appointments, a.person_id will be NULL, and COUNT(a.person_id) will return 0 for that person. This provides a complete count for all persons, accurately reflecting zero-appointment cases. The GROUP BY clause ensures the count is aggregated per person.

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