在mysql和php中，如果第一个查询为空，如何执行第二个查询？

我有一个html表单，它将值传递给执行查询并显示结果的php文件。
现在我想知道，如果在第一个查询中，结果是空的（0行），则执行完全相同的查询，但要覆盖另一个表并显示结果

以下是执行第一个查询的代码：

<?php 

echo "<table style='border: solid 1px black;'>";
echo "<tr>
<th>R1</th>
<th>R2</th>
<th>R3</th>
<th>R4</th>
<th>R5</th>
</tr>";


class TableRows1 extends RecursiveIteratorIterator {
function __construct($it1) {
    parent::__construct($it1, self::LEAVES_ONLY);
}

function current() {
    return "<td style='width: 70px;'>" . parent::current(). "</td>";
}

function beginChildren() {
    echo "<tr>";
}

function endChildren() {
    echo "</tr>" . "\n";
}
}

if( isset($_POST['submit']) )
{
    $feature = $_POST['R1'];
    $feature2 = $_POST['R2'];
    $feature3 = $_POST['R3'];
    $feature4 = $_POST['R4'];
    $feature5 = $_POST['R5'];
};

$feature = $_POST['R1'];
$feature2 = $_POST['R2'];
$feature3 = $_POST['R3'];
$feature4 = $_POST['R4'];
$feature5 = $_POST['R5'];

$values = [$feature, $feature2, $feature3, $feature4, $feature5];


$servername = "";
$username = "";
$password = "";
$dbname = "";

try {


$conn1 = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
$conn1->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$stmt1 = $conn1->prepare(

"SELECT
R1, 
R2, 
R3, 
R4, 
R5, 
FROM table
WHERE
R1 = ?
AND
R2 = ?
AND
R3 = ?
AND
R4 = ?
AND
R5 = ?");

$stmt1->bindParam(1, $feature, PDO::PARAM_INT);
$stmt1->bindParam(2, $feature2, PDO::PARAM_INT);
$stmt1->bindParam(3, $feature3, PDO::PARAM_INT);
$stmt1->bindParam(4, $feature4, PDO::PARAM_INT);
$stmt1->bindParam(5, $feature5, PDO::PARAM_INT);

$stmt1->execute();


// set the resulting array to associative
$result1 = $stmt1->setFetchMode(PDO::FETCH_ASSOC);

foreach(new TableRows1(new RecursiveArrayIterator($stmt1->fetchAll())) as $k1=>$v1) {
    echo $v1;
}
}
catch(PDOException $e1) {
echo "Error: " . $e1->getMessage();
}
$conn1 = null;
echo "</table>";
?>

---

举例说明；我不是说它很漂亮，但我希望这是数据模型的一个弱点，而不是我的编码技能

DROP TABLE IF EXISTS table_a;
DROP TABLE IF EXISTS table_b;

CREATE TABLE table_a(id SERIAL PRIMARY KEY);
CREATE TABLE table_b(id SERIAL PRIMARY KEY);

INSERT INTO table_b VALUES (NULL),(NULL),(NULL);

SELECT x.* 
  FROM
     ( SELECT 1 source, id FROM table_a
        UNION
       SELECT 2, id FROM table_b
     ) x
  JOIN
     ( SELECT MIN(source) min_source
         FROM 
            ( SELECT 1 source, id FROM table_a
               UNION
              SELECT 2, id FROM table_b
            ) n
     ) y
    ON y.min_source = x.source;
    
+--------+----+
| source | id |
+--------+----+
|      2 |  1 |
|      2 |  2 |
|      2 |  3 |
+--------+----+

根据@草莓的建议，我通过以下查询解决了这个问题：

SELECT 
NULL AS field1,
table1.R1,
table1.R2,
table1.R3,
table1.R4,
table1.R5,
FROM 
table1
WHERE
table1.R1 = ?
AND
table1.R2 = ?
AND
table1.R3 = ?
AND
table1.R4 = ?
AND
table1.R5 = ? 
UNION ALL
SELECT
table2.field1, 
table2.R1,
table2.R2,
table2.R3,
table2.R4,
table2.R5
FROM table2
WHERE 
table2.R1 = ?
AND
table2.R2 = ?
AND
table2.R3 = ?
AND
table2.R4 = ?
AND
table2.R5 = ?;

为什么不在两个表上只执行一个查询，而在第一个表为空的情况下只显示第二个表的结果呢？要对多个表执行查询，据我所知，它们必须有共同的行，在这种情况下，两个表没有共同的行。情况并非如此。