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In Excel 2010: Data Validation in Depth, author Dennis Taylor shows how to use the data validation tools in Excel to control how users can input data into workbooks and ensure data is entered consistently and accurately. The course covers creating dropdown lists, preventing duplicate entries, and controlling the format of numeric data, dates and times, and text entered into worksheets. Exercise files are included with the course.
For certain kinds of situations, you might want to preclude duplicate entries. In other words, you might prefer unique entries--say, for example, in column B. The first employee ID and the third one happens to be identical. That's the kind of thing you sometimes need to prevent. There's no obvious choice here under Data Validation, Data Tab > Data Validation. None of these choices here seems to tackle that issue head on. However, a custom formula will allow us to get there.
Let's build this outside of the Data Validation dialog box first. The idea is, every time an entry is made, we want to compare this with other entries in the same column, to make sure that it exists once and only once. You may be familiar with a function called COUNTIF. We're looking in column B, throughout column B, comma. And let's just imagine for the moment we're talking about cell B2. You can click on it or type it.
So what we're going to get out of this is the count of how often the B2 entry appears in column B. It appears there twice. Now, all we really need to do is to take a variation on this and in effect say, 'We want the count of any cell that gets entered here. We want to make sure that it's there once and only once,' the actual current entry. So this our starting point and will change a bit, because we will be referring to B1 in our formula.
Let's simply copy this, Ctrl+C, press Escape, go to column B, and set up a data validation criteria formula under Allow > Custom, a custom formula. I'm going to press Ctrl+V now to paste this in. It's not quite there. We want to make sure that the count of B1, since that's currently the active cell, it acts as a substitute for all the others. And anytime we make an entry, we want to compare that entry with all the entries in column B. So the number of times that this item should appear here must be equal to 1.
In other words, it's there once and only once. Click OK. So, test it out again here. I'm going to type in '134267'. The entry is already there. As always, we might want to customize the error alert message to indicate what's happening here. Slight change. Maybe it was a missed type entry. Put an 8 in. That should work just fine. Remember, the prior entries don't get screened out yet. That's up to you to clean those up, if you're applying the rule with data already here.
We'll change that to a 5. It shouldn't be a problem. That's what it should have been all along. There here we ago. But we certainly can't enter in that top number, 219876. It's there once. This would be twice. Can't do it. So you can restrict unique entries using this data validation formula here, once again, display of it, as we see it right here. And of course, in different columns use different letters there. It works fine, easy. Familiarity with the COUNTIF function helps you understand it better, but it's easy to use.
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