SQL - Difference between COALESCE and ISNULL?

Comparing COALESCE and ISNULL

The ISNULL function and the COALESCE expression have a similar purpose but can behave differently.

1. Because ISNULL is a function, it is evaluated only once. As described above, the input values for the COALESCE expression can be evaluated multiple times.
2. Data type determination of the resulting expression is different. ISNULL uses the data type of the first parameter, COALESCE follows the CASE expression rules and returns the data type of value with the highest precedence.
3. The NULLability of the result expression is different for ISNULL and COALESCE. The ISNULL return value is always considered NOT NULLable (assuming the return value is a non-nullable one) whereas COALESCE with non-null parameters is considered to be NULL. So the expressions ISNULL(NULL, 1) and COALESCE(NULL, 1) although equivalent have different nullability values. This makes a difference if you are using these expressions in computed columns, creating key constraints or making the return value of a scalar UDF deterministic so that it can be indexed as shown in the following example.

> USE tempdb; 
> GO

> -- This statement fails because the PRIMARY KEY cannot accept NULL values
> -- and the nullability of the COALESCE expression for col2 
> -- evaluates to NULL. 

> CREATE TABLE #Demo  (  col1 integer NULL,  col2 AS COALESCE(col1, 0) PRIMARY KEY,  col3 AS ISNULL(col1, 0)  ); 
> 
> -- This statement succeeds because the nullability of the 
> -- ISNULL function evaluates AS NOT NULL.
> 
> CREATE TABLE #Demo  (  col1 integer NULL,  col2 AS COALESCE(col1, 0), 
> col3 AS ISNULL(col1, 0) PRIMARY KEY  );

Validations for ISNULL and COALESCE are also different. For example, a NULL value for ISNULL is converted to int whereas for COALESCE, you must provide a data type. ISNULL takes only 2 parameters whereas COALESCE takes a variable number of parameters.

Source: BOL

The main difference is, that COALESCE is ANSI-Standard, so you will also find it in other RDBMSs, the other difference is you can give a whole list of values to be checked to COALESCE whereas to ISNULL you can only pass one.

Because ISNULL is a function, it is evaluated only once. As described above, the input values for the COALESCE expression can be evaluated multiple times. COALESCE basically translates to CASE expression and ISNULL is a built-in implemented in the database engine.

MSDN