Split table in (postgreSQL) randomly 50/50 - sql

Question

I have a table with two columns in postgresql: original id and duplicate id.
Sample data:
original_id duplicate_id
1 1
2 2
3 3
4 4
5 5
6 6
I would like to randomly split this table in 50/50, so I can put a specific tag in each
Sample data:
original_id duplicate_id tag
1 1 control
2 2 treatment
3 3 treatment
4 4 control
5 5 treatment
6 6 control
What is important:
1. The selection has to be random
2. The split has to be 50/50 (or the closest to this if the number of rows is odd)

Answer 1

You can select half of the rows in a random order with this query:
select *
from my_table
order by random()
limit (select count(*)/ 2 from my_table)
Use it to tag the rows:
with control as (
select *
from my_table
order by random()
limit (select count(*)/ 2 from my_table)
)
select
*,
case when t in (select t from control t) then 'control' else 'treatment' end
from my_table t;
Working example in rextester.

Answer 2

You can use rownumber() OVER (ORDER BY random()) to assign a random number to each record. Then use it in a CASE to assign either the tag 'control' or 'treatment' depending on the number being less than (or equal) than the half of the count of rows in the table or not.
For a SELECT that looks like this:
SELECT original_id,
duplicate_id,
CASE
WHEN rn <= (SELECT count(*) / 2
FROM elbat) THEN
'control'
ELSE
'treatment'
END tag
FROM (SELECT original_id,
duplicate_id,
row_number() OVER (ORDER BY random()) rn
FROM elbat) x;
If you want an UPDATE (I'm not sure on this), assuming, that the pair of original_id and duplicate_id is unique, this could look like:
UPDATE elbat t
SET tag = CASE
WHEN rn <= (SELECT count(*) / 2
FROM elbat) THEN
'control'
ELSE
'treatment'
END
FROM (SELECT original_id,
duplicate_id,
row_number() OVER (ORDER BY random()) rn
FROM elbat) x
WHERE x.original_id = t.original_id
AND x.duplicate_id = t.duplicate_id;
db<>fiddle
(BTW, that SELECT result on the Fiddle gives a nice example, that the order of the rows returned can be totally different from the physical one, if the optimizer likes it better that way.)

Answer 3

I would use window functions:
select t.*,
(case when seqnum <= cnt / 2
then 'treatment' else 'control
end) as tag
from (select t.*,
count(*) over () as cnt,
row_number() over (order by random() as seqnum
from t
) t;
Actually, random is random. So, you don't need the count. You can use modulo arithmetic instead:
select t.*,
(case when row_number() over (order by random()) % 2 = 1
then 'treatment' else 'control'
end) as tag
from t;

Answer 4

You can make the random() generate the values 1 or 2 using the formula: (random() + 1)::int
select t.*,
case (random() + 1)::int
when 1 then 'treatment'
else 'control'
end as tag
from t;
In general, (random() * (upper_limit - 1) + lower_limit)::int will generate numbers between upper_limit and lower_limit (inclusive). If upper limit is 2 then the multiplication can be removed (because it would be * 1 which doesn't change anything), but if you want to e.g. generate four random values you can use that as well:
select t.*,
case (random() * 3 + 1)::int
when 1 then 'treatment'
when 2 then 'control'
when 3 then 'something'
else 'some other thing'
end as tag
from t;