IFNULLexpression

IFNULLexpression refers to a common SQL expression pattern that returns the first argument if it is not NULL; otherwise it returns a specified fallback value. It is used to replace NULLs with a concrete value in query results, calculations, and data presentation.

Syntax and dialects vary. In MySQL and SQLite, the form is IFNULL(expr1, expr2). In SQL Server, a similar effect is achieved with ISNULL(expression, replacement). Oracle uses NVL(expr1, expr2) for the same purpose, while PostgreSQL offers COALESCE(expr, replacement) as a standard, portable alternative. Although these constructs share the same goal, their exact syntax and behavior differ slightly across systems.

Examples illustrate usage. SELECT IFNULL(email, '[email protected]') FROM users; returns a placeholder email when the stored value is NULL. SELECT COALESCE(price, 0) FROM products; substitutes 0 for missing prices. These expressions are commonly used in SELECT lists, WHERE clauses, and calculations to avoid NULL propagation in results or computations.

Semantics and behavior. The expression evaluates to the first non-NULL value among its arguments (in the case of IFNULL/ISNULL/NVL, the second argument serves as the replacement if the first is NULL). If all arguments are NULL, the result is NULL. Type handling generally follows the first argument’s type, but providers may impose rules for implicit casting.

Considerations. For portability, COALESCE is often preferred because it is part of the SQL standard and can handle more than two arguments. When performance is critical, be aware of short-circuiting behavior in your database and test with representative data.