Understanding CROSS APPLY

Andy Levy has a T-SQL programming breakthrough:

Finally, this week I had a breakthrough. I was working on updating a bunch of data but it was breaking on a small subset of that data. In this case, I was attempting to JOIN two tables on fields that should have been INTs, but in a very small number of cases one side was using a comma-delimited string. The user told me that someone else had done these updates in the past and didn’t encounter the problem I was having (so I knew that it was something i was doing “wrong”), but given that it was only a handful of broken updates she was OK with manually doing the updates (we were scripting it because we were updating potentially tens of thousands of records).

I am not OK with manually fixing this in the future. I wanted to know how the other DBA had done it before. I dug into some history and found CROSS APPLY. My nemesis. I was determined to figure out how to use it this time.

The APPLY operator is extremely powerful in the right set of circumstances.  Andy shows the “classic” use case, but there are a number of other uses for the operator.

Related Posts

Enabling Database-Level Change Tracking

Tim Weigel continues a series on change tracking: If you don’t provide a retention period, SQL Server’s default is 2 days. Auto-cleanup defaults to ON unless you tell it otherwise. Easy! The table level commands aren’t any more complicated. Before we get started, please note that change tracking requires a primary key on the table […]

Read More

Isolation Levels and Dynamic SQL

Max Vernon points out how transaction isolation levels work when combined with sp_executesql: Imagine you have a piece of code where you don’t care about the downsides to the “read uncommitted” isolation level, and do your due diligence by adding SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED; at the start of your code. The code following that statement […]

Read More

Categories