Switch between Select All and Where clause dynamically in SQL Server

Question

I want to write a T-SQL query for below conditions.

If the user provides a value for the parameter, run the query with the where clause.

SELECT * 
FROM TestDB 
WHERE CustomerId = @customer_id

If the user doesn't provide a value for the parameter, I want to return all the data

SELECT * 
FROM TestDB

Can I make it work in a single query, for example using ISNULL() or an alternative, without going for a stored procedure.

Answer 1

Add a condition to the WHERE clause which admits all records in the event the user parameter be null:

SELECT *
FROM TestDB
WHERE CustomerId = @customer_id OR @customer_id IS NULL;