![]() As we have already clarified, the function is self-referring, so when we want to give it something to do, we have to treat it like its own metric, rather than like a function, and use it in the very calculation we have in our mind, connecting it with the data we want to use it on via usual operators. It works – strictly speaking – but there is no data value in it, quite literally. The function PREVIOUS_VALUE alone is more or less pointless, unless you want to do some math (which is fine, of course). There are two things here: the first is something very important that this syntax is not telling us and the other is the expression. The syntax of our function is fairly simple: PREVIOUS_VALUE( expression) ![]() That also means we have a problem when Tableau tries to calculate the very first row of our partition because that one does not have a previous value. That means we have to consider the scope and direction of the table calculation when we implement it in our view. Still, Tableau takes the whole column into account when calculating, not just a single cell. Of course, calculations, filters, groups, sets and all the other beloved features in Tableau give us the freedom we want. Whatever we do with a cell in a column, we do with all cells in that column. In Excel, we just drag this formula up or down, and we get what we want without any complex functions.īut remember: Tableau is not a cell-based tool like Excel it is table-based. In the lower-right chart on the image above, the cell C5 (for example) contains the following simple formula: = C4+ A5* B5. When we replicate this function in Excel, we don’t have to think about it too much. This is very important, as the PREVIOUS_VALUE function is neither an aggregation function nor a basic row-level function. You may notice two grey zeros on the right charts. This gives us a running sum and is already quite near to where we are going in Tableau. On the right, we add the previous results for each row to the calculation of this row. And while the blue ones on the left are just straight on calculations for each separate row, the red ones on the right are a little different: And since a picture tells more than a thousand words, let’s look at these four tiny tables where the first two columns are calculated somehow in the third column. It sounds far more complicated than it is. Self-referring? Recursive? What are you talking about? ![]() When it was introduced long ago, it probably was the Tableau dream of everyone who fell in love with loops and recursive functions in other tools or languages. PREVIOUS_VALUE is a self-referring function. Ready? Let’s get right into it and do the boring theory at the beginning (so we don’t have to bring it up later when it gets exciting). So, what does this function do? There is no better way to learn something than to teach it, so here is a new blog that reduces a five-year learning path down to 15 minutes. That does the trick.) Three Foundational Principles (To show previous values, just use the LOOKUP function with an offset of -1. And let’s get one thing straight right away: If you want to show the previous values of your field, don’t bother with this function because that’s the very thing it can’t do. Have a guess at when I finally understood the PREVIOUS_VALUE function … It was yesterday. You know what? I have been working with (and teaching) Tableau for over five years. ![]()
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