How to query rows from 2 tables in a single query? - mysql

Question

TABLE 1
student_id | name
-----------------
1 | A
2 | B
3 | C
4 | D
TABLE 2
vote_id | student_id | vote_for
------------------------------
1 | 1 | 2
2 | 1 | 3
3 | 2 | 1
4 | 1 | 4
How can I get records from TABLE 1 (student names B C D) based on student_id in TABLE 2 (student_id 1) in a single query? I managed to do it but in multiple queries like below:
$students = array();
$query = "SELECT vote_for FROM table2 WHERE student_id=?";
$stmt = $this->con->prepare($query);
$stmt->bind_param("i",$student_id);
$stmt->execute();
$stmt->bind_result($vote_for);
$votes = array();
while($stmt->fetch()){
$votes[] = $vote_for;
}
$stmt->close();
if (!empty($votes)) {
$query = "SELECT name FROM table1 WHERE student_id=?";
foreach ($votes as $vote) {
$stmt = $this->con->prepare($query);
$stmt->bind_param("i",$vote);
$stmt->execute();
$stmt->bind_result($name);
while($stmt->fetch()){
$temp = array();
$temp['name'] = $name;
$students[] = $temp;
}
$stmt->close();
}
}

Answer 1

You can use a JOIN query to get the names of the students that were voted for by a given student_id. For example:
SELECT s.name AS voted_for
FROM table2 v
JOIN table1 s ON s.student_id = v.vote_for
WHERE v.student_id = 1
Demo on dbfiddle
In PHP:
$students = array();
$query = "SELECT s.name AS voted_for
FROM table2 v
JOIN table1 s ON s.student_id = v.vote_for
WHERE v.student_id = ?";
$stmt = $this->con->prepare($query);
$stmt->bind_param("i",$student_id);
$stmt->execute();
$stmt->bind_result($name);
while($stmt->fetch()) {
$students[] = array('name' => $name);
}
$stmt->close();

Answer 2

I believe you can achieve it with the following query:
SELECT T1.STUDENT_ID, T1.NAME
FROM TABLE_1 T1, TABLE_2 T2
WHERE T1.STUDENT_ID = T2.VOTE_FOR
AND T2.STUDENT_ID = ?
And you would just inject the STUDENT_ID for the Table 2. You can remove the T1.STUDENT_ID if necessary.