Need to join 2 columns into only one using PostgreSQL

Question

I need to join 2 columns into only one as follows.

Please read postgresql-performance then edit your question and provide the missing information. — user330315
– user330315, Commented Jan 10, 2018 at 19:31

ScaisEdge · Accepted Answer · 2018-01-10 18:59:38Z

3

You could use union

If both the columns are in the same table

select connection from my_table
union 
select followed_id from my_table

or change the table name if are in different tables

select connection from my_table1
union 
select followed_id from my_table2

answered Jan 10, 2018 at 18:59

ScaisEdge

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Ward · Accepted Answer · 2018-01-10 18:59:31Z

2

select * from (
    select follower_id from T
    union
    select followed_id from T
)

answered Jan 10, 2018 at 18:59

Ward

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Radim Bača · Accepted Answer · 2018-01-10 19:09:00Z

1

If the values are in one table then use cross join with lateral. It uses just one table scan.

SELECT v.*
FROM   table, LATERAL (
   VALUES
      (follower_id )
    , (followed_id)  -- data types must be compatible
   ) v ("connections")

similar question: Select distinct on multiple columns

edited Jan 10, 2018 at 19:09

answered Jan 10, 2018 at 19:05

Radim Bača

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2 Comments

Juan Carlos Oropeza Over a year ago

How is this different than UNION ?

Radim Bača Over a year ago

@JuanCarlosOropeza I believe it uses just one table scan

Tim OMalley · Accepted Answer · 2018-01-10 19:50:44Z

0

Union works, I would also add "order by"

select a.follower_id "Connections" 
from table_name a 
union 
select b.followed_id
from table_name b
order by "Connections"

answered Jan 10, 2018 at 19:50

Tim OMalley

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