Question

I have 3 tables as described below ,

table1 'ads'

 id     userid
 1       47
 2       47                    //ads.id  = adcat.adid
 3       45
 4       47

table2  'adcat'

  adid    catid
   1        1
   2        3
   3        3                 // adcat.catid  = categories.id
   4        3

 table3  'categories'

   id        name
    1        mathematic
    2        biolog
    3        leteratur
    4        chemi

What I want is to get categories.id from ads.id.

I have tried this

SELECT categories.id, ads.id 
FROM ads 
INNER JOIN adcat ON ads.id = adcat.adid 
INNER JOIN categories ON categories.id = adcat.catid 

But it does not come by the exact value.

obs : I don’t get any error just not exact value.
any help will be much appreciated.

Answer 1

Seems to be no reason to go in to categories table if adcat.catid = categories.id and that is the only value you need.

SELECT ads.id,adcat.catid
FROM ads
INNER JOIN adcat
ON ads.id=adcat.adid

Use $row['catid'] with this query.

If in the future you need more columns from categories then you will need a double join:

SELECT categories.id *AS id*,ads.id *AS ads_id*
FROM ads 
INNER JOIN adcat ON ads.id = adcat.adid  
INNER JOIN categories ON categories.id = adcat.catid

Use $row['id'] with this query.

See *AS id* and *AS ads_id* – remove the stars and you are giving the columns an ALIAS which is basically like a nickname, this can be handy in many cases but especially important in this case since the two columns both have the same names. So now you can call $row['id'] and get categories.id