Arrays

Postgres columns can hold lists — query them directly.

A column that holds a list

Postgres columns can be arrays. Each book carries a tags text[]. Work with them:

'x' = ANY(tags) — does the array contain x?
tags @> ARRAY['a','b'] — does it contain all of these?
array_length(tags, 1) — how many elements (1 = first dimension)
unnest(tags) — expand the array into one row per element

SELECT title FROM books WHERE tags @> ARRAY['science'];

(Note: array_length of an empty array {} is NULL, not 0 — a classic gotcha.)

Two different absences sit in this column: Rivers of Salt has an array with nothing in it, Saltwater Hymns has no array at all. array_length returns NULL for both — so COALESCE(array_length(tags, 1), 0) is what you actually want when counting.