Sql Server equivalent of a COUNTIF aggregate function

I'm building a query with a GROUP BY clause that needs the ability to count records based only on a certain condition (e.g. count only records where a certain column value is equal to 1).

SELECT  UID, 
        COUNT(UID) AS TotalRecords, 
        SUM(ContractDollars) AS ContractDollars,
        (COUNTIF(MyColumn, 1) / COUNT(UID) * 100) -- Get the average of all records that are 1
FROM    dbo.AD_CurrentView
GROUP BY UID
HAVING  SUM(ContractDollars) >= 500000

The COUNTIF() line obviously fails since there is no native SQL function called COUNTIF, but the idea here is to determine the percentage of all rows that have the value '1' for MyColumn.

Any thoughts on how to properly implement this in a MS SQL 2005 environment?

You could use a SUM (not COUNT!) combined with a CASE statement, like this:

SELECT SUM(CASE WHEN myColumn=1 THEN 1 ELSE 0 END)
FROM AD_CurrentView

Note: in my own test NULLs were not an issue, though this can be environment dependent. You could handle nulls such as:

SELECT SUM(CASE WHEN ISNULL(myColumn,0)=1 THEN 1 ELSE 0 END)
FROM AD_CurrentView

I usually do what Josh recommended, but brainstormed and tested a slightly hokey alternative that I felt like sharing.

You can take advantage of the fact that COUNT(ColumnName) doesn't count NULLs, and use something like this:

SELECT COUNT(NULLIF(0, myColumn))
FROM AD_CurrentView

NULLIF - returns NULL if the two passed in values are the same.

Advantage: Expresses your intent to COUNT rows instead of having the SUM() notation. Disadvantage: Not as clear how it is working ("magic" is usually bad).

I would use this syntax. It achives the same as Josh and Chris's suggestions, but with the advantage it is ANSI complient and not tied to a particular database vendor.

select count(case when myColumn = 1 then 1 else null end)
from   AD_CurrentView

How about

SELECT id, COUNT(IF status=42 THEN 1 ENDIF) AS cnt
FROM table
GROUP BY table

Shorter than CASE :)

Works because COUNT() doesn't count null values, and IF/CASE return null when condition is not met and there is no ELSE.

I think it's better than using SUM().

Adding on to Josh's answer,

SELECT COUNT(CASE WHEN myColumn=1 THEN AD_CurrentView.PrimaryKeyColumn ELSE NULL END)
FROM AD_CurrentView

Worked well for me (in SQL Server 2012) without changing the 'count' to a 'sum' and the same logic is portable to other 'conditional aggregates'. E.g., summing based on a condition:

SELECT SUM(CASE WHEN myColumn=1 THEN AD_CurrentView.NumberColumn ELSE 0 END)
FROM AD_CurrentView

Not product-specific, but the SQL standard provides

SELECT COUNT() FILTER WHERE <condition-1>,
       COUNT() FILTER WHERE <condition-2>, ...
  FROM ...

for this purpose. Or something that closely resembles it, I don't know off the top of my hat.

And of course vendors will prefer to stick with their proprietary solutions.

Why not like this?

SELECT count(1)
FROM AD_CurrentView
WHERE myColumn=1

I had to use COUNTIF() in my case as part of my SELECT columns AND to mimic a % of the number of times each item appeared in my results.

So I used this...

SELECT COL1, COL2, ... ETC
       (1 / SELECT a.vcount 
            FROM (SELECT vm2.visit_id, count(*) AS vcount 
                  FROM dbo.visitmanifests AS vm2 
                  WHERE vm2.inactive = 0 AND vm2.visit_id = vm.Visit_ID 
                  GROUP BY vm2.visit_id) AS a)) AS [No of Visits],
       COL xyz
FROM etc etc

Of course you will need to format the result according to your display requirements.

SELECT COALESCE(IF(myColumn = 1,COUNT(DISTINCT NumberColumn),NULL),0) column1,
COALESCE(CASE WHEN myColumn = 1 THEN COUNT(DISTINCT NumberColumn) ELSE NULL END,0) AS column2
FROM AD_CurrentView