Question

I need to count all distinct values from table LDS where status = 'ok' AND date >= '2012-01-01' AND if there are multiple identical IDs then only check the one with the newest date: if this id has a status of “ok” then count it.

Table LDS:

ID | STATUS  | DATE
1  | ok      | 2012-01-01
2  | bad     | 2012-01-01
1  | bad     | 2012-02-02
3  | ok      | 2012-01-01
4  | ok      | 1999-01-01

Result should be “1” (ID 3)

Answer 1

I’m assuming that since you want to count distinct IDs, that it doesn’t matter which ID is counted in the case of duplicates, since this doesn’t affect the final count:

EDIT: Updated query to exclude any ID which has a subsequent 'bad' status:

SELECT COUNT(DISTINCT lds1.ID)
FROM 
    LDS lds1
    LEFT JOIN LDS lds2
        ON lds1.ID = lds2.ID
        AND lds1.Date < lds2.Date
        AND lds2.Status = 'bad'
WHERE 
    lds1.Date > '2012-01-01'
    AND lds1.Status = 'ok'
    AND lds2.ID IS NUL