COALESCE() vs. ISNULL(): What are the Key Differences in SQL NULL Handling?

Differences Between COALESCE() and ISNULL()

In SQL, COALESCE() and ISNULL() are expressions used to handle NULL values. While they serve a similar purpose, there are several key differences between the two functions.

Behavior

ISNULL() is a function that evaluates its arguments only once. COALESCE(), on the other hand, is an expression that evaluates its arguments multiple times. This can affect performance if the arguments involve complex computations.

Data Type Determination

ISNULL() returns the data type of the first argument. COALESCE() follows the CASE expression rules and returns the data type of the argument with the highest precedence.

Nullability

The nullability of the result expression differs between the two functions. ISNULL() returns a NOT NULL result, while the nullability of COALESCE() with non-NULL arguments is considered NULL. This distinction is important when using these functions in computed columns, key constraints, or UDFs.

Validation

ISNULL() requires only two arguments, while COALESCE() can take a variable number of arguments. Additionally, NULL values in ISNULL() are converted to the specified data type, while COALESCE() requires explicit data type conversion.

Example Usage

When avoiding NULL values in SQL concatenations, it is generally recommended to use COALESCE() over ISNULL(). This is because COALESCE() allows you to specify multiple backup values, which can be useful when dealing with multiple columns or complex expressions.

For example, the following query uses COALESCE() to return the first non-NULL value among three columns:

SELECT COALESCE(column_a, column_b, column_c) AS result_column
FROM table_name;

Conclusion

COALESCE() and ISNULL() are both useful functions for handling NULL values in SQL. Understanding the differences between them can help you choose the appropriate function for your specific needs.

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