Insuring all values from a parent subquery/CTE are present in a child query

Question

Insuring all values from a parent subquery/CTE are present in a child query

DaveBriCam

SSCarpal Tunnel

Points: 4294
More actions
August 30, 2022 at 6:08 pm

#4082117
I need a random sampling of 1% of a population of last names and spoken languages in a county. The lastname does not have to correspond to the person's language, immaterial. My problem is sometimes all the last names do not appear in the results. I need a way of making sure (even if it slightly exceeds 1%) that all the names are returned. I consistently get 167 names but sometimes rarer names like 'Poindexter' fall out of the results. If I run the query again 'Poindexter' may be present but 'Hermann' drops out. Any ideas? The end data needs to look something like:
ID language lastname
1515 Spanish Heath
1620 English Edwards
1452 Korean Smith
1350 Japanese Smith
```
with cte1 -- to build set of languages - all five need to appear
as (select distinct language from county.reporting.dbo.2022),

    cte2 -- to build set of lastnames - all 35 must appear, some can be multiples
as (select distinct lastname from county.reporting.dbo.2022),

   cte3 -- build final dataset prior to sampling
as (
    select * from county.reporting.dbo.2022
    where exists (select * from cte1)

    union 

    select * from county.reporting.dbo.2022
    where exists (select * from cte2)
)

select top 1 PERCENT * from cte3 order by newID();
```
- This topic was modified 1 year, 8 months ago by DaveBriCam.

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Johan Bijnens SSC Guru Points: 135265 More actions · Answer 1

First problem:

This cte will return all rows of county.reporting.dbo.2022 whenever there are results in cte1 ! ( no where clause in exists query )

Same for the second union part

deduplicated by the fact that you are using UNION in stead of UNION ALL.

cte3 -- build final dataset prior to sampling
as (
    select * from county.reporting.dbo.2022
    where exists (select * from cte1)

    union 

    select * from county.reporting.dbo.2022
    where exists (select * from cte2)
)

That's the logic of 1% of a set, if all languages must be present and you determine the 1% sorting a random uniqueidentifier ?

( btw since it is a linked server reference, guess how the engine will handle such query ???!!! )

This reply was modified 1 year, 8 months ago by Johan Bijnens.

Johan

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