Running Total until specific condition is true - sql

Question

I'm having a table representing the dealers cards and their rank. I'm now trying to make a query (as fast as possible) to set status on the game.
(As said before, only the dealer cards is shown)
W = Win
S = Stand
L = Loss
B = Blackjack (in two cards)
About the rules:
The dealer wins at 21, if it's in two cards its blackjack. If the rank is between 17 and 20 it's S = stand. Over 21 is a loss.
Ranks:
1 (ACE) - 1 or 11 rank. Counted as 11.
2-10 - 2-10 rank
11-13 (knight - king) - 10 rank
╔════╦══════╦════════╗
║ Id ║ Rank ║ Status ║
╠════╬══════╬════════╣
║ 1 ║ 1 ║ ║
║ 2 ║ 5 ║ ║
║ 3 ║ 8 ║ L ║ //24 = Loss
║ 4 ║ 3 ║ ║
║ 5 ║ 1 ║ ║
║ 6 ║ 7 ║ W ║ //21 = Win
║ 7 ║ 10 ║ ║
║ 8 ║ 1 ║ B ║ //21 = Blackjack
║ 9 ║ 10 ║ ║
╚════╩══════╩════════╝
I've tried to use a counter to check if it's blackjack and then I'm using a "RunningPoint" to check the sum of the cards.
I have now a solution bad it shows very bad performance when it's a lot of data. How would you do this and what can I do to optimize my query? When using more data I also need to use option (maxrecursion 0)
(When having 1 million rows it's not even possible to run this...)
My example: http://sqlfiddle.com/#!6/3855e/1

Answer 1

There's no efficient solution using plain SQL (including Windowed Aggregate Functons), at least nobody found one, yet :-)
Your recursive query performs bad because it's way too complicated, this is a simplified version:
Edit: Fixed the calculation (Fiddle)
WITH ctePoints AS
(
SELECT 1 AS id
,rank
,CASE
WHEN rank >= 10 THEN 10
WHEN rank = 1 THEN 11
ELSE rank
END AS Point
,1 AS Counter
FROM dbo.BlackJack
WHERE Id = 1
UNION ALL
SELECT t2.Id
,t2.rank
,CASE WHEN t1.Point < 17 THEN t1.Point ELSE 0 END
+ CASE
WHEN t2.rank >= 10 THEN 10
WHEN t2.rank = 1 THEN 11
ELSE t2.rank
END AS Point
,CASE WHEN t1.Point < 17 THEN t1.Counter + 1 ELSE 1 END AS Counter
FROM dbo.BlackJack AS t2
INNER JOIN ctePoints AS t1 ON t2.Id = t1.Id + 1
)
SELECT ctepoints.*
,CASE
WHEN Point < 17 THEN ''
WHEN Point < 20 THEN 'S'
WHEN Point > 21 THEN 'L'
WHEN Point = 21 AND Counter = 2 THEN 'B'
ELSE 'W'
END AS DealerStatus
FROM ctePoints
It's probably still too slow, because it processes row by row.
I usually use recursive SQL to replace cursor logic (because in my DBMS it's usually much faster) but a cursor update might actually be faster (Demo):
CREATE TABLE #BlackJack
(
id INT PRIMARY KEY CLUSTERED
,Rank INT
,DealerStatus CHAR(1)
);
insert into #BlackJack (Id, Rank)
values
(1, 1),(2, 5), (3, 8), (4, 3), (5, 1), (6, 7), (7, 10), (8, 1),(9, 10), (10, 10), (11,1);
DECLARE #Counter INT = 0
,#Point INT = 0
,#id int
,#Rank int
,#DealerStatus char(1)
DECLARE c CURSOR
FOR
SELECT id, Rank
FROM #BlackJack
ORDER BY id FOR UPDATE OF DealerStatus
OPEN c
FETCH NEXT FROM c INTO #id, #Rank
WHILE ##FETCH_STATUS = 0
BEGIN
SET #counter = #counter + 1
SET #Rank = CASE
WHEN #Rank >= 10 THEN 10
WHEN #Rank = 1 THEN 11
ELSE #Rank
END
SET #Point = #Point + #Rank
SET #DealerStatus = CASE
WHEN #Point < 17 THEN ''
WHEN #Point < 20 THEN 'S'
WHEN #Point > 21 THEN 'L'
WHEN #Point = 21 AND #Counter = 2 THEN 'B'
ELSE 'W'
END
IF #Point >= 17
BEGIN
UPDATE #BlackJack
SET DealerStatus = #DealerStatus
WHERE CURRENT OF c;
SET #Point = 0
SET #Counter = 0
END
FETCH NEXT FROM c INTO #id, #Rank
END
CLOSE c
DEALLOCATE c
SELECT * FROM #BlackJack ORDER BY id
Still #lad2025's "quirky update" is the fastest way to get the expected result, but it's using an undocumented feature and if a Service Pack breaks it there's no way to complain about it :-)

Answer 2

This solution is based on quirky update. More info here.
LiveDemo
Data and structures:
CREATE TABLE #BlackJack
(
id INT
,Rank INT
,running_total INT
,result NVARCHAR(100)
);
CREATE CLUSTERED INDEX IX_ROW_NUM ON #BlackJack(id);
insert into #BlackJack (Id, Rank)
values (1, 1),(2, 5), (3, 8), (4, 3), (5, 1),
(6, 7), (7, 10), (8, 1),(9, 10), (10, 10), (11,1);
Main query:
DECLARE #running_total INT = 0
,#number_of_cards INT = 0
,#prev_running_total INT = 0;
UPDATE #BlackJack
SET
#prev_running_total = #running_total
,#running_total = running_total = IIF(#running_total >= 20, 0, #running_total)
+ CHOOSE(Rank,11,2,3,4,5,6,7,8,9,10,10,10,10)
,result = CASE WHEN #running_total = 20 THEN 'S'
WHEN #running_total = 21 AND #number_of_cards = 2 THEN 'B'
WHEN #running_total = 21 THEN 'W'
WHEN #running_total > 21 THEN 'L'
ELSE NULL
END
,#number_of_cards = IIF(#prev_running_total >= 20, 0, #number_of_cards) + 1
FROM #BlackJack WITH(INDEX(IX_ROW_NUM))
OPTION (MAXDOP 1);
SELECT *
FROM #BlackJack
ORDER BY id;
Warning
If you use SQL Server < 2012 you need to replace IIF and CHOOSE with CASE. I don't check all Blackjack rules, only for provided sample. If something is wrong feel free to change CASE logic.
Second I extend base table BlackJack with auxiliary columns, but you can create any new table, if needed.
The key point is to read data sequentially based on clustered key ascending and do not allow parallel execution. Before you use it in production check how it behaves with large data set.