从同一个表联接

Question

I am trying to join the same table "travel_plan" twice, as the value(s) I need are location_from & location_to in which I can then join the value to my cities table to grab the city name.

SELECT * FROM travel_plan 
LEFT JOIN Cities ON Cities.CityID = travel_plan.location_to AS plan_to 
LEFT JOIN Cities ON Cities.CityID = travel_plan.location_from AS plan_from
LEFT JOIN user_table ON travel_plan.user_id = user_table.id 
ORDER BY date_from DESC LIMIT 0,4") or die(mysql_error());

Answer 1

You need to use table aliases correctly when you're joining the same table more than once, as you're doing with Cities in this query.

     SELECT * 
       FROM travel_plan  AS tr
  LEFT JOIN Cities       AS C1     ON  C1.CityID = tr.location_to 
  LEFT JOIN Cities       AS C2     ON  C2.CityID = tr.location_from 
  LEFT JOIN user_table   AS us     ON tr.user_id = us.id 
   ORDER BY date_from DESC 
      LIMIT 0,4

The way you wrote your query, the LEFT JOIN AS clauses were misplaced and not used for qualifying the column names.

This use of SELECT * is really suboptimal, however. From this four-table JOIN, SELECT * kicks back lots of columns with duplicate names, which fouls up _fetch_assoc() methods in php.

Your best bet is to enumerate the columns you fetch, and provide aliases so they don't end up with the same names. I don't know the names of your columns so I have to guess, but it would go something like this.

SELECT us.name, us.id AS userid, 
       C1.cityname AS to_cityname,
       C2.cityname AS from_cityname,
  FROM ....

Then you'll find the values in $result['from_cityname'] after you fetch each row.

Answer 2

You misuse the AS keyword, it can be only used in the select part of the query (before FROM), or optionally as alias for table references. But not in the ON part of a join. I guess what you want is:

SELECT *, c1.City as toCity, c2.City as fromCity  FROM travel_plan LEFT JOIN Cities c1 ON c1.CityID = travel_plan.location_to LEFT JOIN Cities c2 ON c2.CityID = travel_plan.location_from LEFT JOIN user_table ON travel_plan.user_id = user_table.id ORDER BY date_from DESC LIMIT 0,4

Now you can access the the column aliases toCity and fromCity in your resultset, even though the the original column names are the same.