Summarize large fact tables first, then join the result.
Section: Aggregation after joins
Pre-aggregate before joining
sql
sql
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;Explanation
This often improves both correctness and performance.
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Related commands
Same sheet · prioritizing Aggregation after joins
Use COUNT(DISTINCT ...) when joins multiply rows
Protect aggregates from one-to-many duplication.
Use COALESCE for outer-join aggregates
Replace `NULL` with a friendlier value in results.
Expect NULLs on the optional side of outer joins
Missing matches appear as NULL-valued columns.