Cross Join | PostgreSQL Tutorial

Cross Join in Join Tables helps you write SQL that is easier to test, review, and operate at scale.

Introduction to Cross Join

Use Cross Join to combine related tables while preserving correct cardinality.

Commonly paired with: SELECT, FROM, JOIN, INNER JOIN.

Practical examples with Cross Join in PostgreSQL

Reference pattern: start from canonical syntax and keep it explicit.

SELECT p.product_id, d.day
FROM products p
CROSS JOIN dim_day d;

Production-style scenario: apply the same concept to realistic application data.

SELECT
  o.order_id,
  o.total_amount,
  o.placed_at
FROM orders o
WHERE o.placed_at >= now() - interval '30 days'
ORDER BY o.placed_at DESC
LIMIT 50;

Additional example: use a variation to validate behavior and edge cases.

SELECT c.customer_id, c.company_name, COUNT(o.order_id) AS order_count
FROM customers c
LEFT JOIN orders o ON o.customer_id = c.customer_id
GROUP BY c.customer_id, c.company_name
ORDER BY order_count DESC;

Production tips

Prefer explicit column lists and deterministic ordering when results feed APIs or batch jobs.
Validate plans with EXPLAIN before adding indexes, then re-check after schema changes.
Keep DDL, data backfills, and cleanups in transactions when possible to avoid partial state.
Use isolated environments for risky changes so query tuning and schema experiments stay safe.

Vela workflow tip

Test this pattern in an isolated branch database, share the result with your team, and promote only after query plans and row counts look correct.

Reference: PostgreSQL official documentation.