Though there is a difference between a NULL and an "empty" value.
In this example we will examine the above issue.
Consider the following table:
CREATE TABLE [dbo].[table_A](
[id] [int] NOT NULL,
[name] [varchar](50) NULL,
CONSTRAINT [PK_table_A] PRIMARY KEY CLUSTERED
) ON [PRIMARY]
Let's populate the table with some data:
insert into table_A(id,name)
values (1,'A'), (2,'B'), (3,''), (4,NULL)
Note that the third record (id=3) contains an empty string for the column "name".
Also note that the fourth (id=4) record contains a NULL value for the same column.
Let's see how can we handle the two above "special" cases.
First of all if we select all the records from table_A we will get:
select id, name from table_A
Then let's try to handle the record having the NULL value and set as a new value the string "NewValue" for the result set of our select statement.
SQL Server provides 2 functions for doing this; (i) the ISNULL; and (ii) the COALESCE.
Even though the two functions are quite similar, still they have some differences:
(1) ISNULL takes only two parameters as input; (a) the expression to be checked and (b) the replacement value
(2) COALESCE takes N parameters as input (N>=2). By having N expressions as input parameters it returns the first expression that IS NOT NULL. If only all expressions are NULL it then returns a NULL value. It is like a more enhanced version of ISNULL.
Let's try to handle the above scenario with these two functions:
--USE of ISNULL--
select id,ISNULL(name,'NewValue') from table_A
--USE of COALESCE--
select id,COALESCE(name,'NewValue','NewValue2') from table_A
Hmmm, we can see that even though for the record with id=4 we got the "NewValue" string instead of NULL, the record with id=3 still returned an empty string value.
In this case it seems that the ISNULL and COALESCE functions had no effect on the third record's result. This was expected because that record does not contain a NULL value but an empty value instead. An empty value cannot be considered as a NULL of course.
So, how do we deal with this? How can we handle empty string values?
Unfortunately there is not a "magic" formula for that. The only suggestion is not to use empty strings but use NULL values instead which can be handled by the ISNULL, NULLIF and COALESCE functions.
Additionally you can always create your own user-defined scalar-valued function that can deal with the issue of the empty string values. In my example, I created the following function and called it ISEMPTY:
CREATE FUNCTION ISEMPTY
-- Input Parameters
@input as varchar(250),
-- Output parameter
RETURNS varchar (250)
-- First handle the case where the input value is a NULL
Declare @inputFiltered as varchar(250)
-- The main logic goes here
RETURN (case rtrim(ltrim(@inputFiltered)) when '' then rtrim(ltrim(@newValue)) else rtrim(ltrim(@inputFiltered)) end)
My function takes as input two parameters; (a) the input string and (b) the replacement string.
Then by using a combination of the ISNULL function and the CASE statement it handles both NULL and EMPTY string values. Though, the above user-defined function just handles strings and not any other expressions.
Now if we try running the ISEMPTY function for the same example we get the following:
select id, dbo.ISEMPTY(name,'NewValue') from table_a
Well, I guess that's it! :)
You can find more information regarding the abovementioned built-in SQL Server functions that deal with NULLs on the following MSDN Library links: ISNULL, NULLIF, COALESCE.