How to use the value from one select query into another in union?

Question

I am trying to insert rows made from random values in a table 'Users' where the the email is dependent on the value of the username:

The row 'username' is not seen by the last SELECT query (doesn't exist)

How can i fix this?

SELECT(
  SELECT
  string_agg(substr(characters, (random() * length(characters) + 1)::integer, 1), '') as username
  from (values('abcdefghijklmnopqrstuvwxyz0123456789')) as symbols(characters) join generate_series(1, 15) on 1 = 1
  UNION
  SELECT
  string_agg(substr(characters, (random() * length(characters) + 1)::integer, 1), '') as password
  from (values('ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789*')) as symbols(characters) join generate_series(1, 15) on 1 = 1
  UNION
  SELECT
  string_agg(substr(characters, (random() * length(characters) + 1)::integer, 1), '') as phonenumber
  from (values('0123456789')) as symbols(characters) join generate_series(1, 9) on 1 = 1
  UNION
  SELECT
    (username || '@' || (
    CASE (RANDOM() * 2)::INT
      WHEN 0 THEN 'gmail'
      WHEN 1 THEN 'hotmail'
      WHEN 2 THEN 'yahoo'
    END
  ) || '.com') AS email
    ) INTO "User" from generate_series(1,10000)

This also doesn't work:

SELECT(
  SELECT
  string_agg(substr(characters, (random() * length(characters) + 1)::integer, 1), '') as username
  from (values('abcdefghijklmnopqrstuvwxyz0123456789')) as symbols(characters) join generate_series(1, 15) on 1 = 1
  UNION
  SELECT
  string_agg(substr(characters, (random() * length(characters) + 1)::integer, 1), '') as password
  from (values('ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789')) as symbols(characters) join generate_series(1, 15) on 1 = 1
  as select1
  UNION
  SELECT
  string_agg(substr(characters, (random() * length(characters) + 1)::integer, 1), '') as phonenumber
  from (values('0123456789')) as symbols(characters) join generate_series(1, 9) on 1 = 1
  UNION
  SELECT
    (select1.username || '@' || (
    CASE (RANDOM() * 2)::INT
      WHEN 0 THEN 'gmail'
      WHEN 1 THEN 'hotmail'
      WHEN 2 THEN 'yahoo'
    END
  ) || '.com') AS email
    ) INTO "User" from generate_series(1,10000)

Answer 1

Consider a recursive CTE query and removing subqueries for top level columns:

WITH RECURSIVE main AS
   (
     SELECT 
         string_agg(substr(shortchars, (random() * length(shortchars) + 1)::integer, 1),'') AS "username"
       , string_agg(substr(longchars, (random() * length(longchars) + 1)::integer, 1), '') AS "password"
       , substr(string_agg(substr(nums, (random() * length(nums) + 1)::integer, 1), ''),1,9) AS "phonenumber"
       , 1 AS n

       FROM (values('ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789!@#$%^&*')) as longchars(longchars) 
       CROSS JOIN (values('abcdefghijklmnopqrstuvwxyz0123456789')) as shortchars(shortchars)
       CROSS JOIN (values('0123456789')) as num(nums)
       CROSS JOIN generate_series(1, 15)

     UNION ALL

     SELECT 
         string_agg(substr(shortchars, (random() * length(shortchars) + 1)::integer, 1),'') AS "username"
       , string_agg(substr(longchars, (random() * length(longchars) + 1)::integer, 1), '') AS "password"
       , substr(string_agg(substr(nums, (random() * length(nums) + 1)::integer, 1), ''),1,9) AS "phonenumber"
       , n + 1

       FROM (values('ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789!@#$%^&*')) as longchars(longchars) 
       CROSS JOIN (values('abcdefghijklmnopqrstuvwxyz0123456789')) as shortchars(shortchars)
       CROSS JOIN (values('0123456789')) as num(nums)
       CROSS JOIN generate_series(1, 15)
       CROSS JOIN (SELECT n FROM main LIMIT 1) AS mn

     WHERE n < 10
     GROUP BY mn.n
  )


SELECT CONCAT(main."username", 
           '@',
           ( CASE (RANDOM() * 2)::INT 
                  WHEN 0 THEN 'gmail'
                  WHEN 1 THEN 'hotmail' 
                  WHEN 2 THEN 'yahoo' 
              END ), 
           'com') AS email
     , main."username"
     , main."password"
     , main."phonenumber"

FROM main

Online Demo