til/postgres/compute-median-instead-of-average.md

Compute Median Instead Of Average

One of the first aggregate functions we might use in PostgreSQL, besides sum, is avg.

select avg(book_count) as average_books_read
from (
  select users.id, count(books.id) as book_count
  from users
  left join books
    on books.user_id = users.id
  where books.read_in_year = 2025
  group by users.id
) as user_book_counts;

This computes the average of the set of values which sums them all up and divides by the count. The average (maybe you've heard this also called the mean) is not always the best way to understand data, especially when there are outliers.

Instead, we might want to compute the median value of our set of data. There is no easily identifiable median aggregate function. Instead, we can use percentile_cont with a value of 0.5. This gets us the 50th percentile of our set of data which is the definition of the median.

select percentile_cont(0.5) within group (
  order by book_count
) as median_books_read
from (
  select users.id, count(books.id) as book_count
  from users
  left join books on books.user_id = users.id and books.read_in_year = 2025
  group by users.id
) as user_book_counts;

The full syntax for percentile_cont is percentile_cong(precision) within group (order by ...) because this is an aggregiate that has to work with an ordered-set of data.

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