Unable to calculate median - SQL Server 2017

Question

I am trying to computer the median number of transactions in each category. A few notes (as the dataset below is a small snippet of a much larger dataset):

• An employee can belong to multiple categories
• Each transaction's median should be > 0
• Not every person appears in every category

The data is set up like this:

|  Person | Category | Transaction |
|:-------:|:--------:|:-----------:|
| PersonA |   Sales  |      27     |
| PersonB |   Sales  |      75     |
| PersonC |   Sales  |      87     |
| PersonD |   Sales  |      36     |
| PersonE |   Sales  |      70     |
| PersonB |   Buys   |      60     |
| PersonC |   Buys   |      92     |
| PersonD |   Buys   |      39     |
| PersonA |    HR    |      59     |
| PersonB |    HR    |      53     |
| PersonC |    HR    |      98     |
| PersonD |    HR    |      54     |
| PersonE |    HR    |      70     |
| PersonA |   Other  |      46     |
| PersonC |   Other  |      66     |
| PersonD |   Other  |      76     |
| PersonB |   Other  |      2      |

An ideal output would look like:

| Category | Median | Average |
|:--------:|:------:|:-------:|
|   Sales  |   70   |    59   |
|   Buys   |   60   |    64   |
|    HR    |   59   |    67   |
|   Other  |   56   |    48   |

I can get the average by:

SELECT
    Category,
    AVG(Transaction) AS Average_Transactions
FROM
    table
GROUP BY
    Category

And that works great!

This post tried to help me find the median. What I wrote was:

SELECT
    Category,
    PERCENTILE_DISC(0.5) WITHIN GROUP (ORDER BY Transaction) OVER (PARTITION BY Category) AS Median_Transactions
FROM
    table
GROUP BY
    Category

But I get an error:

Msg 8120: Column 'Transactions' is invalid in the select list because it is not contained in either an aggregate function or the **GROUP BY** clause

How can I fix this?

Answer 1

You can do what you want using SELECT DISTINCT:

SELECT DISTINCT Category,
       PERCENTILE_DISC(0.5) WITHIN GROUP (ORDER BY Transaction) OVER (PARTITION BY Category) AS Median_Transactions
FROM table;

Unfortunately, SQL Server doesn't offer the PERCENTILE_ functions as window functions and doesn't have a MEDIAN() aggregation function. You can also do this using subqueries and counts.