SQL query - Join that returns the first two records of joining table

Question

I have two tables:

Patient

• pkPatientId
• FirstName
• Surname

PatientStatus

• pk

Answer 1

Here is my attempt - It should work on SQL Server 2005 and SQL Server 2008 (Tested on SQL Server 2008) owing to the use of a common table expression:

WITH CTE AS
(
    SELECT  fkPatientId
          , StatusCode
          -- add more columns here
          , ROW_NUMBER() OVER
    (
    PARTITION BY fkPatientId ORDER BY fkPatientId desc) AS [Row_Number] 
    from PatientStatus
    where fkPatientId in
    (
        select fkPatientId
        from PatientStatus
        group by fkPatientId
        having COUNT(*) >= 2
    )
)
SELECT p.pkPatientId,
    p.FirstName,
    CTE.StatusCode  
FROM [Patient] as p
    INNER JOIN CTE
        ON p.[pkPatientId] = CTE.fkPatientId
WHERE CTE.[Row_Number] = 1 
or CTE.[Row_Number] = 2

Answer 2

I did not try but this could work;

SELECT /*(your select columns here)*/, row_number() over(ORDER BY ps.fkPatientId, ps.StartDate) as rownumber FROM Patient p INNER JOIN PatientStatus ps ON p.pkPatientId = ps.fkPatientId
where rownumber between 1 and 2

if this did not work, see this link.

Answer 3

A CTE is probably your best bet if you're in SQL Server 2005 or greater, but if you want something a little more compatible with other platforms, this should work:

SELECT
     P.pkPatientID,
     P.FirstName,
     P.LastName,
     PS1.StatusCode AS FirstStatusCode,
     PS1.StartDate AS FirstStatusStartDate,
     PS1.EndDate AS FirstStatusEndDate,
     PS2.StatusCode AS SecondStatusCode,
     PS2.StartDate AS SecondStatusStartDate,
     PS2.EndDate AS SecondStatusEndDate
FROM
     Patient P
INNER JOIN PatientStatus PS1 ON
     PS1.fkPatientID = P.pkPatientID
INNER JOIN PatientStatus PS2 ON
     PS2.fkPatientID = P.pkPatientID AND
     PS2.StartDate > PS1.StartDate
LEFT OUTER JOIN PatientStatus PS3 ON
     PS3.fkPatientID = P.pkPatientID AND
     PS3.StartDate < PS1.StartDate
LEFT OUTER JOIN PatientStatus PS4 ON
     PS4.fkPatientID = P.pkPatientID AND
     PS4.StartDate > PS1.StartDate AND
     PS4.StartDate < PS2.StartDate
WHERE
     PS3.pkPatientStatusID IS NULL AND
     PS4.pkPatientStatusID IS NULL

It does seem a little odd to me that you would want the first two statuses instead of the last two, but I'll assume that you know what you want.

You can also use WHERE NOT EXISTS instead of the PS3 and PS4 joins if you get better performance with that.

Answer 4

Ugly, but this one does not rely on uniqueness of StartDate and works on SQL 2000

select * 
from Patient p 
join PatientStatus ps on p.pkPatientId=ps.fkPatientId
where pkPatientStatusId in (
 select top 2 pkPatientStatusId 
 from PatientStatus 
 where fkPatientId=ps.fkPatientId 
 order by StartDate
) and pkPatientId in (
 select fkPatientId
 from PatientStatus
 group by fkPatientId
 having count(*)>=2
)

Answer 5

Check if your server supports windowed functions:

SELECT * 
FROM Patient p
LEFT JOIN PatientStatus ps ON p.pkPatientId = ps.fkPatientId
QUALIFY ROW_NUMBER() OVER (PARTITION BY ps.fkPatientId ORDER BY ps.StartDate) < 3

Another possibility, which should work with SQL Server 2005:

SELECT * FROM Patient p
LEFT JOIN ( 
    SELECT *, ROW_NUMBER(PARTITION BY fsPatientId ORDER by StartDate) rn
    FROM PatientStatus) ps
ON p.pkPatientId = ps.fkPatientID 
and ps.rn < 3

Answer 6

Adding this WHERE clause to the outer query of Tomalak's first solution will prevent Patients with less than 2 status records from being returned. You can also "and" it in the WHERE clause of the second query for the same results.

WHERE pkPatientId IN (
    SELECT pkPatientID 
    FROM Patient JOIN PatientStatus ON pkPatientId = fkPatientId
    GROUP BY pkPatientID HAVING Count(*) >= 2
)