Get the cancelling rate of a vendor ignoring a certain state, in one query

Let’s say I have a table representing sales with these attributes:

  • a state that can be sold, waiting, unknown or cancelled
  • a vendor
  • a buyer
  • a cancellor that is either the vendor or the buyer in case the sale’s state is cancelled, and null for other states.

I want to compute the cancelling rate for Joe. Right now I’m doing two SQL requests. The first one gets the number of cancelled sales by Joe:

SELECT COUNT(*) AS n_cancelled FROM sales WHERE cancellor = "Joe"

The second one gets the number of sales where Joe was involved, excluding the ones whose state is unknown or waiting:

SELECT COUNT(*) AS n_sales FROM sales
WHERE
  STATE in ("sold", "cancelled")
  AND (vendor = "Joe" OR buyer = "Joe")

I then compute my cancelling rate by dividing the results of these two requests.

I’m interested in doing so in one request that would output the cancelling rate and also the number of sales used to compute it (n_sales here). How would I do that?

Answer

You can use conditional aggregation:

SELECT SUM(cancellor = 'Joe') as n_cancelled,
       SUM(state in ('sold', 'cancelled') AND 'Joe' IN (vendor, buyer)) AS n_sales
FROM sales
WHERE ( (cancellor = 'Joe') or
        (state in ('sold', 'cancelled') AND 'Joe' IN (vendor, buyer))
      );

You can do the division in the query as well.

Note: This keeps the WHERE clause, which is not strictly necessary. But reducing the number of rows before aggregating the data can improve performance.

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