Postgres 13

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create table foo as
    select n, '' as bar
    from generate_series(1, 10) gs(n)

SELECT 10


update foo
    set bar = array_to_string(array(select string_agg(substring('0123456789bcdfghjkmnpqrstvwxyz', round(random() * 30)::integer, 1), '')
                                    from generate_series(1, 9)
                                    where foo.bar is distinct from 'something'
                                   ), '');
 

UPDATE 10


select *
from foo


n
bar


1
5ky05vqc0

2
w28n7m2bs

3
jg278bw6n

4
c45d63f8f

5
hc1vqzxv9

6
gytb1tn2c

7
sts7b83rd

8
f1wyjwcv0

9
0yq4xhsc

10
w3jy9fhfr



SELECT 10

n	bar
1	5ky05vqc0
2	w28n7m2bs
3	jg278bw6n
4	c45d63f8f
5	hc1vqzxv9
6	gytb1tn2c
7	sts7b83rd
8	f1wyjwcv0
9	0yq4xhsc
10	w3jy9fhfr