Tag: T-SQL

Recently I needed to add a computed column to a table and realized that I didn’t remember the syntax. This short post show how to do this.

Another post for me that is simple and hopefully serves as an example for people trying to get blogging as #SQLNewBloggers.

Adding the Column

I had a table, OrderHeader, and wanted to add a new column, OrderedByDate. I can do this with a simple ALTER statement and an ADD. The inserting part is the computation. You use as AS clause with the computation instead of a datatype. For example, my code was:

ALTER TABLE dbo.OrderHeader 
   ADD OrderedBy AS OrderDate;
GO

This added a copy of my OrderDate column with a new name. This is useful for zero downtime deployments in some cases, and in this case, I wanted to just have a copy. However, if I wanted some calculation, I could easily do that by specifying this as I would in a SELECT statement. For example, if I wanted the computed column to be a week later, I could do this:

ALTER TABLE dbo.OrderHeader 
   ADD OrderedBy AS DATEADDD(DAY, 7, OrderDate);
GO

This would add a week to the original value and return that. I can likewise do any sort of string or numeric manipulation I want. A common one is adding or multiplying two columns together for a new value. For example, adding various charges for a total in an order.

When you do this as a computed column that is not persisted, no space is taken in the actual table rows. If you persist this, then space is used.

More information on Microsoft Learn.

SQL New Blogger

I realized that I needed to double check the syntax in the docs, and when I did, I took this as an opportunity to write a short blog post. I grabbed a link, wrote some code, and then spent 10 minues knocking out this post.

If some employer does this a lot, they might search your blog to see if you can do this. A nice few posts on how to do this, what persisted does, how you index this, etc. Take a few minutes and start blogging on topics like this throughout your week.

Does Context Info work across databases? This post shows it does.

Another post for me that is simple and hopefully serves as an example for people trying to get blogging as #SQLNewBloggers. Here are some hints to get started.

The Demo

Someone asked the question, would a trigger in another database see context info from a different database. I thought it should work, but decided to test it.

Here I’m going to create a table and trigger in database compare2. This is looking for a context value.

USE compare2

GO

CREATE TABLE TriggerTest (myid INT, mychar CHAR(1))

GO

CREATE TRIGGER tri_triggertest ON  dbo.TriggerTest FOR INSERT

AS

BEGIN
     IF CONTEXT_INFO() = 0x1256698456
         PRINT 'caught'
     ELSE
         UPDATE dbo.TriggerTest
          SET mychar = 'X'
          FROM inserted i
          WHERE i.myid = dbo.TriggerTest.myid

END

GO

Now, back in DB 1, I’m going to set CONTEXT_INFO and insert a value into the database. This should give me a result where the trigger updates the table. A “normal” action.

USE compare1

GO

SET CONTEXT_INFO 0x000

GO

INSERT compare2.dbo.TriggerTest (myid, mychar) VALUES (1, NULL)

GO

This does, as the table contains a 1 and X.

Now, same connection, let’s set the magic value for context and insert a row. Now the trigger should avoid the update, letting my bypass the normal action. This is what someone was trying to do.

SET CONTEXT_INFO 0x1256698456;  

GO  

SELECT CONTEXT_INFO();  

GO  

INSERT compare2.dbo.TriggerTest (myid, mychar) VALUES (2, NULL)

GO

When I look at the results, I have the “caught” message. The final results from the table are shown here:

As you can see here, the context is with the connection, not the database. The database doesn’t matter for this value, it’s whether or not the connection that sets the context (the session really) is still alive when it accesses the other database.

SQL New Blogger

This was a quick test for me to answer a question and prove this to someone (and myself). I thought this would work, but I spent 5 minutes devising a test. It took me less than 10 minutes to put this post together.

This shows volunteerism (helping someone), testing ability, and diligence to prove something I suspected was true. I didn’t assume, I tested. Lots of employers love that.

You can raise your brand and be a SQL New Blogger like this, showing your knowledge.