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This is the same question as problem #10 in the SQL Chapter of Ace the Data Science Interview!
The table below contains information about tweets over a given period of time. Calculate the 3-day rolling average of tweets published by each user for each date that a tweet was posted. Output the user id, tweet date, and rolling averages rounded to 2 decimal places.
Note: Rolling average is a metric that helps us analyze data points by creating a series of averages based on different subsets of a dataset. It is also known as a moving average, running average, moving mean, or rolling mean.
User 111 made 2 tweets on 06/01/2022, and 1 tweet the next day. By 06/02/2022, the user had made in total 3 tweets over the course of 2 days; thus, the rolling average is 3/2=1.5. By 06/04/2022, there are 4 tweets that were made during 3 days: 4/3 = 1.33 rolling average.
The dataset you are querying against may have different input & output - this is just an example!
First, we need to obtain the total number of tweets created by each user on each day using either a common table expression (CTE) or a subquery. Either way, the query must apply a clause to the field and then group the data by and .
Output showing the first 5 rows:
represents the number of tweets that the user tweeted on the particular date.
The query above is then converted into a CTE or subquery called . Then, we use a window function on the resulting CTE or subquery to retrieve the number of tweets over a time period that contains the current row and the previous two days, thus yielding the 3-day rolling average. Finally, we order by user id and tweet date.
This is how the window function is interpreted:
Below are two methods of solving the question, one with a common table expression (CTE) and another with a subquery.
A CTE is a temporary data set to be used as part of a query and it exists during the entire query session. A subquery is a nested query. It’s a query within a query and unlike CTE, it can be used within that query only. Read here and here for more understanding.
Both methods give the same output and perform fairly similarly. Differences are CTE is reusable during the entire session and more readable, whereas subquery can be used in FROM and WHERE clauses and can act as a column with a single value. We share more resources here (1, 2, 3 on their use cases.
Solution #1: Using CTE
Solution #2: Using Subquery