COALESCE vs. ISNULL: When Should I Use Each SQL Function for NULL Handling?

Understanding the Differences Between COALESCE and ISNULL

COALESCE and ISNULL are commonly used SQL functions to handle NULL values. While they appear to have similar functionality, there are subtle differences that may impact their usage.

Key Distinctions:

1. Evaluation: ISNULL is a function that is evaluated once, while COALESCE is evaluated multiple times, once for each non-NULL parameter.
2. Data Type Determination: ISNULL returns the data type of its first parameter, whereas COALESCE follows CASE expression rules and returns the data type of the non-NULL parameter with the highest precedence.
3. NULLability: ISNULL always returns a NOT NULL value, even if its first parameter is NULL. COALESCE, on the other hand, returns a NULL value if all its parameters are NULL.

Practical Implications:

When concatenating fields to avoid NULL values, the choice between COALESCE and ISNULL depends on specific requirements:

• If data type consistency is needed, COALESCE is preferred as it preserves data types.
• If non-NULL values are desired, ISNULL may be suitable as it always returns a NOT NULL value.

Example:

Consider the following query:

SELECT COALESCE(first_name, '', last_name) AS full_name
FROM table

If there are any NULL values for both first_name and last_name, COALESCE will return an empty string ('') for full_name, resulting in a non-NULL value. On the other hand, the following query:

SELECT ISNULL(first_name, last_name) AS full_name
FROM table

will return NULL if both first_name and last_name are NULL, ensuring NULLability semantics.

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