mysql - 通过将两个查询合并为一个而不是循环来提​​高 SQL 的效率

Question

我有以下代码，想知道如何将它简化为一条 MYSQL 语句

$sql = "SELECT sesionID 
FROM  `juegos_sesiones` 
WHERE  `usuarioID` = ".$_SESSION['MM_UserID'];


$query = mysql_query($sql, $cnx) or die(mysql_error());
//$row_rsPaises = mysql_fetch_assoc($rsPaises);
$exercises = mysql_num_rows($query);

echo $exercises;

//find total points

while( $row = mysql_fetch_array($query)){

$sql2 = 'SELECT correct_answers FROM `juegos_sesiones_detalle`  WHERE  `sesionID`  = '.$row['sesionID'];


$query2 = mysql_query($sql2, $cnx) or die(mysql_error());
$details2 = mysql_fetch_assoc($query2);
$counter = $counter + $details2['correct_answers'];

}

echo '<br />'.$counter;

Answer 1

SELECT sum(correct_answers) as total_correct_answers
FROM `juegos_sesiones_detalle` d
INNER JOIN  `juegos_sesiones` s on s.`sesionID` = d.`sesionID`
WHERE  `usuarioID` = $_SESSION['MM_UserID']

您可以阅读有关联接的信息 here