Performing Count() and Avg() within a Case statement - tsql

Question

I am using this CASE statement to show a total count of visits and average length of visit grouped by age groups:
CASE WHEN AgeCalcSort = 0 AND AgeCalcSort <= 1 then (
Count(VisitID),
AVG(DATEDIFF(dd,StartDate,EndDate ))
)
WHEN AgeCalcSort >= 2 AND AgeCalcSort <= 17 then (
Count(VisitID),
AVG(DATEDIFF(dd,StartDate,EndDate ))
)
WHEN AgeCalcSort >= 18 AND AgeCalcSort <= 64 then (
Count(VisitID),
AVG(DATEDIFF(dd,StartDate,EndDate ))
)
WHEN AgeCalcSort >= 65 then (
Count(VisitID),
AVG(DATEDIFF(dd,StartDate,EndDate ))
)
END,
The result should look like this:
Age 1 Count Age 1 Avg LOS Age 2 Count Age 1 Avg LOS Age 3 Count Age 3 Avg LOS Age 4 Count Age 4 Avg LOS
5 5.3 18 9.2 20 12 0 0
Can anyone adivse what am I doing wrong, or a better way to achieve the end result? Thanks In advance.

Answer 1

You have mentioned 2 points:
1) Can anyone adivse what am I doing wrong
You are using the CASE expression in your query incorrectly.
You cannot have 2 columns returned when some condition is satisfied in CASE statement.
As per Microsoft documentation, CASE evaluates a list of conditions and returns one of multiple possible result expressions.
Check this Microsoft white paper for further study Ref. URL
2) a better way to achieve the end result
There can be many ways to achieve this.
Below is one of the simple way. try the query below:
SELECT
SUM(CASE WHEN AgeCalcSort = 0 AND AgeCalcSort <= 1 THEN 1 ELSE 0 END) [Age 1 Count],
AVG(CASE WHEN AgeCalcSort = 0 AND AgeCalcSort <= 1 THEN DATEDIFF(DAY,StartDate,EndDate) ELSE NULL END) [Age 1 Avg LOS]
SUM(CASE WHEN AgeCalcSort = 2 AND AgeCalcSort <= 17 THEN 1 ELSE 0 END) [Age 2 Count],
AVG(CASE WHEN AgeCalcSort = 2 AND AgeCalcSort <= 17 THEN DATEDIFF(DAY,StartDate,EndDate) ELSE NULL END) [Age 2 Avg LOS]
SUM(CASE WHEN AgeCalcSort = 18 AND AgeCalcSort <= 64 THEN 1 ELSE 0 END) [Age 3 Count],
AVG(CASE WHEN AgeCalcSort = 18 AND AgeCalcSort <= 64 THEN DATEDIFF(DAY,StartDate,EndDate) ELSE NULL END) [Age 3 Avg LOS]
SUM(CASE WHEN AgeCalcSort >= 65 THEN 1 ELSE 0 END) [Age 4 Count],
AVG(CASE WHEN AgeCalcSort >= 65 THEN DATEDIFF(DAY,StartDate,EndDate) ELSE NULL END) [Age 4 Avg LOS]
FROM [YourTableName]
Query explanation
I have achieved count by using SUM, if you look into any of count logic you will notice it says when it satisfies condition 1 else 0. and that is summed later using SUM. So that all 1's ultimately gives count.
for average you will notice that then condition is not satisfied I am using NULL, which is intentional, to avoid wrong math.
for in TSQL query AVG of (2,4,NULL) is 3, while AVG of (2,4,0) is 2. so in your case NULL will help to avoid messing average value.

Answer 2

A case expression returns a single value, which you may then count or average.
It is not possible to produce multiple columns of output from a singe case expression. So, you need one case expression for each wanted column of output, like this:
select
COUNT(CASE WHEN AgeCalcSort = 0 AND AgeCalcSort <= 1 then VisitID end)
, AVG(CASE WHEN AgeCalcSort = 0 AND AgeCalcSort <= 1 then DATEDIFF(dd,StartDate,EndDate ) end)
, COUNT(CASE WHEN AgeCalcSort = 2 AND AgeCalcSort <= 17 then VisitID end)
, AVG(CASE WHEN AgeCalcSort = 2 AND AgeCalcSort <= 17 then DATEDIFF(dd,StartDate,EndDate ) end)
, COUNT(CASE WHEN AgeCalcSort = 18 AND AgeCalcSort <= 64 then VisitID end)
, AVG(CASE WHEN AgeCalcSort = 18 AND AgeCalcSort <= 64 then DATEDIFF(dd,StartDate,EndDate ) end)
, COUNT(CASE WHEN AgeCalcSort = 65 then VisitID end)
, AVG(CASE WHEN AgeCalcSort = 65 then DATEDIFF(dd,StartDate,EndDate ) end)
from ...
Notes:
place each case expression inside the relevant aggregate function
the count() function ignores NULLs so if the when conditions are not met the count will not increment
whilst T-SQL documentation can sometimes use the term "case statement" technically that is incorrect; e.g. a complete select query is a "statement", and "expressions" evaluate to single values, hence the correct term is case expression