Question

Imagine I have these columns in a table:

id int NOT NULL IDENTITY PRIMARY KEY, 
instant datetime NOT NULL,
foreignId bigint NOT NULL

For each group (grouped by foreignId) I want to delete all the rows which are 1 hour older than the max(instant). Thus, for each group the parameter is different.

Is it possible without looping?

Answer 1

DELETE
FROM    mytable
FROM    mytable mto
WHERE   instant <
        (
        SELECT  DATEADD(hour, -1, MAX(instant))
        FROM    mytable mti
        WHERE   mti.foreignid = mto.foreignid
        )

Note double FROM clause, it’s on purpose, otherwise you won’t be able to alias the table you’re deleting from.

The sample data to check:

DECLARE @mytable TABLE
        (
        id INT NOT NULL PRIMARY KEY,
        instant DATETIME NOT NULL,
        foreignID INT NOT NULL
        )

INSERT
INTO    @mytable
SELECT  1, '2009-22-07 10:00:00', 1
UNION ALL
SELECT  2, '2009-22-07 09:30:00', 1
UNION ALL
SELECT  3, '2009-22-07 08:00:00', 1
UNION ALL
SELECT  4, '2009-22-07 10:00:00', 2
UNION ALL
SELECT  5, '2009-22-07 08:00:00', 2
UNION ALL
SELECT  6, '2009-22-07 07:30:00', 2

DELETE
FROM    @mytable
FROM    @mytable mto
WHERE   instant <
        (
        SELECT  DATEADD(hour, -1, MAX(instant))
        FROM    @mytable mti
        WHERE   mti.foreignid = mto.foreignid
        )

SELECT  *
FROM    @mytable

1   2009-07-22 10:00:00.000 1
2   2009-07-22 09:30:00.000 1
4   2009-07-22 10:00:00.000 2