如何将查询结果添加到 SQL 中的表中？

Question

I am trying to add the results of a query to an existing table, dependent on the values of an existing column. For example, using the table below

Store	Sales	Weekday
10	11000	Weekday
11	5000	Weekday
12	8000	Weekday
10	19000	Weekend
11	20000	Weekend
12	5000	Weekend

I want the averages per store and weekday which I can get using the following:

SELECT AVG(Sales) AS weekday_avg, store
FROM store_sales
WHERE Weekday = 'Weekday'
GROUP BY store;

But then I'd like to add these results to the same table and store in a column named 'weekday_avg'.

I've tried the following, and while I don't get an error, the column doesn't have any values added:

ALTER TABLE store_sales ADD COLUMN weekday_avg numeric;
UPDATE store_sales SET weekday_avg = (
    SELECT AVG(Sales) AS weekday_avg
    FROM store_sales
    WHERE Weekday = 'Weekday'
    GROUP BY store
);

I know this probably isn't best database practice, but I'm working with what has been provided and all I need is to end up with a table with columns for averages per store / weekday type that I can export into R for further analysis.

Many thanks in advance!

Answer 1

Don't store the values. They are easily calculated on the fly:

select ss.*,
       avg(sales) over (partition by store, weekday) as weekday_avg_sales
from store_sales ss;

Perhaps if you have a really big table, you might want to store the summary values. But even so, I would recommend a second table for the summaries and joins. With indexes, that would be much more efficient.

Note that calculating the data on the fly means that it is always up-to-date when existing data is updated or new data is inserted.

Answer 2

As mentioned in other answer, don't store calculated values in table.

But if you want to know how it can be done, then one of the options is to use the corelated query as follows:

UPDATE store_sales s SET s.weekday_avg = (
    SELECT AVG(ss.Sales) AS weekday_avg
    FROM store_sales ss
    WHERE s.Weekday = ss.weekday
      And ss.store=s.store
);