Question

If i have two tables like

user table-“u”

userid | name
 1     |  lenova
 2     |  acer
 3     |  hp

pass table-“p”

userid | password
 1     | len123
 2     | acer123
 3     | hp123

as for as i learnt from tutorials I can join these 2 tables using many joins available in
mysql as said here

If i have a table like

role table-“r”

roleid | rname
 1     | admin
 2     | user
 3     | dataanalyst

token table-“t”

tokenid| tname
 1     | xxxx
 2     | yyyy
 3     | zzzz

tole_token_association table-“a”

roleid | tokenid
 1     |  1
 1     |  2
 3     |  1
 3     |  3
 3     |  1

I have to make a join such that I have to display a table which corresponds
like this “rolename” has all these tokens.How to make this? I am confused. Is it possible to make a join? I am liking mysql a lot. I wish to play with queries such that not playing. I want to get well versed. Any Suggestions Please?

Answer 1

It’s easiest to see when the column names that need to be joined are named identically:

SELECT r.rname,
       t.tname
  FROM ROLE r
  JOIN ROLE_TOKEN_ASSOCIATION rta ON rta.roleid = r.roleid
  JOIN TOKEN t ON t.tokenid = rta.tokenid

This will return only the roles with tokens associated. If you have a role that doesn’t have a token associated, you need to use an OUTER join, like this:

   SELECT r.rname,
          t.tname
     FROM ROLE r
LEFT JOIN ROLE_TOKEN_ASSOCIATION rta ON rta.roleid = r.roleid
     JOIN TOKEN t ON t.tokenid = rta.tokenid

This link might help — it’s a visual representation of JOINs.