How to Use Joins in Redshift

Galaxy Glossary

How do I correctly use joins in Amazon Redshift?

Joins in Amazon Redshift combine rows from multiple tables based on related columns to return a unified result set.

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Description

Example H2

Example H3

How do joins work in Redshift?

Joins match rows from two or more tables through equality conditions between key columns. Redshift first redistributes or broadcasts data so matching rows are on the same node, then applies hash or merge algorithms to produce the final set.

Which join types are available?

Redshift supports INNER, LEFT (OUTER), RIGHT (OUTER), FULL (OUTER), CROSS, and self-joins. NATURAL and USING clauses simplify matching when columns share the same name.

What is the basic INNER JOIN syntax?

INNER JOIN returns only rows with matching keys: SELECT ... FROM A INNER JOIN B ON A.key = B.key;

How do I include non-matching rows with LEFT JOIN?

LEFT JOIN keeps all rows from the left table and NULL-fills unmatched right rows: SELECT ... FROM A LEFT JOIN B ON A.key = B.key;

When should I use FULL JOIN?

FULL JOIN returns every row from both tables, NULL-filling where no match exists—ideal for reconciling data.

How do I join three or more tables?

Chain joins by adding additional JOIN clauses: Customers c JOIN Orders o ON ... JOIN OrderItems oi ON .... Evaluate join order for clarity; Redshift’s optimizer can rearrange for speed.

How can I optimize join performance?

Align DISTKEYs on join columns, pick EVEN distribution when keys differ, and set SORTKEYs on frequently joined columns. Use EXPLAIN to inspect data redistribution.

Can I filter before joining?

Yes—add WHERE clauses or CTEs to reduce row counts before expensive joins, lowering network and memory use.

Best practices recap

Pick the minimal join type, standardize key data types, qualify columns, analyze SVL_QLOG for skew, and test with realistic dataset sizes.

Why How to Use Joins in Redshift is important

How to Use Joins in Redshift Example Usage


-- Revenue by customer including customers with no orders
SELECT c.id,
       c.name,
       COALESCE(SUM(o.total_amount), 0) AS lifetime_value
FROM Customers c
LEFT JOIN Orders o ON o.customer_id = c.id
GROUP BY c.id, c.name
ORDER BY lifetime_value DESC;

How to Use Joins in Redshift Syntax


-- INNER JOIN
SELECT c.id, c.name, o.id AS order_id, o.total_amount
FROM Customers c
INNER JOIN Orders o ON o.customer_id = c.id;

-- LEFT JOIN with USING
SELECT c.name, o.id AS order_id, o.total_amount
FROM Customers c
LEFT JOIN Orders o USING (id);

-- RIGHT JOIN
SELECT o.id, p.name, p.price
FROM OrderItems oi
RIGHT JOIN Products p ON p.id = oi.product_id;

-- FULL JOIN for audit
SELECT c.id, c.email, o.id AS order_id
FROM Customers c
FULL JOIN Orders o ON o.customer_id = c.id;

-- CROSS JOIN (cartesian)
SELECT p.name, d.date
FROM Products p CROSS JOIN calendar_dates d;

Common Mistakes

Ignoring table distribution styles. When join keys are not DISTKEYs, Redshift shuffles data across nodes, causing slow hash joins. Align DISTKEYs or apply EVEN distribution to cut network cost.
Leaving column names unqualified. If both tables contain an "id" column, queries error or return wrong data. Always prefix with table aliases, e.g., c.id, o.id.