Add support, custom, and other queries

src/Mapper/ExistsMapper.php

@@ -0,0 +1,26 @@
+<?php declare(strict_types=1);
+
+namespace BitBadger\PDODocument\Mapper;
+
+use BitBadger\PDODocument\Configuration;
+use BitBadger\PDODocument\DocumentException;
+use BitBadger\PDODocument\Mode;
+
+/**
+ * Map an EXISTS result to a boolean value
+ */
+class ExistsMapper implements Mapper
+{
+    /**
+     * @inheritDoc
+     * @throws DocumentException If the database mode has not been set
+     */
+    public function map(array $result): bool
+    {
+        return match (Configuration::$mode) {
+            Mode::PgSQL  => (bool)$result[0],
+            Mode::SQLite => (int)$result[0] > 0,
+            default      => throw new DocumentException('Database mode not set; cannot map existence result'),
+        };
+    }
+}