Finally, make sure to change columns that don't make sense as numbers into text. For example, unless you want Power BI to calculate sums of your flight numbers -- and you don't -- turn them into text. Otherwise, attempting to make graphs with these fields as categories won't work, and you'll end up wasting a fair amount of time (don't ask me how I know). You can convert FL_NUM and MONTH into text by right-clicking the column header and choosing Change Type > Text. Another tweak: The CANCELLED column shows 0 for false and 1 for true -- if you want, you can change that type to True/False.
Now comes the fun part.
Close out of the Query Editor with Close & Apply to get back to the main Power BI application. (Because these files are so large, this may take a little while.) At this point you might also want to do a File > Save to save your work as a Power BI project so that if something happens, you won't need to re-create your steps.
Your Power BI blank canvas should look something like this:
A Power BI blank canvas.
It would be nice to get rid of all the other query files that we merged into 2015_SUMMER_ONTIME, since we won't be using them again. If you go to the Fields panel on the right and try to right-click and choose Delete, you'll get a warning that you can't delete those queries because they're referenced by another query. However, you can right-click and then Hide them to get them out of the way.
Do that for all the queries except 2015_SUMMER_ONTIME. Now that there's only one query table left, the 2015_SUMMER_ONTIME table will automatically expand to show its column names. (When there are multiple queries in the menu, the column names don't appear by default; click the triangle next to one to expand it; right-click and choose Expand all to see all the queries' fields.)
Want to graph departure delays? Click the check box next to DEP_DELAY. You'll get a bar graph with one bar summing all the delay time in your data. (Power BI tries to guess what visualization will work best with your data if you don't specify.) Click Airline, and the graph will morph into a bar graph of each airline's total flight delays.
A sum of total delays by airline isn't that useful, though; the more flights an airline has, the more minutes delayed it's likely to have. But if you look under the Visualizations panel and find DEP_DELAY under Value, you can click the triangle drop-down menu and change the Sum of DEP_DELAY minutes to Average (or Median, if you prefer).
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