带有LEFT JOIN的SELECT返回NULL值

Question

I've two tables:

posts :

id | id_author | content | year
1  | 1         | hello   | 2015
2  | 1         | world   | 2016
3  | 2         | hi      | 2016

favorites :

id | id_author | id_post | year
1  | 3         | 2       | 2016

And I want to do a SELECT to get the posts data by the favorites using LEFT JOIN . So, I did: ( taking into account that I am logged in with the user with id_author = 3)

SELECT p.id, p.id_author, p.content, p.year, f.id_post
FROM favorites f
LEFT JOIN posts p
ON f.id_post = p.id
WHERE f.id_author = 3
ORDER BY f.id
DESC

So, I did: ( $sql is equal to the query above)

if(count($sql) > 0) {
    var_dump($sql);
}

But the output is:

array(1) {
  [0]=>
  array(5) {
    ["id"]=>
    NULL
    ["id_author"]=>
    NULL
    ["content"]=>
    NULL
    ["year"]=>
    NULL
    ["id_post"]=>
    string(1) "2"
  }
}

Why is returning NULL ?

Answer 1

Given the schema and data of the OP the following query is not syntactically valid:

SELECT p.id, p.id_author, p.content, p.year, f.id_post
FROM favorites f
LEFT JOIN posts p
  ON f.id_post = p.id
WHERE id_author = 3
ORDER BY f.id DESC

as there is an ambiguity concerning the field used by the predicate of the WHERE clause: id_author = 3 .

You most probably have p.id_author = 3 , which means the field of the right table of the LEFT JOIN operation is used by the WHERE clause. This turns the LEFT JOIN into an INNER JOIN operation, yielding no results. So, you probably have to use:

WHERE f.id_author = 3