Aggregation After Joins and NULL Handling

SELECT c.customer_id,
       COUNT(DISTINCT 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;

# Protect aggregates from one-to-many duplication.

WITH order_totals AS (
  SELECT customer_id, SUM(total_amount) AS total_spent
  FROM orders
  GROUP BY customer_id
)
SELECT c.customer_id, c.name, ot.total_spent
FROM customers c
LEFT JOIN order_totals ot ON ot.customer_id = c.customer_id;

# Summarize large fact tables first, then join the result.

SELECT c.customer_id,
       COALESCE(SUM(o.total_amount), 0) AS total_spent
FROM customers c
LEFT JOIN orders o ON o.customer_id = c.customer_id
GROUP BY c.customer_id;

# Replace `NULL` with a friendlier value in results.

LEFT JOIN → right-side columns may be NULL
RIGHT JOIN → left-side columns may be NULL
FULL OUTER JOIN → either side may be NULL

# Missing matches appear as NULL-valued columns.