Question

I have two tables

Table1 with list of users comma separated

Name    UserID
abc     A,B,C,D
def     A,B,C

Table2

Name   UserID
abc    A
abc    B
abc    C
def    A
def    B

I need to find the users that are in table1 for each Name but not in table2 (There won’t ever be an instance when a UserID to Name pair is present in table2 but not in table1 as CSV).

The output should be

Name    UserID
abc     D
def     C

I can do this with PHP but is there a way this can be done through a query? I am not sure where to begin in case I’m doing this as a query. Can I parse in MySQL using comma as delimiter?

Answer 1

I plugged your test data into a test schema in SQLFiddle and ran the query that follows.

Here’s the link to SQLFiddle with the test and positive results:
http://sqlfiddle.com/#!2/83dfd/4/0

Here’s the query:

SELECT
COALESCE(NORMALIZED_TABLE1.NAME, TABLE2.NAME) AS NAME,
COALESCE(NORMALIZED_TABLE1.USERID, TABLE2.USERID) AS USERID
FROM (
     SELECT NAME,
     SUBSTRING(
          USERID
          FROM CASE
               WHEN INDEX_TABLE.POS = 1 THEN 1
               ELSE INDEX_TABLE.POS + 1
               END
          FOR CASE LOCATE(',', USERID, INDEX_TABLE.POS + 1)
              WHEN 0 THEN CHARACTER_LENGTH(USERID) + 1
              ELSE LOCATE(',', USERID, INDEX_TABLE.POS + 1)
              END
              - CASE
                WHEN INDEX_TABLE.POS = 1 THEN 1
                ELSE INDEX_TABLE.POS + 1
                END
     ) AS USERID
     FROM TABLE1
     INNER JOIN (
          SELECT @rownum:=@rownum+1 POS
          FROM (
             SELECT 0 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3
             UNION SELECT 4 UNION SELECT 5 UNION SELECT 6
             UNION SELECT 7 UNION SELECT 8 UNION SELECT 9
          ) a, (
             SELECT 0 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3
             UNION SELECT 4 UNION SELECT 5 UNION SELECT 6
             UNION SELECT 7 UNION SELECT 8 UNION SELECT 9
          ) b, (
             SELECT 0 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3
             UNION SELECT 4 UNION SELECT 5 UNION SELECT 6
             UNION SELECT 7 UNION SELECT 8 UNION SELECT 9
          ) c, (SELECT @rownum:=0) r
     ) INDEX_TABLE
     ON INDEX_TABLE.POS <= CHAR_LENGTH(TABLE1.USERID)
     AND (
          INDEX_TABLE.POS = 1
          OR SUBSTRING(USERID FROM INDEX_TABLE.POS FOR 1) = ','
     )
) AS NORMALIZED_TABLE1
LEFT OUTER JOIN TABLE2
ON NORMALIZED_TABLE1.NAME = TABLE2.NAME
AND NORMALIZED_TABLE1.USERID = TABLE2.USERID
WHERE TABLE2.NAME IS NULL;

If you have very long column width in table1 you might need to expand the “INDEX_TABLE” subquery. You can copy and paste over it with the code at this link to do so:
http://www.experts-exchange.com/Database/MySQL/A_3573-A-MySQL-Tidbit-Quick-Numbers-Table-Generation.html