SQL select from table with an update subquery - sql-server

Question

I'm trying to update a table and at the same time (in the same transaction) select from the table and perform a count on one of its columns.
See below for example code. The last 2 querys (the UPDATE and the SELECT) I want to merge them into one query somehow.
CREATE TABLE ChildProcesses
(
Id INT IDENTITY(1,1) PRIMARY KEY,
ParentId INT,
ProcessIsFinished BIT
);
INSERT INTO ChildProcesses (ParentId, ProcessIsFinished) VALUES (1234, 0);
INSERT INTO ChildProcesses (ParentId, ProcessIsFinished) VALUES (1234, 0);
INSERT INTO ChildProcesses (ParentId, ProcessIsFinished) VALUES (1234, 0);
INSERT INTO ChildProcesses (ParentId, ProcessIsFinished) VALUES (1234, 0);
UPDATE ChildProcesses SET ProcessIsFinished = 1 WHERE Id = 4;
SELECT
COUNT(*) AS CountProcessesStillRunning
FROM
ChildProcesses
WHERE
ParentId = 1234 AND
ProcessIsFinished = 0;
The reason for all of this is, I have a distributed process where my code is running in many places. This code performs processing on my "ChildProcesses" and when they're done they set the "ChildProcesses.ProcessIsFinished" to "1". When all of the child processes have finished I need to perform some cleanup steps, but only when ALL of the child processes have finished and I only want to perform the cleanup steps once. So I'm trying to find a way to both "mark the process as finished" and at the same time "check to see if this was the last child process to finish". If this is the last child process to finish then i'll have that child process perform the cleanup steps.
I thought to have the UPDATE query as a subquery to the SELECT, but I get errors saying that's not allowed. Here's what I tried that's not working:
SELECT
COUNT(*) AS CountProcessesStillRunning
FROM
ChildProcesses
WHERE
ParentId = (UPDATE ChildProcesses SET ProcessIsFinished = 1 OUTPUT INSERTED.ParentId WHERE Id = 4) AND
ProcessIsFinished = 0;
Any help would be appreciated.

Answer 1

Took inspiration from #CharlieFace on this one.
I was originally trying to SELECT with a subquery having UPDATE...OUTPUT, which isn't allowed.
However, I can UPDATE...OUTPUT...INTO a temporary table. So I decided to UPDATE all ChildProcesses but use a CASE to just update my particular ChildId. Then output all of those modified entries into a temporary table. This temporary table I could then compute if all child processes had completed or if there were still running child processes.
Here's the new query.
DECLARE #ParentId INT = 1234;
DECLARE #ChildId INT = 1;
DECLARE #Output TABLE (Id INT, ParentId INT, ProcessIsFinished BIT);
UPDATE
ChildProcesses
SET
ProcessIsFinished =
CASE Id
WHEN #ChildId THEN 1 -- update only 'this' child process
ELSE ProcessIsFinished -- leave the other entries alone
END
OUTPUT
INSERTED.*
INTO
#Output
WHERE
ParentId = #ParentId;
SELECT
COUNT(*) As StillHasRunningChildProcesses
FROM
#Output
WHERE
ProcessIsFinished = 0;

Answer 2

You cannot nest UPDATE...OUTPUT except inside a INSERT...SELECT, you are not allowed a bare SELECT and you are not allowed to group.
Instead you can use a table variable
DECLARE #output TABLE (ParentId int);
UPDATE ChildProcesses
SET ProcessIsFinished = 1
OUTPUT inserted.ParentId
INTO #output (ParentId)
WHERE Id = 4;
SELECT
COUNT(*) AS CountProcessesStillRunning
FROM
ChildProcesses cp
JOIN
#output u ON u.ParentId = cp.ParentId
WHERE
cp.ProcessIsFinished = 0;
To avoid race conditions, you might want a transaction. To enure you lock the ParentId rows that you are going to read afterwards, use a SELECT with a UPDLOCK and a SERIALIZABLE hint
SET XACT_ABORT, NOCOUNT ON;
BEGIN TRAN;
DECLARE #dummy int;
SELECT
#dummy = COUNT(*)
FROM
ChildProcesses cp
JOIN ChildProcesses cp2 WITH (UPDLOCK, SERIALIZABLE)
ON cp2.ParentId = cp.ParentId
WHERE
cp.Id = 4;
DECLARE #output TABLE (ParentId int);
UPDATE ChildProcesses WITH (SERIALIZABLE)
SET ProcessIsFinished = 1
OUTPUT inserted.ParentId
INTO #output (ParentId)
WHERE Id = 4;
SELECT
COUNT(*) AS CountProcessesStillRunning
FROM
ChildProcesses cp
JOIN
#output u ON u.ParentId = cp.ParentId
WHERE
cp.ProcessIsFinished = 0;
COMMIT;
If ParentId is actually the primary key for this table, and you just want the number of rows affected then you can just use ##ROWCOUNT. It doesn't seem that you actually want this logic, but you should get the idea.
UPDATE ChildProcesses
SET ProcessIsFinished = 1
WHERE Id = 4
AND ProcessIsFinished = 0;
SELECT
##ROWCOUNT AS CountProcessesStillRunning;