ISNULL vs COALESCE: Key Differences in SQL Server

As per my experience, to better understand...

ISNULL() handles two values; COALESCE() handles many. COALESCE is more flexible and follows standard SQL with type precedence.

Example:

-- ISNULL example
SELECT ISNULL(NULL, 'Default') -- Returns 'Default'
-- COALESCE example
SELECT COALESCE(NULL, NULL, 'FirstNonNull') -- Returns 'FirstNonNull'

SQL Server: Key Differences

Feature	`ISNULL()`	`COALESCE()`
Arguments	Accepts only 2 arguments	Accepts 2 or more arguments
Return Type	Returns the data type of the first argument	Returns the data type with highest precedence
Standard Compliance	Proprietary to SQL Server (T-SQL)	ANSI SQL standard compliant
Evaluation	Both arguments always evaluated	Stops evaluating once a non-NULL is found
Nullable Result	May return non-nullable	Returns nullable if any input is nullable
Performance	Slightly faster in some SQL Server versions	May be slightly slower due to type resolution

More Examples

1. Basic NULL Replacement
SELECT ISNULL(NULL, 'Default') -- Output: Default
SELECT COALESCE(NULL, 'Default') -- Output: Default
2. Multiple Null Checks with COALESCE
SELECT COALESCE(NULL, NULL, 'SQL', 'Server') -- Output: SQL
ISNULL() cannot do this as it only takes 2 arguments.
3. Data Type Precedence Difference
-- Example 1: ISNULL returns INT
SELECT ISNULL(NULL, 10) -- Output: 10 (INT)
-- Example 2: COALESCE resolves based on type precedence
SELECT COALESCE(NULL, CAST(10.5 AS FLOAT)) -- Output: 10.5 (FLOAT)
4. Nullable Column Comparison
DECLARE @val VARCHAR(10) = NULL;
-- ISNULL treats result as NOT NULL if second argument is NOT NULL
SELECT ISNULL(@val, 'N/A') -- Output: N/A
-- COALESCE respects input nullability
SELECT COALESCE(@val, 'N/A') -- Output: N/A
5. Usage in Calculations
-- Avoid division by NULL
SELECT ISNULL(NULL, 0) + 5 -- Output: 5
SELECT COALESCE(NULL, 0) + 5 -- Output: 5

Conclusion

Use ISNULL() when working with just one fallback value and simple replacement is needed. Choose COALESCE() when checking multiple values or when working in environments that follow standard SQL. Both are useful for handling NULL values effectively in different scenarios.