Question

I am trying to join two tables: table A and table B. Table A has unique ID’s for each row and has corresponding ID’s in Table B but in Table B the ID’s can appear multiple times.

Whenever I try an inner join I get duplicate rows because of table B so I’m wondering how I can join without having those duplicates.

Basically I only want one record from Table B for each row of Table A but its not happening.

edit:

SELECT p.post_title, tt.cntaccess FROM wp_posts as p INNER JOIN wp_top_ten as tt ON p.ID = tt.postnumber WHERE p.post_date > '2000-01-02 00:00:00' AND tt.cntaccess > '10'

Answer 1

This will join a with the corresponding b having the latest date and if there are multiple b’s matching that criteria, the b with the highest id. b.id and a.id are assumed to be unique (or primary key).

SELECT a.id
     , b.id
     , b.date
  FROM a
  JOIN b
    ON a.id = b.a_id
  LEFT JOIN b AS b2
    ON b.a_id = b2.a_id
   AND (b.date, b.id) < (b2.date, b2.id)
 WHERE b2.id IS NULL
;

Here’s my attempt to use your tables with your logic. Note that I refer to tt.pk. This pk represents a unique or primary key for the intended behavior. You didn’t show enough detail to know this information. I haven’t tested the following SQL. The above test SQL has been tested.

SELECT p.post_title
     , tt.cntaccess
     , p.ID
     , tt.pk 
  FROM wp_posts AS p
  JOIN wp_top_ten AS tt
    ON p.id = tt.postnumber 
   AND tt.cntaccess > '10'
  LEFT JOIN wp_top_ten as tt2
    ON tt.postnumber = tt2.postnumber 
   AND tt2.cntaccess > '10'
   AND (tt.date, tt.pk) < (tt2.date, tt2.pk) 
 WHERE tt2.postnumber IS NULL 
   AND p.post_date > '2000-01-02 00:00:00'
;