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Search | ASP FAQ Tutorials :: Databases :: Other Articles :: How do I calculate the median in a table? How do I calculate the median in a table?MEAN is easy (using the aggregate function AVG). MEDIAN, on the other hand, can be a little more difficult to calculate. (MEDIAN is one of the aggregate functions typically handled by statistics packages or by OLAP / Analysis Services.) SQL Server Say we have the following table:
If we had an odd number of rows, we could calculate the median by simply finding the row where the number of values greater than that splunge value is equal to the number of values less than that splunge value:
Or this way, by just taking the greatest value from the top 50% of the table, ordered by splunge:
With an even number of rows, however, the first query returns no results, and the second query might not return the desired result (in the case above, it will return the 4th of 8 rows, where splunge = 5). We need to do a little more work to calculate the *true* median, including implicitly converting to decimal, and taking the average of two nested subqueries and a union:
No, it's not pretty, but it gets the job done. There are similar ways to approach this problem that require fewer subqueries, but add the requirement of dynamic SQL. For added fun, you could determine beforehand which method you need, so—in the case where there are an odd number of rows—the overall work required by the procedure will likely be more efficient.
Keep in mind that I had a CONSTRAINT here that prevented splunge from containing a NULL (this could break some of the logic above). If the column you are using can have NULL values, you'll want to add AND splunge IS NOT NULL to all WHERE clauses. Microsoft Access In Access, we can run the same single subquery example as above, and the same CREATE TABLE statement.
In the case of an even number of rows, we might want to round to the 5th instead of 4th value, we can just swap it around:
However, we can also calculate the mathematical median in Access, which is basically the average of the two median values. Instead of nesting a bunch of sub-queries and a union, let's create a peripheral table to store the results temporarily.
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