sql server - SQL - LAG to get previous value if condition using multiple previous columns satisfied

Question

I have a table created by:

CREATE TABLE #test_table 
(
id INT
,EventName VARCHAR(50)
,HomeTeam VARCHAR(25)
,Metric INT
)

INSERT INTO #test_table VALUES
(1, 'Team A vs Team B', 'Team A', 5),
(2, 'Team A vs Team B', 'Team A', 7),
(3, 'Team C vs Team D', 'Team C', 6),
(4, 'Team Z vs Team A', 'Team Z', 8),
(5, 'Team A vs Team B', 'Team A', 9),
(6, 'Team C vs Team D', 'Team C', 3),
(7, 'Team C vs Team D', 'Team C', 1),
(8, 'Team E vs Team F', 'Team E', 2)

Which results in:

id  EventName           HomeTeam    Metric
------------------------------------------
1   Team A vs Team B    Team A      5
2   Team A vs Team B    Team A      7
3   Team C vs Team D    Team C      6
4   Team Z vs Team A    Team Z      8
5   Team A vs Team B    Team A      9
6   Team C vs Team D    Team C      3
7   Team C vs Team D    Team C      1
8   Team E vs Team F    Team E      2

A want to calculate a new column PreviousMetricN where N can be 1, 2, 3, ... which shows the previous value for Metric, but only if the HomeTeam was involved in the previous event. For example:

id  EventName           HomeTeam    Metric  PreviousMetric1 PreviousMetric2
------------------------------------------------------------------------
1   Team A vs Team B    Team A      5       NULL            NULL
2   Team A vs Team B    Team A      7       5               NULL
3   Team C vs Team D    Team C      6       NULL            NULL
4   Team Z vs Team A    Team Z      8       NULL            NULL
5   Team A vs Team B    Team A      9       8               7
6   Team C vs Team D    Team C      3       6               NULL
7   Team C vs Team D    Team C      1       3               6
8   Team E vs Team F    Team E      2       NULL            NULL

I have been trying variations of LAG with a new grouping variable in the PARTITION BY clause such as

LAG(Metric) OVER(Partition by (CASE WHEN CHARINDEX(HomeTeam, EventName)>0 THEN 1 ELSE 0 END) ORDER BY id)

but without any success. How can this be done?

question from:https://stackoverflow.com/questions/65905861/sql-lag-to-get-previous-value-if-condition-using-multiple-previous-columns-sat

Answer 1

I see no answer here that uses window functions and a single scan of the table. We can do this query in a single scan as follows:

Let us assume you have the AwayTeam in another column.

If you don't have this yet and you wanted to parse it out of EventData:
We could use: SUBSTRING(EventData, CHARINDEX(' vs ', EventData) + 4)
I urge you to follow proper normalization and create this as a proper column in your table.

Our algorithm runs like this:

1. Multiply out (unpivot) the two teams as separate rows, using CROSS APPLY
2. Calculate the previous Metrics using LAG, partitioning by the merged Team column
3. Filter back down the doubled up rows, so that we only get a single row for each of our original ones

SELECT id, HomeTeam, AwayTeam, Metric, Prev1, Prev2, Prev3
FROM (

  SELECT *
    ,Prev1 = LAG(Metric, 1) OVER (PARTITION BY v.Team ORDER BY id)
    ,Prev2 = LAG(Metric, 2) OVER (PARTITION BY v.Team ORDER BY id)
    ,Prev3 = LAG(Metric, 3) OVER (PARTITION BY v.Team ORDER BY id)
    -- more of these ......
  FROM test_table
  CROSS APPLY (VALUES (HomeTeam, 1),(AwayTeam, 0)) AS v(Team,IsHome)
) AS t

WHERE IsHome = 1
-- ORDER BY id  --if necessary

Importantly, we can do this without the use of multiple different sorts, partitions or ordering, and without the use of a self-join. Just a single scan.

Result:

id	HomeTeam	AwayTeam	Metric	Prev1	Prev2	Prev3
1	Team A	Team B	5	(null)	(null)	(null)
2	Team A	Team B	7	5	(null)	(null)
3	Team C	Team D	6	(null)	(null)	(null)
4	Team Z	Team A	8	(null)	(null)	(null)
5	Team A	Team B	9	8	7	5
6	Team C	Team D	3	6	(null)	(null)
7	Team C	Team D	1	3	6	(null)
8	Team E	Team F	2	(null)	(null)	(null)