Randomly insert 1 of 3 declared variables - sql

Question

I have three variables that are declared and have an integer value assigned to them.
I am trying to randomly assign the integer value to a field in an UPDATE statement, but get an error.
This is statement I am trying to execute:
FOR user_record IN (SELECT * FROM users_to_add) LOOP
UPDATE
customer."user"
SET
primary_site_id = ({site_GRO, site_WHS, site_SHR}[])[ceil(random()*3)],
WHERE
userid = (SELECT userID FROM customer.user
WHERE emailaddress=user_record.email_address);
END LOOP;
I am getting:
SyntaxError: syntax error at or near "{"
This same format works if the value being randomly selected is a string but since these are variables, the inside curly brackets can't be enclosed in quotes.

Answer 1

Use an ARRAY constructor instead of the (invalid) array literal.
(ARRAY[site_GRO, site_WHS, site_SHR])[ceil(random()*3)]
However, a set-based solution is typically more efficient than looping:
UPDATE customer."user" u
SET primary_site_id = CASE trunc(random()*3)::int
WHEN 0 THEN site_gro -- your variables here
WHEN 1 THEN site_whs
WHEN 2 THEN site_shr
END
FROM users_to_add ua
WHERE u.userid = ua.email_address;
Should achieve the same. Works inside a PL/pgSQL block or as standalone SQL DML command (then you need to interpolate variable values yourself).
A single multi-row UPDATE is much cheaper than many updates in a loop.
trunc() is slightly more correct than ceil(), as random() returns a value in the domain [0,1) (1 excluded). It's also faster.
And a CASE construct is substantially faster than building an array just to extract a single element from it.
Asides:
Avoid reserved words like user as identifiers. Always requires double-quoting, and can lead to confusing errors when forgotten.
Also avoid random capitalization in identifiers. This goes for SQL as well as for PL/pgSQL. See:
Are PostgreSQL column names case-sensitive?

Answer 2

Perhaps you can try splitting the index and array out into their own vars?
FOR user_record IN (SELECT * FROM users_to_add) LOOP
a := ARRAY[site_GRO, site_WHS, site_SHR];
i := ceil(random()*3);
UPDATE
customer."user"
SET
primary_site_id = a[i]
WHERE
userid = (SELECT userID FROM customer.user WHERE emailaddress=user_record.email_address);
END LOOP;