Daniel Hutmacher gives us a head-scratcher:

Performance tuning the other day, I was stumped by a query plan I was looking at. Even though I had constructed a covering index, I was still getting a Key Lookup operator in my query plan. What I usually do when that happens is to check the operator’s properties to see what its output columns are, so I can include those columns in my covering index.

Here’s the interesting thing: there weren’t any output columns. What happened?

The answer makes perfect sense, and shows that looking at the SELECT clause isn’t enough.

Aaron Bertrand discusses procedures which satisfy all potential search conditions:

This table has a lot of columns, obviously. Now envision an interface where employees at World Wide Importers could perform searches for specific customers. They may want to perform different searches based on unique combinations of criteria, including (and this is not an exhaustive list by any means, but enough for my purposes today):

• CustomerID (e.g. pick the customer from a list);
• actual “bill to” CustomerID;
• name;
• category;
• buying group;
• when the account was opened;
• city; or,
• whether the account is on credit hold.

And any combination of the above. I know you’ve seen and probably built interfaces like this before, but just as a quick mockup, the employees would have a screen something like this:

There are plenty of ways to solve this problem, and Aaron shows a couple methods (including one which has major problems).