I know that those letters mean Extract, Transform, and Load.

But, when I used it at first, I thought that during the Transform phase I could do plenty of different joins on data that I've extracted from data sources, later on I realized that doing a join on a different ETL is not that handy.

  • So what do we do in Transform phase ?
  • Calculate and output the result ?
  • String transformation?
  • Should input data sources only be csv, xml or plain file?
  • If joins are not that handy, should we only do high level transformation within an ETL ?

Thank you

  • 9
    This post badly needs editing. – Brian Ballsun-Stanton Jan 4 '11 at 8:57
  • ETL is a process, not a thing. You probably have multiple ETL processes, and multiple ETL sources and destinations. – Jon of All Trades Jan 14 '14 at 22:33

Extract Transform and Load is the preparation of foreign data to be inserted into your database or data warehouse

Looking at the basics of the ETL, noted Data Warehouse Designer Bill Inmon notes:

Once upon a time in the not so distant past, there was no ETL (extract, transform and load) software. If you wanted to build a data warehouse, you had to write code in order to get data from one source to the appropriate target. There was lots of code – lots of repetitive code.

After you wrote your code, you had to maintain it. Every time a legacy system changed, you did manual maintenance to your code. Every time a target definition changed, you had to do manual maintenance to your code. Every time an end user wanted something new, you had to do maintenance to your code.

From here, a plethora of ETL products proliferated, as Inmon describes in his brief history of ETL products. They were popular because they were software tools designed to extract data from changing systems, transform it according to specific rules, and load it into data warehouses. This software process meant that humans were involved only in the critical element of the loop: untangling loaded data errors. By automating to the maximum possible extent, the ETL process provided companies a seamless way of not only loading their current databases into a data warehouse, but the ability to load future data sets of the same databases in, so that the data warehouse can continuously provide future results.

To answer your question specifically, different databases provide different extracts. Transformations are applied to normalize the data. Normalization is both in the database-specific sense, changing the patterns of the data to match the receiving data warehouse, but also in the human sense, insuring that the same data in different systems appears the same to the incoming system.

Data sources can be anything you can code a transform for, as the purpose of the transform is to apply rules to the incoming data such that it fits your data model. Joining different data sets should only be performed if necessary. Rely on your recipient database instead to synchronize results.

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Here are a few uses:

  • Data Cleansing (dates from old system do not have date times and you must validate integer date fields.
  • Check for orphans
  • normalize data (We have taken 17 loan tables and output 5)
  • Merge data from multiple source systems
  • Create aggregate tables
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Besides what's been mentioned, a large amount of my "Transform" is done doing unit conversion, as most of my databases track scientific data that's come from different sources. So when I "normalize" the data, it's what our field calls "data harmonization" (putting things on similar scales/reference frames so they can be directly compared), not necessarily "database normalization", although I might have to do some extra massaging to get the data organized around different concepts to fit within our system.

My common problems are:

  • time : add/subtract due to different epochs, possibly having to deal with leap seconds (eg, postgres always assumes Dec 31st is the same number of seconds, so I'd have to subtract out a few seconds so it'd display the time accurately)
  • spectral ranges : nanometers vs. Ångstrom, wavelength vs. frequency vs. electron volts
  • coordinate systems : (this one really sucks, as what's stationary in one reference frame might be moving in another)
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