Re: Please advice on a query

On Wed, Jul 11, 2018 at 9:05 AM, JORGE MALDONADO <[email protected]>
wrote:

> I have a table more or less like this:
>
> -----------------------------------------------------
> crs_id                some data                req
> -----------------------------------------------------
> 25                       aaaaaaaa                 21
> 25                       aaaaaaaa                 19
> 17                       ccccccccc                 null
> 8                         dddddddd                 null
> 19                       eeeeeeee                 15
> 19                       xxxxxxxxx                  37
> 15                       fffffffffffffffff                  10
> 10                       gggggggg                   null
>
> I need to delete rows according to the following criteria:
>
> * Row with "crs_id = 25" must be deleted (only the second row) because
> "req = 19" and there is a row with "crs_id = 19"
>
> * Row with "crs_id = 19" must be deleted (only the first row) because "req
> = 15" and there is a row with "crs_id = 15"
>
> * Row with "crs_id = 15" must be deleted because "req = 10" and there is a
> row with "crs_id = 10"
>
> I have been struggling with this issue without a successful result.
>
>
​Maybe...

DELETE FROM tbl
WHERE EXISTS (SELECT 1 FROM tbl AS cmp WHERE tbl.req_id = cmp.crs_id)
​
That probably can be written with USING though I find that syntax a bit
hard to read personally.  Might want to compare plans if performance
matters.

David J.