Question

I have two tables that I am trying to join in a query, but I’m having some difficulty getting the required output.

Table: Scans

ScanId  ScanTime
1        8:00
2        8:15
3        9:00
4        9:30
6       10:00
7       10:45
8       11:00
9       11:10

Table: Responses

ScanId  RespA  RespB
3       ABC    X
7       DEF    Y
9       GHI    Z

There is a foreign key on Responses.ScanId that references Scans.ScanId.

The Responses table may or may not have a corresponding row for each row in the Scans table.

What I need to do is produce the output shown below, each row in the Scans table is returned with the most recent previous response values.

For Scans.ScanId 1 and 2, there is no previous response so the Response columns are null.

For Scans.ScanId 3, 4 and 6, the most recent previous RespA is ABC and RespB is X (ScanId=3)

For Scans.ScanId 7 and 8, the most recent previous RespA is DEF and RespB is Y (ScanId=7)

For Scan.ScanId 9, the most recent previous RespA is GHI and RespB is Z (ScanId=9)

Desired Output:

ScanId  ScanTime  RespScanId  RespA  RespB
1        8:00     NULL        NULL   NULL
2        8:15     NULL        NULL   NULL
3        9:00     3           ABC    X
4        9:30     3           ABC    X
6       10:00     3           ABC    X
7       10:45     7           DEF    Y
8       11:00     7           DEF    Y
9       11:10     9           GHI    Z

I’m having difficulty figuring out how to write the join clause for this one. It needs to run on Sql Server 2005 and above.

Answer 1

Here’s a solution using a CTE…

DECLARE @Scans TABLE (
    ScanID INT,
    ScanTime DATETIME
)

DECLARE @Responses TABLE (
    ScanID INT, 
    RespA VARCHAR(50),
    RespB VARCHAR(50)
)

INSERT INTO @Scans VALUES (1, '8:00')
INSERT INTO @Scans VALUES (2, '8:15')
INSERT INTO @Scans VALUES (3, '9:00')
INSERT INTO @Scans VALUES (4, '9:30')
INSERT INTO @Scans VALUES (6, '10:00')
INSERT INTO @Scans VALUES (7, '10:45')
INSERT INTO @Scans VALUES (8, '11:00')
INSERT INTO @Scans VALUES (9, '11:10')

INSERT INTO @Responses VALUES (3, 'ABC', 'X')
INSERT INTO @Responses VALUES (7, 'DEF', 'Y')
INSERT INTO @Responses VALUES (9, 'GHI', 'Z')

;WITH ResponsesScan AS
(
    SELECT
        r.*, s.ScanTime
    FROM
        @Responses r
        JOIN @Scans s ON s.ScanID = r.ScanID
)
SELECT
    s.ScanID,
    s.ScanTime,
    rs.ScanID AS RespScanId,
    rs.RespA,
    rs.RespB
FROM
    @Scans s 
    LEFT JOIN ResponsesScan rs
        ON rs.ScanID = (
            SELECT TOP 1 ScanID
            FROM ResponsesScan
            WHERE ScanTime <= s.ScanTime
            ORDER BY ScanTime DESC
        )