How to select rows after using row_number() in postgresql - postgresql

Question

I have to select the null values one by one from a column with some null values in it. I have to select them one by one because I want to update all of them with different values.
I chose to do it with row_number() after running out of possible options in my mind, So here is the query that I executed
select cid, ROW_NUMBER () OVER (ORDER BY random()) as row from aa_dev.calls where cid is null;
How can I pick each row without storing it in any temp table and update each row?
This column has 100 values 96 are populated with integers with only 4 nulls.
I need to populate row_number as follows. for example there are total 10 values in this data and 3 of them are null values.
cid
row
1
0
54
0
null
1
26
0
86
0
45
0
null
2
56
0
null
3
5
0

Answer 1

Two possible ways came to mind:
demos:db<>fiddle
Using the row_number() over partitions which are (cid = NULL) and (cid != NULL). And this just execute for those records that are NULL, all others are set to 0:
SELECT
*,
CASE WHEN cid IS NULL THEN
row_number() OVER (PARTITION BY cid IS NULL)
ELSE 0 END
FROM
mytable
Second way is simply using a UNION construct over (cid = NULL) and (cid != NULL):
SELECT
cid,
row_number() OVER ()
FROM mytable
WHERE cid IS NULL
UNION
SELECT
cid,
0
FROM mytable
WHERE cid IS NOT NULL