SQL - COALESCE

COALESCE (Transact-SQL)

Evaluates the arguments in order and returns the current value of the first expression that initially doesn't evaluate to NULL. For example, SELECT COALESCE(NULL, NULL, 'third_value', 'fourth_value'); returns the third value because the third value is the first value that isn't null.

CREATE TABLE dbo.wages 

( 

    emp_id        tinyint   identity, 

    hourly_wage   decimal   NULL, 

    salary        decimal   NULL, 

    commission    decimal   NULL, 

    num_sales     tinyint   NULL 

); 

GO 

INSERT dbo.wages (hourly_wage, salary, commission, num_sales) 

VALUES 

    (10.00, NULL, NULL, NULL), 

    (20.00, NULL, NULL, NULL), 

    (30.00, NULL, NULL, NULL), 

    (40.00, NULL, NULL, NULL), 

    (NULL, 10000.00, NULL, NULL), 

    (NULL, 20000.00, NULL, NULL), 

    (NULL, 30000.00, NULL, NULL), 

    (NULL, 40000.00, NULL, NULL), 

    (NULL, NULL, 15000, 3), 

    (NULL, NULL, 25000, 2), 

    (NULL, NULL, 20000, 6), 

    (NULL, NULL, 14000, 4); 

GO 

SET NOCOUNT OFF; 

GO 

SELECT CAST(COALESCE(hourly_wage * 40 * 52,  

   salary,  

   commission * num_sales) AS money) AS 'Total Salary'  

FROM dbo.wages 

ORDER BY 'Total Salary'; 

GO 

1.      Because ISNULL is a function, it's evaluated only once. 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). By contrast, COALESCE with non-null parameters is considered to be NULL. So, the expressions ISNULL(NULL, 1) and COALESCE(NULL, 1), although equal, have different nullability values.

4.      Validations for ISNULL and COALESCE are also different. For example, a NULL value for ISNULL is converted to int though for COALESCE, you must provide a data type.

5.      ISNULL takes only two parameters. By contrast COALESCE takes a variable number of parameters.