quietone
--- a/core/tests/Drupal/KernelTests/Core/Database/MergeTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Database/MergeTest.php
    $this->assertEquals('2', $person->id);
  }

+  /**
+   * @covers \Drupal\Core\Database\Query\Merge::__toString
+   */
+  public function testMergeToString(): void {
--- a/core/tests/Drupal/KernelTests/Core/Database/MergeTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Database/MergeTest.php
    $this->assertEquals('2', $person->id);
  }

+  /**
+   * @covers \Drupal\Core\Database\Query\Merge::__toString
+   */
+  public function testMergeToString(): void {
+    $query = $this->connection->merge('test_people');
+    $this->assertEmpty((string) $query);
-    $this->assertEmpty((string) $query);
+    $this->assertSame(‘’, $query);
--- a/core/lib/Drupal/Core/Database/Query/Query.php
+++ b/core/lib/Drupal/Core/Database/Query/Query.php
   * statement.
   *
   * @return string
-   *   A prepared statement query string for this object.
+   *   A prepared statement query string for this object. If the query cannot
+   *   be represented as a single statement an empty string is returned.
--- a/core/lib/Drupal/Core/Database/Query/Query.php
+++ b/core/lib/Drupal/Core/Database/Query/Query.php
   * statement.
   *
   * @return string
-   *   A prepared statement query string for this object.
+   *   A prepared statement query string for this object. If the query cannot
+   *   be represented as a single statement an empty string is returned. When
+   *   the operation is a Merge, an exception is thrown instead.