UPDATE baseusers
SET FirstNameUa = subquery.FirstNameUa,
LastNameUa = subquery.LastNameUa
FROM (
SELECT
User_Id,
CONCAT(
SUBSTRING('abcdefghijklmnopqrstuvwxyz', CAST(FLOOR(RAND(CHECKSUM(NEWID())) * 26) + 1 AS INT), 1),
SUBSTRING('abcdefghijklmnopqrstuvwxyz', CAST(FLOOR(RAND(CHECKSUM(NEWID())) * 26) + 1 AS INT), 1),
SUBSTRING('abcdefghijklmnopqrstuvwxyz', CAST(FLOOR(RAND(CHECKSUM(NEWID())) * 26) + 1 AS INT), 1),
SUBSTRING('abcdefghijklmnopqrstuvwxyz', CAST(FLOOR(RAND(CHECKSUM(NEWID())) * 26) + 1 AS INT), 1),
SUBSTRING('abcdefghijklmnopqrstuvwxyz', CAST(FLOOR(RAND(CHECKSUM(NEWID())) * 26) + 1 AS INT), 1),
SUBSTRING('abcdefghijklmnopqrstuvwxyz', CAST(FLOOR(RAND(CHECKSUM(NEWID())) * 26) + 1 AS INT), 1)
) AS FirstNameUa,
CONCAT(
SUBSTRING('abcdefghijklmnopqrstuvwxyz', CAST(FLOOR(RAND(CHECKSUM(NEWID())) * 26) + 1 AS INT), 1),
SUBSTRING('abcdefghijklmnopqrstuvwxyz', CAST(FLOOR(RAND(CHECKSUM(NEWID())) * 26) + 1 AS INT), 1),
SUBSTRING('abcdefghijklmnopqrstuvwxyz', CAST(FLOOR(RAND(CHECKSUM(NEWID())) * 26) + 1 AS INT), 1),
SUBSTRING('abcdefghijklmnopqrstuvwxyz', CAST(FLOOR(RAND(CHECKSUM(NEWID())) * 26) + 1 AS INT), 1),
SUBSTRING('abcdefghijklmnopqrstuvwxyz', CAST(FLOOR(RAND(CHECKSUM(NEWID())) * 26) + 1 AS INT), 1),
SUBSTRING('abcdefghijklmnopqrstuvwxyz', CAST(FLOOR(RAND(CHECKSUM(NEWID())) * 26) + 1 AS INT), 1)
) AS LastNameUa
FROM baseusers
) AS subquery
WHERE baseusers.User_Id = subquery.User_Id
AND baseusers.User_Id IN (46090, 46051);