External Tables To Hadoop

I have a post looking at creating external tables in Polybase to hit a Hadoop folder:

The DATA_SOURCE and DATA_FORMAT options are easy:  pick you external data source and external file format of choice.

The last major section deals with rejection.  We’re going from a semi-structured system to a structured system, and sometimes there are bad rows in our data, as there are no strict checks of structure before inserting records.  The Hadoop mindset is that there are two places in which you can perform data quality checks:  in the original client (pushing data into HDFS) and in any clients reading data from HDFS.  To make things simpler for us, the Polybase engine will outright reject any records which do not adhere to the quality standards you define when you create the table.  For example, let’s say that we have a Age column for each of our players, and that each age is an integer.  If the first row of our file has headers, then the first row will literally read “Age” and conversion to integer will fail.  Polybase rejects this row (removing it from the result set stream) and increments a rejection counter.  What happens next depends upon the reject options.

Creating an external table is pretty easy once you have the foundation prepared.

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