select row from multiple tables failed - php

Question

i have the following code.
i already try'd some sulotions from the search option, but nothing helped.
all tables have a row called: user_id with data in it, but still the query gives false
The $user variable has the value 29 and in my tables look like this:
+---------+---------+-------+-----------+
+ id | user_id | jaar + kenmerk |
+---------+---------+-------+-----------+
+ 1 | 29 | 2015 + standaard |
+---------+---------+-------+-----------+
$query = "SELECT buitenklimaat.user_id, kasklimaat.user_id, watermangement.user_id, energie.user_id FROM buitenklimaat, kasklimaat, watermangement, energie WHERE user_id = ".$user." AND kenmerk = 'standaard' AND jaar = ".date("Y")."";
$result = mysqli_query($conn, $query);
if($result === false) {
echo 'Query failed';
die();
}
else {
// do something
}

Answer 1

If the field user_id exists in more than one table, you need to prefix the field with the table name in your where clause, too. Otherwise your database will not know in which table it should look for the user_id field. But even after you fixed this error the query will probably not return what you want it to. Have a look at some SQL tutorials that cover the join syntax.
$query = "SELECT buitenklimaat.user_id, kasklimaat.user_id, watermangement.user_id, energie.user_id FROM buitenklimaat, kasklimaat, watermangement, energie WHERE <TABLENAME MISSING>.user_id = ".$user." AND kenmerk = 'standaard' AND jaar = ".date("Y")."";

Answer 2

You may need to join all those tables together to get the user_id condition to work properly:
$query = "SELECT buitenklimaat.user_id, kasklimaat.user_id,
watermangement.user_id, energie.user_id FROM buitenklimaat, kasklimaat,
watermangement, energie
WHERE buitenklimaat.user_id = kasklimaat.user_id
AND buitenklimaat.user_id = watermangement.user_id
AND buitenklimaat.user_id = energie .user_id
AND buitenklimaat.user_id = ".$user." AND kenmerk = 'standaard'
AND jaar = ".date("Y")."";