SQL Server Non-Null Non-Empty Check

Understanding the Concept:

• Is not Null: This condition ensures that a column value is not null. A null value represents the absence of data.
• Is not Empty string: This condition verifies that a string value is not empty. An empty string contains zero characters.

Methods to Check:

1. Using the IS NOT NULL and LEN Functions:

   • IS NOT NULL: Directly checks if the value is not null.
   • LEN: Determines the length of a string. If the length is greater than zero, the string is not empty.

   SELECT *
   FROM your_table
   WHERE your_column IS NOT NULL AND LEN(your_column) > 0;
   

2. Using the COALESCE Function:

   • COALESCE: Returns the first non-null value from a list of expressions.
   • If the column value is null, COALESCE can be used to replace it with an empty string or another default value.
   • Then, check if the resulting value is not equal to the empty string.

   SELECT *
   FROM your_table
   WHERE COALESCE(your_column, '') <> '';
   

3. Using a Case Expression:

   • A CASE expression can be used to conditionally check for null and empty values.

   SELECT *
   FROM your_table
   WHERE CASE WHEN your_column IS NULL OR your_column = '' THEN 0 ELSE 1 END = 1;
   

Example:

Assuming you have a table named Customers with a column named Name, you can use the following query to retrieve rows where the Name column is not null and not empty:

SELECT *
FROM Customers
WHERE Name IS NOT NULL AND LEN(Name) > 0;

Choosing the Best Method:

• The IS NOT NULL and LEN method is often considered the most straightforward and efficient approach.
• COALESCE can be useful when you need to handle null values in a more flexible way.
• The CASE expression provides a more conditional approach, but it might be less readable for complex conditions.

Understanding the Example Codes

Scenario: We have a table named Customers with a column named Name. We want to retrieve only those customers where the Name column is neither null nor empty.

Method 1: Using IS NOT NULL and LEN

SELECT *
FROM Customers
WHERE Name IS NOT NULL AND LEN(Name) > 0;

Explanation:

• IS NOT NULL: This condition ensures that the Name column doesn't contain a null value.
• LEN(Name): This function calculates the length of the Name column. If the length is greater than 0, it means the string is not empty.

Method 2: Using COALESCE

SELECT *
FROM Customers
WHERE COALESCE(Name, '') <> '';

• COALESCE(Name, ''): This function replaces any null value in the Name column with an empty string.
• <> '': This condition checks if the resulting value (either the original Name or the replaced empty string) is not equal to an empty string.

Method 3: Using a CASE Expression

SELECT *
FROM Customers
WHERE CASE WHEN Name IS NULL OR Name = '' THEN 0 ELSE 1 END = 1;

• The CASE expression evaluates the condition Name IS NULL OR Name = ''.
• If the condition is true (i.e., Name is null or empty), the expression returns 0.
• The WHERE clause filters for rows where the CASE expression returns 1.

Alternative Methods for Checking Non-Null and Non-Empty Strings in SQL Server

While the methods discussed earlier are common and effective, there are a few additional approaches you can consider:

Using TRIM and LEN:

• TRIM: Removes leading and trailing whitespace from a string.
• LEN: Checks the length of the trimmed string. If the length is greater than 0, the string is not null or empty after removing whitespace.

SELECT *
FROM Customers
WHERE LEN(TRIM(Name)) > 0;

Using PATINDEX:

• PATINDEX: Searches for a pattern within a string. In this case, we can search for any non-whitespace character.
• If a match is found, the string is not null or empty.

SELECT *
FROM Customers
WHERE PATINDEX('%[^ ]%', Name) > 0;

Using a User-Defined Function:

• Create a user-defined function that combines the logic of checking for null and empty values.
• This can be useful for reusability and readability.

CREATE FUNCTION IsNotNullOrEmpty (@value nvarchar(MAX))
RETURNS bit
AS
BEGIN
    RETURN CASE WHEN @value IS NULL OR @value = '' THEN 0 ELSE 1 END;
END;

SELECT *
FROM Customers
WHERE IsNotNullOrEmpty(Name) = 1;

Using a Common Table Expression (CTE):

• Create a CTE to filter out null and empty values before performing other operations.

WITH FilteredCustomers AS (
    SELECT *
    FROM Customers
    WHERE Name IS NOT NULL AND Name <> ''
)
SELECT *
FROM FilteredCustomers;

Choosing the Best Method: The most suitable method depends on your specific requirements and preferences. Consider factors like:

• Performance: For large datasets, methods that avoid unnecessary function calls or CTEs might be more efficient.
• Reusability: User-defined functions can be helpful for reusable logic.
• Complexity: If you need to handle more complex scenarios, a CTE or a combination of methods might be necessary.