Find centralized, trusted content and collaborate around the technologies you use most. Connect and share knowledge within a single location that is structured and easy to search. I wrote a T-SQL Statement similar like this the original one looks different but I want to give an easy example here :. But Why? If one or both parts of a comparison is null, the result of the comparison will be UNKNOWN, which is treated like false in a case structure.
The issue is that NULL is not considered to be equal to anything even not to itself, but the strange part is that is also not not equal to itself. In comparison, in general programming languages null is treated is a regular value and is equal to itself, however the is the NAN value which is also not equal to itself, but at least it returns 'false' when comparing it to itself, and when checking for not equals different programming languages have different implementations.
Note however that in the Basic languages i. VB etc. NULL does not equal anything. There are also several system stored procedures that are written incorrectly with your syntax. Wish I knew how to notify Microsoft of those mistakes as I'm not able to change the system stored procs. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. Asked 11 years, 4 months ago. Active 5 months ago.
Viewed k times. Improve this question. Add a comment. Active Oldest Votes. Improve this answer. These values make a difference if you're using these expressions in computed columns, creating key constraints or making the return value of a scalar UDF deterministic so that it can be indexed as shown in the following example:.
This example uses the AdventureWorks database. In the following example, the wages table includes three columns that contain information about the yearly wages of the employees: the hourly wage, salary, and commission. However, an employee receives only one type of pay. Assume for this example that the Products table contains this data:.
Feedback will be sent to Microsoft: By pressing the submit button, your feedback will be used to improve Microsoft products and services. Privacy policy. Skip to main content. This browser is no longer supported. Color column and use two different methods for updating NULL values in this column. The drawback to this method as you can see from the query plan and output below is that it actually updated every record in the table. A better option in this case would be to use the special IS operator in the WHERE clause of the update statement which would allow the query engine to use an index to perform the update.
You can see from the query plan and output below that it is now performing an index seek rather than the scan performed when using the ISNULL function. As said above, a good rule of thumb is to try and avoid using functions in a WHERE clause as it limits the query engine's ability to use an index effectively. But this then begs the question: What do we do if we want to search for some particular value as well as NULL? As you can see from the query plan below this will use the index but has to perform a scan of the entire index as was happening in the update example.
You can use the ISNULL function to specify a hard-coded value or even use another column in the table for the join condition. Below is an example that shows how you can return the personal address of a person and if that does not exist return their business address. A good example of this is using our Person table structure from above.
We know from before that you do not have to specify a businessaddressid as shown below. If you were to write a query to concatenate all the name related columns to display the full name you could do this but it would require manually adding the ISNULL function around every column that is nullable as follows. As you can see this is quite cumbersome and if you are doing the operation a lot a much simpler approach would be to add the following persisted computed column to the table.
0コメント