SQL query to count total ocurrences of values in one column and relative occurrence in another column
This is my first post, so any general corrections to format/content are also welcome. I'm relatively new to SQL.
Say I have a database which collects test results from an classification evaluation. I know what the expected outcome is for each test. I also have a column indicating whether the test was successful, ie the expected value returned matched the expected value. It looks something like this:
Expected_Result Result Success
A A True
A B False
B B True
A A True
B A False
I know I can return the total occurrences of each expected type with
SELECT Expected_Result, COUNT(Expected_Result) FROM Evaluation_Results GROUP BY Expected_ResultI know how to count the number of false detections for a specific expected outcome with
SELECT COUNT(*) FROM Evaluation_Results WHERE Success = 'True' AND Expected_Result = 'A'Where I'm struggling is combining the two. I would like the query to return a list of all distinct expected results, the total of each, the count of successful results, and the percentage of the total, like so:
Expected_Result Total Num_Successful Success_Rate
A 3 2 66.67
B 2 1 50.00
You could use a CASE expression to perform a condition check during aggregation. A case statement identifies a conditional outcome. For instance you could use:
select evaluation_result
, count(*) AS total
, sum(case when success='true' and result='a' then 1 else 0 end) AS num_successful
, sum(case when success='true' and result='a' then 1 else 0 end)/count(*) AS success_rate
from evaluation_results group by evaluation_result;
Basically what's happening there is you're taking a count(*) of all grades, a sum() of a 1 or 0 based on a conditional outcome, then performing the ratio math. There's no need for a join here. The CASE Expression is a powerful conditional statement which can be used in so many diverse ways.
Or for a more flexible solution have a look at this:
select evaluation_result
, count(*) AS total
, sum(case when success='true' and result=evaluation_result then 1 else 0 end) AS num_successful
, sum(case when success='true' and result=evaluation_result then 1 else 0 end)/count(*) AS success_rate
from evaluation_results group by evaluation_result;