COALESCE vs. ISNULL: Which SQL Function Should You Use for Handling NULLs?

COALESCE vs. ISNULL in SQL

In SQL, COALESCE and ISNULL are two functions used to handle missing or NULL values. They share similarities, but there are notable differences in their behavior and implementation.

COALESCE

COALESCE is a function that takes multiple arguments and returns the first non-NULL value. It's useful for concatenating strings or returning a default value when a column is missing. For example:

SELECT COALESCE(name, 'Unknown') FROM table

COALESCE evaluates all its arguments in order, so if the first argument is NULL, it moves on to the next until a non-NULL value is found.

ISNULL

ISNULL is a function that takes two arguments and returns the first argument if it's not NULL, and the second argument otherwise. It's often used to avoid string concatenations returning NULL when one of the strings is NULL. For example:

SELECT ISNULL(name, '') FROM table

ISNULL evaluates its first argument only once and does not proceed to the second argument if the first is non-NULL.

Key Differences

The key differences between COALESCE and ISNULL are:

• Number of arguments: COALESCE takes multiple arguments, while ISNULL takes only two.
• Evaluation order: COALESCE evaluates all arguments in order, while ISNULL stops at the first non-NULL argument.
• Data type: ISNULL uses the data type of the first argument, while COALESCE follows CASE expression rules and returns the data type with the highest precedence.
• Nullability: ISNULL's result is always considered NOT NULLable, while COALESCE's result can be NULL if all arguments are NULL.

Recommendation for Concatenations

When avoiding NULL values in SQL concatenations, it's recommended to use ISNULL. ISNULL ensures that the result will never be NULL, regardless of the nullability of the arguments. For example:

SELECT ISNULL(column1, '') || ISNULL(column2, '') FROM table

This concatenation will always return a non-NULL string, even if both column1 and column2 are NULL.

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