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UNION but avoid duplicated data

Posted on 2010-09-10
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Last Modified: 2012-05-10
Hi,

I have two tables : patients and patients_guest.

I would like to have the patients who have a lastname starting by X (parameter) in both tables. I think that's a simple UNION with: WHERE P.lastname LIKE "' . $x . '%"
The problem is that a patient may be in both tables. I need to check that there is only one lastname-firstname-birthDate returned (note that there may be two patients with same lastname and firstname, that's why they have to have the same birthday to be considered as duplicated).

The fields to return are :
id
lastname
firstname
date_of_birth
is_guest
(this must be generated, should be true if the record is from patients_guest, false otherwise).

Thank you for any help
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Question by:matthew016
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Expert Comment

by:cyberkiwi
Comment Utility
Just change UNION to UNION ALL
It will remove the duplicates
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Assisted Solution

by:packratt_jk
packratt_jk earned 100 total points
Comment Utility
select distinct id, lastname, firstname, date_of_birth from (
select distinct id, lastname, firstname, date_of_birth from table1
union
select distinct id, lastname, firstname, date_of_birth from table2
)
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Accepted Solution

by:
cyberkiwi earned 400 total points
Comment Utility
This will give you no duplicates, but when there are duplicates, it will RANDOMLY/ARBITRARILY show one of the two IDs and one of the is_guest values.

select lastname,firstname,date_of_birth, id, is_guest
from
(
select id,lastname,firstname,date_of_birth,'false' is_guest
from patients
union
select id,lastname,firstname,date_of_birth,'true' is_guest
from patients_guest
) both
group by lastname,firstname,date_of_birth
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Author Comment

by:matthew016
Comment Utility
how will UNION ALL know that duplicated data is data with same lastname, firstname and date_of_birth (id may be differnet) ?
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Author Comment

by:matthew016
Comment Utility
@cyberkiwi

Thanks, is it possible to have the record from table "patients" when there is a duplicate, and not from "patients_guest" ?

Maybe a AND NOT EXISTS or something should be added to the second part of the union ?
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Expert Comment

by:cyberkiwi
Comment Utility
Because the UNION is a "virtual" dataset, this very complicated query is needed to properly return just 1 row from duplicates.
But even this is not perfect, if you have

firstname, lastname, date_of_birth AND id duplicated in both tables, the query will give you both rows since it cannot distinguish between the two.
select B.*

from

(

select min(id) min_id, lastname,firstname,date_of_birth

from

(

select id,lastname,firstname,date_of_birth,'false' is_guest

from patients

union

select id,lastname,firstname,date_of_birth,'true' is_guest

from patients_guest

) both

group by lastname,firstname,date_of_birth

) A

inner join

(

select id,lastname,firstname,date_of_birth,'false' is_guest

from patients

union

select id,lastname,firstname,date_of_birth,'true' is_guest

from patients_guest

) B on A.lastname=B.lastname and a.date_of_birth=b.date_of_birth and a.firstname=b.firstname and A.min_id=B.id

Open in new window

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Expert Comment

by:cyberkiwi
Comment Utility
> Thanks, is it possible to have the record from table "patients" when there is a duplicate, and not from "patients_guest" ?
Brilliant - that makes it easier if there is a preference.

FYI - I just woke up - UNION is the one that removes duplicates and UNION ALL does not.
-- all patients

-- patients_guest only when not already in patients by (lastname,firstname,date_of_birth)

select id,lastname,firstname,date_of_birth,'false' is_guest

from patients

union all

select A.id,A.lastname,A.firstname,A.date_of_birth,'true' is_guest

from patients_guest A

left join patients B on A.lastname=B.lastname and a.date_of_birth=b.date_of_birth and a.firstname=b.firstname

where B.id is null

Open in new window

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Author Comment

by:matthew016
Comment Utility
Thanks,

So what should I use in my case, UNION or UNION ALL ?

I'll post a second question about a problem with the data returned, this is enough for now :)
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Expert Comment

by:cyberkiwi
Comment Utility
Normally if you want duplicate data (defined as begin equal on every single column) removed, use UNION.
However it has a cost, since it is performing a DISTINCT filter.

In the case of the last query presented above, we already know (by design) that the two parts are distinct, so using UNION ALL is just slightly faster to avoid the additional duplicate filtering process.
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