Adding values to a column in SQL(Snowflake) from another table

Question

I have two tables A,B

Table A:

uid  category  
 1     a     
 1     b     
 1     c     
 2     b     
 2     d     

Table B:

category
  d       
  e       

Table A contains user id and category
Table B contains top 2 most categories selected by the user

How can I add categories from table B to category column in table A but only the distinct value.

Final result

uid  category
 1      a     
 1      b     
 1      c     
 1      d
 1      e     
 2      b     
 2      d     
 2      e     

Answer 1

It is possible to generate missing rows by perfroming CROSS JOIN of distinct UID from tableA and categories from tableB:

WITH cte AS (
   SELECT A.UID, B.CATEGORY
   FROM (SELECT DISTINCT UID FROM tableA) AS A
   CROSS JOIN tableB AS B
)
SELECT A.UID, A.CATEGORY
FROM tableA AS A
UNION ALL 
SELECT C.UID, C.CATEGORY
FROM cte AS c
WHERE (c.UID, c.category) NOT IN (SELECT A.UID, A.CATEGORY
                                  FROM tableA AS A)
ORDER BY 1,2;

Sample input:

CREATE OR REPLACE TABLE tableA(uid INT, category TEXT)
AS
SELECT 1,'a' UNION ALL     
SELECT 1,'b' UNION ALL     
SELECT 1,'c' UNION ALL     
SELECT 2,'b' UNION ALL     
SELECT 2,'d';

CREATE OR REPLACE TABLE tableB(category TEXT)
AS
SELECT 'd' UNION ALL SELECT 'e';

Output:

Answer 2

Let union take care of duplicates

select uid, category
from t1
union 
select uid, category
from (select distinct uid from t1) t1 cross join t2
order by uid, category