从 mysql JOIN 获取所有结果，即使没有连接匹配

Question

I am doing a JOIN operation between a table and itself. The table schema is something like:

| id | name | parent |
|  0 | .... |   ...  |
|  1 | .... |   ...  |
|  2 | .... |   ...  |

The query looks like:

$qMarks = str_repeat('?,', count($arr) - 1) . '?';
$stmt = $db->prepare("SELECT t1.id AS t1id, t1.name AS t1name, t2.name AS t2name 
                            FROM cats t1 
                            JOIN cats t2 ON t1.parent = t2.id
                            WHERE t1.name IN ($qMarks)");
$stmt->execute($arr);
$result = $stmt->fetchAll();

So, I'm passing in an array of names $arr , and I am getting back the rows where there is a matching name to one of the items in the parameter array. This works fine, so long as there is also a matching id somewhere for parent .

But, sometimes the value for parent will be blank (empty cell). I still want to get those results, as long as the t1.name IN ($qMarks) condition is met.

How do I return the values, even if the t1.parent value in the table is blank?

Answer 1

Use a left join .

$stmt = $db->prepare("SELECT t1.id AS t1id, t1.name AS t1name, t2.name AS t2name 
                        FROM cats t1 
                        LEFT JOIN cats t2 ON t1.parent = t2.id
                        WHERE t1.name IN ($qMarks)");