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Table of Contents  


What is Tableau?
Before we delve into this, lets turn the question around and ask: what is Excel?
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When first introduced to Excel (and other spredsheet spreadsheet tools such as Googe Sheets and OpenOffice Calc), we don't really question the tool, we just get to work, learning as we go, yet still never too sure about what a spreadsheet really is: Is it a calculator? A data entry and manipulation tool? An analysis tool? A visualisation tool? Knowing this, I hope to help you settle much quicker than without it. So we know that Excel is a spreadsheet tool. So really, this isn't so much a question of what Excel is but rather, a question of what a spreadsheet is, and how does this compare to Tableau. So according to Collins, a spreadsheet is:
So spreadsheets are multipupose purpose tools that allow for data entry, data maipulationmanipulation, analytics and visualisation. So going back to the original question What is Tableau, we can say that Tableau neither allows for data entry nor data manipulation (at least not directly), though through calculations, we can alter the output and meaning of the data, whilst allowing the orginal original data to persist. Tableau is a presentation layer. Or rather, a pivotingtool, in that it presents the output of your data model, whether this be a direct read of your data, or where you have manipulated the meaning of the data through calculations This is the key answer and the one that should always remain on your mind when you are working with either tool: Tableau is a presentation layer which means, it is the final step in any datawork whether this be:
Where you would use a spreadsheet to enter and manipulate data and then analyse data, Tableau simply visualises data and the output of your calculations. This means that any alterations to the data through calculation, editing and manipulation are either semipermanent, or permanent and this able to be used by other tools, whereas the data that Tableau display's has only been read by Tableau, it's state has not altered. This brings us neatly onto the next section: Where are my calculations processed? 
Where are my calculations processed?
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Spreadsheets (Excel, OpenOffice Calc, Google Sheets etc) contain all the necessary programming to evaluate and execute the expressions you enter be it arithmetic, logic, string, date, manipulation, or a combination of any of these. Tableau has only a rudimentary calculation function for Table Calculations, which are largely intersect and lookup calculations; other than these, all other calculations are sent as SQL queries to the dataengine of the datasource (such as Excel or SQL Server) for execution. Once executed, the sourceengine returns the resultset back to Tableau for Tableau to consume and plot as your visual representation. With Tabeau Tableau however, we have a third type of datasource: the Tableau extract (Hyper), this is the process of snapshotting your data as it stands today, either as a prefiltered portion, or in its entirety, into an extract layer, saved as a .hyper archive file (or .tde for pre v10.5). Although the data has been extracted by Tableau, and exists in the same archive as the Tableau workbook, Hyper (and tde) are processed by a different module than that used for the workbook and visualisation. The extract is expanded into a temporary memoryspace as a columnstore NOSQL database when your workbook is opened, and expressions formed in the visualisation layer are then sent outside the workbook to the dataprocessing layer exactly as they are for external sources such as to a database, or flatfile such as text, Excel or Access. 
Can Tableau write data?
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The short answer (in 2020) is no, Tableau cannot writedata. Excel users are used to being able to enter data into cells  short of connecting to a database or OLAP cube, or importing data from an alternate source, this is the only other way (and tends to be the primary method), of getting data into Excel, furthermore, once the data has been written into Excel, it can be made available for other workbooks to read from almost as a writeable database. Natively, being a presentation layer, Tableau can only read from a datasource:

There appears to be loads of things I can easily do with Excel but not or not very easily if at all with Tableau
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So you keep saying, but this still doesn't help me to understand why Tableau cannot do seemingly simple tasks As mentioned above, spreadsheets are selfcontained programs that have all the necessary functions and libraries needed to allow them to calculate the expressions that you enter into the cells, the result is, you can do a lot more with Excel in this way than you can with Tableau. Tableau is a presentation layer  a very pretty presentation layer but nothing more. The easiest way to understand Tableau is to think of it as a pivottable and pivotchart. When using either of these in Excel, you define a source typically several rows and columns of data, you then drag the defined elements into place as a table and then, if you need to, place a chart atop the pivoted data. Tableau is no different as such, Tableau is only really restricted by two items (which are largely the same two items that block full creativity in Excel):

So if Tableau is nothing more than a Pivot Table and I can already use this in Excel, why do I need Tableau?
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.. if your data is already in Excel, and you are comfortable with the pivottable functions there however, with Tableau you can:
All of which is perfectly accomplishable with an Excel pivottable however, to do so with Excel, you must first define your pivottable, then use this pivottable as the basis for new pivottables that will provide your analyses and then build against the new tables eg:
So as you can see, it can be done although the question becomes one of ease of use, ease to maintain and, data availability, where Tableau is unrestricted on the numbers of rows it can read (restricted only on the spool size and pipeline width from the source). If however this is a question of whether Tableau is right for you when you have no Tableau expertise, are very proficient with Excel, have sharepoint installed and configured and, are proficient using PowerBI, then this becomes a question of do you want to move from one BI tool to another? 
So, if I can do more with Tableau than I can with Excel, can't I just ditch Excel?
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This would be inadvisable unless you were thinking about ditching and moving to an alternate spreadsheet tool. Excel (spreadsheet tools) are more than just a means of presenting data, they are data storage, data manipulation, and data calculation tools; they are your first port of call for most things data and as your visualisation career progresses, will be where you can quickly test ideas on sample sets of data to ensure correct functionality before moving the calculations to Tableau. In short, Tableau should be working with Excel, and it will never be a replacement for it. 
Tableau is NEVER wrong
That's a bold statement, I hope you have some rocksolid evidence to back this up.
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