How do you replace NULL with a default value for a column in a SELECT?

• A. IFNULL(phone, 'N/A')
• B. ISNULL(phone, 'N/A')
• C. COALESCE(phone, 'N/A')
• D. CASE WHEN phone IS NULL THEN 'N/A' ELSE phone END

Answer: (C)

Replacing NULL with a default value in a SELECT hinges on choosing a fallback value when a column is NULL. The best tool for this is COALESCE(phone, 'N/A'). COALESCE returns the first argument that is not NULL, so if phone has a real value it is shown; if it’s NULL, 'N/A' is shown. This works across many database systems because COALESCE is standard SQL, making your query portable and concise. The other forms can work in certain databases: IFNULL is common in MySQL for the same purpose; ISNULL exists in some systems with similar intent but varying behavior; and a CASE expression can express the same logic but is more verbose. So COALESCE is the clean, reliable choice for substituting a default when a value is NULL.