ISNULL vs COALESCE

Handling NULL values is a common task in database programming. SQL Server supports two functions for this purpose: ISNULL and COALESCE. Those functions are similar but still not identical. Here are the differences between ISNULL and COALESCE in SQL Server:

• ISNULL is a SQL Server specific function. COALESCE is a part of SQL standard supported by Oracle, PostgreSQL and other popular database management systems.
• ISNULL(expression, replacement) function accepts two parameters - expression to validate for NULL and replacement value that is returned if expression is NULL. COALESCE(expression1, expression2, ..., expressionN) accepts multiple expressions and returns the first having non-NULL value.
• The replacement value for ISNULL must have the same data type as the expression being checked. In COALESCE the data types of the expressions in the list can be different, the resulting data type will be the data type with the highest precedence among the expressions.
• COALESCE evaluates expressions from the list in the specified order until it finds first non-NULL value. ISNULL evaluates the second argument even if the first argument is not NULL.

In summary, ISNULL is more suitable for handling a single expression, while COALESCE is useful when dealing with multiple expressions. COALESCE provides more flexibility in terms of data types, but ISNULL may offer better performance in certain scenarios.

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