This community-built FAQ covers the “LEAD” exercise from the lesson “Window Functions”.
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This exercise can be found in the following Codecademy content:
Analyze Data with SQL
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Given that you can use LEAD or LAG to calculate changes to weekly streams and changes to chart position, in what instances would you typically use one over the other?
For example the following provide the same results:
LEAD(streams_millions, 1) OVER (
PARTITION BY artist
ORDER BY week
) - streams_millions AS ‘streams_millions_change’
streams_millions - LAG(streams_millions, 1, streams_millions) OVER (
PARTITION BY artist
ORDER BY week
) AS ‘streams_millions_change’
I am a beginner using Windows functions. In what situations would you use LEAD and in what situations would you use LAG?
Thanks for your advice!
My sense is that the function written with LEAD creates a value that anticipates the change over the upcoming week, and the function written with LAG creates a value that represents the change since last week. I think the LEAD change is kind of confusing and unhelpful.
For LAG and LEAD concepts, I recommend not throwing in additional complexities, like switching the subtraction for chart position (where “reducing” from spot 2 to spot 1 is GOOD). It’s disheartening to try over and over, thinking you understood the concept, only to read the answer and realize that the only thing stopping it flagging yours as correct is where you place the subtraction.
The way the LAG results are structured makes sense to me. Typically, in business, you review week-over-week, or month-over-month, or quarter-over-quarter, or year-over-year. In these kinds of reports you show (if using monthly numbers) your monthly results and the change from prior month (that’s month-over-month) or change from same month last year (that’s year-over-year). I don’t believe I’ve ever seen a report structured to show January’s results and then change from February on January’s line. The change from January to February would always be on February’s line. Because of this, I found the LEAD examples very confusing. It just isn’t the way most financial statements are structured.
I’m sure there are plenty of useful applications of LEAD, but I found this example difficult to get my head around.
I do want to say that I’ve really enjoyed using Codecademy.