Which SQL function returns the first non-null value among arguments?

• A. COALESCE (a, b)
• B. CASE WHEN a IS NULL THEN b ELSE a END
• C. NULLIF(a, NULL)
• D. NVL(a, b)

Answer: (A)

COALESCE is used to pick the first non-null value among its arguments. It evaluates from left to right and returns the first value that isn’t null; if all inputs are null, it yields null. This makes it ideal for combining several potential sources of data and choosing the best available one, for example COALESCE(city, address, 'Unknown').

The two-argument CASE expression achieves the same result for just two inputs, but COALESCE is simpler and portable across database systems, and it scales to many arguments. The other option, NULLIF(a, NULL), doesn’t select a non-null value; it returns null only if the two expressions are equal (which for a compared to NULL is never true). The final option, NVL(a, b), behaves similarly to COALESCE for two arguments but is Oracle-specific and not part of the standard SQL.