Hi Vinoth Kumar,
As per my understanding, tableau join is just like JOIN in SQL. the command join two tables in same datasource.
As for data blending, it is used when you blend data from multiple data sources on a single worksheet. For each data source that is used on the sheet, a query is sent to the database and the results are processed. Then all the results are left joined on the common dimensions.(which means that you must define common dimensions between the primary and secondary data sources )
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Tableau join is a plain SQL join and acts the same way as it does in the SQL.
However talking about Blend its an amazing feature of tableau. While using Joins your data gets joined immediately and rows are matched to each other on row level as per the join type(Inner, Left or Right) and join condition, however while using Tableau Blend it acts a bit different, tableau blend only works as Left Join and the data blended post aggregation. The data will be coming from both the sources will be first aggregated and then it'll be blended together. Lets assume you have one data source which as ID, OrderDate, Sales, OrderQty and other as ID and Cost. Now you want to calculate profit out of it which is Sales-(OrderQty*Cost), now your first source will carry multiple rows for each ID but the second one has only one row for each ID when the data will be blended Tableau will first Sum the Sales and OrderQty and then Sum the Cost and then blend these both. But a very important thing to keep in mind while working with Blend is that it only supports one to many relationship no many to many. The only place where many to many relationship can be used is where you have to bring a coloumn from the secondary data source in filters only.
Hope this helps..!!
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Joining is where you are able to combine data from the SAME data source, for example worksheets in an Excel file or tables in an Oracle database. You need a common field, also called a key, that tells Tableau how to join the data. You also need a key to do blending.
Blending is where you able to combine data from DIFFERENT data sources, for example a worksheet in Excel with a table from an Oracle database. This is very powerful because in the past you would typically need to have someone with the specific technical skills create a new data set with the combined information.
A blend is a post-aggregate left join. That is, the data on the secondary source is grouped by the dimension being joined, and then left joined with the primary. The left part of the join means members of the secondary that have no equivalent in the primary do no get put in the results, but if there are members of the primary without a corresponding member of the secondary, then are joined (or blended) with NULL.
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You may find this article I wrote helpful
In this I take an example with 2 datasources...I Blend and Join them to show the differences in behavior.
Hope it's of use