ISNULL vs COALESCE

ISNULL	COALESCE
The ISNULL Function is a built-in function to replace nulls with specified replacement values.	The COALESCE() function returns the first NON-NULL value.
ISNULL() is a T-SQL (Transact SQL) function and only work with Microsoft products e.g. Microsoft SQL Server	The COALESCE function is defined by the ANSI SQL standard and supported in all major databases e.g. MySQL, Oracle, PostgreSQL, DB2
The ISNULL() method takes only two parameters, it returns the first parameter if it’s not null and return the second parameter if the first parameter is null	COALESCE can take multiple parameters, and return the NOT NULL parameter starting from first to last
ISNULL ( check_expression , replacement_value )	COALESCE ( expression [ ,...n ] )
ISNULL is faster than COALESCE in SQL Server because of its a built-in function implemented in the Database engine	COALESCE slow compare to ISNULL
ISNULL just looks the data type of the first argument and makes everything of that type	COALESCE correctly promotes its arguments to the highest data type in the expression list
SELECT 17 / ISNULL(CONVERT(INT,NULL), 3.00); Output 5	SELECT 17 / COALESCE(CONVERT(INT,NULL), 3.00) Output 5.666666
ISNULL can only work with two values	COALESCE is more flexible and allows you to provide multiple columns and default values

 

DECLARE @x VARCHAR(10)

DECLARE @y VARCHAR(10)

DECLARE @z VARCHAR(10)

DECLARE @a VARCHAR(10)

SELECT @a = 'SQL'

SELECT COALESCE(@x,@y,@z,@a)

SELECT ISNULL(@x,@y);