left join same field select all rows

Question

How to select rows in table UsrArticle which UsrId = 1? I'm new to use join, below is my code is there something wrong it only print empty array?

table Usr

id    UsrId
0     0
1     1
2     2
3     3

table UsrArticle

ArticleId   UsrId
0           1
1           0
2           1
3           1
4           1
5           3

php

$UsrId = 1;

$sth = $db->prepare("SELECT * FROM Usr LEFT JOIN UsrArticle ON (Usr.UsrId = UsrArticle.UsrId) WHERE UsrId = :UsrId");
$sth->bindParam(':UsrId',$UsrId,PDO::PARAM_INT);
$sth->setFetchMode(PDO::FETCH_ASSOC);
$sth->execute();

$ArticleRows = $sth->fetchAll();
print_r($ArticleRows);

Answer 1

It will be failing because UsrID is ambiguous (there are multiple possibilities for which column you are referring to). I know it's silly because your conditions are specifying that they should be the same, but that's how SQL works. Add a table alias to the prefix of that field name:

... WHERE Usr.UsrId = :UsrId