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.)
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.
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.
Related ArticlesHow do I build a query with optional parameters?
How do I create a store locator feature?
How do I deal with MEMO, TEXT, HYPERLINK, and CURRENCY columns?
How do I deal with multiple resultsets from a stored procedure?
How do I debug my SQL statements?
How do I determine if a column exists in a given table?
How do I enable or disable connection pooling?
How do I enumerate through the DSNs on a machine?
How do I find a stored procedure containing <text>?
How do I get a list of Access tables and their row counts?
How do I get the latest version of the JET OLEDB drivers?
How do I handle alphabetic paging?
How do I handle BIT / BOOLEAN columns?
How do I handle error checking in a stored procedure?
How do I ignore common words in a search?
How do I page through a recordset?
How do I present one-to-many relationships in my ASP page?
How do I prevent duplicates in a table?
How do I prevent my ASP pages from waiting for backend activity?
How do I prevent NULLs in my database from mucking up my HTML?
How do I protect my Access database (MDB file)?
How do I protect my stored procedure code?
How do I protect myself against the W32.Slammer worm?
How do I remove duplicates from a table?
How do I rename a column?
How do I retrieve a random record?
How do I return row numbers with my query?
How do I send a database query to a text file?
How do I simulate an array inside a stored procedure?
How do I solve 'Could not find installable ISAM' errors?
How do I solve 'Operation must use an updateable query' errors?
How do I temporarily disable a trigger?
How do I use a SELECT list alias in the WHERE or GROUP BY clause?
How do I use a variable in an ORDER BY clause?
Should I index my database table(s), and if so, how?
Should I store images in the database or the filesystem?
Should I use a #temp table or a @table variable?
Should I use a view, a stored procedure, or a user-defined function?
Should I use recordset iteration, or GetRows(), or GetString()?
What are all these dt_ stored procedures, and can I remove them?
What are the limitations of MS Access?
What are the limitations of MSDE?
What are the valid styles for converting datetime to string?
What datatype should I use for my character-based database columns?
What datatype should I use for numeric columns?
What does "ambiguous column name" mean?
What is this 'Multiple-step OLE DB' error?
What is wrong with 'SELECT *'?
What naming convention should I use in my database?
What should I choose for my primary key?
What should my connection string look like?
When should I use CreateObject to create my recordset objects?
Where can I get this 'Books Online' documentation?
Where do I get MSDE?
Which database platform should I use for my ASP application?
Which tool should I use: Enterprise Manager or Query Analyzer?
Why are there gaps in my IDENTITY / AUTOINCREMENT column?
Why can I not 'open a database created with a previous version...'?
Why can't I access a database or text file on another server?
Why can't I use the TOP keyword?
Why do I get 'Argument data type text is invalid for argument [...]'?
Why do I get 'Not enough space on temporary disk' errors?
Why does ASP give me ActiveX errors when connecting to a database?
Should I use COALESCE() or ISNULL()?
Where can I get basic info about using stored procedures?