How to Handle NULL Values in Redshift

Galaxy Glossary

How do I handle NULL values in Amazon Redshift?

Techniques and functions for testing, replacing, and preventing NULL values in Amazon Redshift queries.

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Description

Why do NULL values matter in Redshift?

NULL represents unknown data. Aggregations, joins, and comparisons ignore or mis-evaluate NULLs, so query results can surprise you unless you handle them explicitly.

Which functions replace NULLs?

When should I use COALESCE?

Use COALESCE(expr1, expr2, ...) to return the first non-NULL value. It supports unlimited arguments and stops scanning once a non-NULL appears, making it ideal for fallback logic across many columns.

How does NVL differ from COALESCE?

NVL(expr, replacement) is a two-argument shortcut.Both arguments must share a compatible type. Prefer NVL for simple defaults and COALESCE for multi-level fallbacks.

How to test for NULLs?

IS NULL vs IS NOT NULL explained

Use column IS NULL to filter missing data and column IS NOT NULL to keep known values. Never use the equality operator (=) for NULL comparisons; it always returns NULL, not TRUE.

How to prevent NULLs in calculations?

Using NULLIF to avoid divide-by-zero

NULLIF(a, b) returns NULL when a equals b, letting you skip invalid arithmetic.Combine it with COALESCE to substitute safe defaults.

Best practices for NULL handling

1) Replace NULLs as late as possible to preserve data fidelity. 2) Pick default values that won’t distort analytics (e.g., 0 for totals, empty string for text). 3) Document NULL logic in views so downstream users understand the intent.

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Why How to Handle NULL Values in Redshift is important

How to Handle NULL Values in Redshift Example Usage


-- Show each customer's most recent order total; 0 if none
SELECT c.id,
       c.name,
       COALESCE(MAX(o.total_amount), 0) AS last_order_total
FROM Customers c
LEFT JOIN Orders o ON o.customer_id = c.id
GROUP BY c.id, c.name;

How to Handle NULL Values in Redshift Syntax


-- Replace NULLs with defaults
COALESCE(value1 [, value2, ...])
NVL(value, default_value)

-- Return NULL when two expressions match
NULLIF(value1, value2)

-- Test for presence or absence of NULL
expression IS NULL
expression IS NOT NULL

Common Mistakes

Using = NULL in filters. In SQL, "= NULL" evaluates to NULL, not TRUE, so no rows return. Fix by writing column IS NULL.
Relying on COALESCE with mismatched data types, causing implicit casts or errors. Ensure all arguments share a compatible type or cast them explicitly.

Frequently Asked Questions (FAQs)

Can COALESCE slow down my query?

COALESCE stops at the first non-NULL value, so its cost is minimal. Index usage remains intact because Redshift evaluates functions row by row.

Is NVL just an Oracle alias?

Redshift supports NVL for Oracle familiarity. Internally, it rewrites to COALESCE with two arguments, so performance and semantics are identical.

How do I count NULLs?

COUNT(column) ignores NULLs. Use COUNT(*) - COUNT(column) to measure how many NULLs a column contains.

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