Delete rows where date was least updated - sql

Question

How can I delete rows where dateupdated was least updated ?
My table is
Name Dateupdated ID status
john 1/02/17 JHN1 A
john 1/03/17 JHN2 A
sally 1/02/17 SLLY1 A
sally 1/03/17 SLLY2 A
Mike 1/03/17 MK1 A
Mike 1/04/17 MK2 A
I want to be left with the following after the data removal:
Name Date ID status
john 1/03/17 JHN2 A
sally 1/03/17 SLLY2 A
Mike 1/04/17 MK2 A

Answer 1

If you really want to "delete rows where dateupdated was least updated" then a simple single-row subquery should do the trick.
DELETE MyTable
WHERE Date = (SELECT MIN(Date) From MyTable)
If on the other hand you just want to delete the row with the earliest Date per person (as identified by their ID) you could use:
DELETE MyTable
FROM MyTable a
JOIN (SELECT ID, MIN(Date) MinDate FROM MyTable GROUP BY ID) b
ON a.ID = b.ID AND a.Date = b.MinDate
The idea here is you create an aggregate query that returns rows containing the columns that would match the rows you want deleted, then join to it. Because it's an inner join, rows that do not match the criteria will be excluded.
If people are uniquely identified by something else (e.g. Name then you can just substitute that for the ID in my example above.
I am thinking though that you don't want either of these. I think you want to delete everything except for each person's latest row. If that is the case, try this:
DELETE MyTable
WHERE EXISTS (SELECT 0 FROM MyTable b WHERE b.ID = MyTable.ID AND b.Date > MyTable.Date)
The idea here is you check for existence of another data row with the same ID and a later date. If there is a later record, delete this one.
The nice thing about the last example is you can run it over and over and every person will still be left with exactly one row. The other two queries, if run over and over, will nibble away at the table until it is empty.
P.S. As these are significantly different solutions, I suggest you spend some effort learning how to articulate unambiguous requirements. This is an extremely important skill for any developer.

Answer 2

This deletes rows where the name is a duplicate, and deletes all but the latest row for each name. This is different from your stated question.
Using a common table expression (cte) and row_number():
;with cte as (
select *
, rn = row_number() over (
partition by Name
order by Dateupdated desc
)
from t
)
/* ------------------------------------------------
-- Remove duplicates by deleting rows
-- where the row number (rn) is greater than 1
-- leaving the first row for each partition
------------------------------------------------ */
delete
from cte
where cte.rn > 1
select * from t
rextester: http://rextester.com/HZBQ50469
returns:
+-------+-------------+-------+--------+
| Name | Dateupdated | ID | status |
+-------+-------------+-------+--------+
| john | 2017-01-03 | JHN2 | A |
| sally | 2017-01-03 | SLLY2 | A |
| Mike | 2017-01-04 | MK2 | A |
+-------+-------------+-------+--------+
Without using the cte it can be written as:
delete d
from (
select *
, rn = row_number() over (
partition by Name
order by Dateupdated desc
)
from t
) as d
where d.rn > 1

Answer 3

This should do the trick:
delete
from MyTable a
where not exists (
select top 1 1
from MyTable b
where b.name = a.name
and b.DateUpdated < a.DateUpdated
)
i.e. remove any entries from the table for which there is no record on the same name with a date earlier than the record to be deleted's.

Answer 4

Your Name column has Mike and Mik2 which is different for each other.
So, if you did not make a mistake, standard column to group by must be ID column without last digit.
I think following is more accurate if you did not mistaken.
delete a
from MyTable a
inner join
(select substring(ID, 1, len(ID) - 1) as ID, min(Dateupdated) as MinDate
from MyTable
group by substring(ID, 1, len(ID) - 1)
) b
on substring(a.ID, 1, len(a.ID) - 1) = b.ID and a.Dateupdated = b.MinDate
You can test it at SQLFiddle: http://sqlfiddle.com/#!6/9c440/1