How to update table from another table, with 2 columns - sql

Question

I am using SQL Server database and want a way to update MachinesSummary.ShareCount.
Here are my two tables
MachinesSummary
ID Machine1 Machine2 ShareCount
-------------------------------
1 A J NULL
2 K S NULL
3 A E NULL
4 J A NULL
5 Y U NULL
6 S W NULL
7 G A NULL
8 W S NULL
The other table is MachineDetails
ProcessNo Machine
------------------
1 A
1 H
1 W
2 A
2 J
2 W
3 Y
3 K
4 J
4 A
I want to update ShareCount in the MachineSummary table with the count of processes that both Machine1 and Machine2 share.
For record 1 in the MachineSummary table, I want the number of processes both share in MachineDetails which is 1 in this case
While for record 4 the ShareCount is 2
I tried this
UPDATE M
SET ShareCount = COUNT(DISTINCT X.ProcessNo)
FROM
(SELECT ProcessNo, ',' + STRING_AGG(Machine,',') + ',' Machines
FROM MachineDetails
GROUP BY ProcessNo) X
INNER JOIN MachinesSummary M ON X.Machines LIKE '%'+ M.Machine1 + '%'
AND X.Machines LIKE '%'+ M.Machine2 + '%'
But I wonder if there is an easier high performance way
The MachineDetails table has 250 million rows.

Answer 1

Well, I would use a self-join to get the number of combinations:
UPDATE M
SET ShareCount = num_processes
FROM MachinesSummary M JOIN
(SELECT md1.Machine as machine1, md2.Machine as machine2, COUNT(*) as num_processes
FROM MachineDetails md1 JOIN
MachinesDetails md2
ON md1.processno = md2.processno
GROUP BY md1.Machine, md2.Machine
) md
ON md.Machine1 = M.machine1 AND md.Machine2 = M.machine2;

Answer 2

I would use an updatable CTE here:
WITH cte AS (
SELECT Machine, COUNT(*) AS cnt
FROM MachineDetails
GROUP BY Machine
),
cte2 AS (
SELECT ShareCount, COALESCE(t1.cnt, 0) AS m1_cnt, COALESCE(t2.cnt, 0) AS m2_cnt
FROM MachineSummary ms
LEFT JOIN cte t1 ON t1.Machine1 = ms.Machine
LEFT JOIN cte t2 ON t2.Machine2 = ms.Machine
)
UPDATE cte2
SET ShareCount = m1_cnt + m2_cnt;
The logic of the first CTE involving the MachineDetails table is to get the counts for every machine. The second CTE joins this counts CTE to the MachineSummary table twice, once for each of machine 1 and 2. Then, we update this second CTE and assign the sum of counts.