Add classes and tests from common project

tests/unit/Query/ExistsTest.php

@@ -0,0 +1,32 @@
+<?php declare(strict_types=1);
+
+namespace Test\Unit\Query;
+
+use BitBadger\PDODocument\Field;
+use BitBadger\PDODocument\Query\Exists;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * Unit tests for the Exists class
+ */
+class ExistsTest extends TestCase
+{
+    public function testQuerySucceeds(): void
+    {
+        $this->assertEquals('SELECT EXISTS (SELECT 1 FROM abc WHERE def)', Exists::query('abc', 'def'),
+            'Existence query not generated correctly');
+    }
+
+    public function testByIdSucceeds(): void
+    {
+        $this->assertEquals("SELECT EXISTS (SELECT 1 FROM dox WHERE data->>'id' = @id)", Exists::byId('dox'),
+            'Existence query not generated correctly');
+    }
+
+    public function testByFieldsSucceeds(): void
+    {
+        $this->assertEquals("SELECT EXISTS (SELECT 1 FROM box WHERE data->>'status' <> @status)",
+            Exists::byFields('box', [Field::NE('status', 'occupied', '@status')]),
+            'Existence query not generated correctly');
+    }
+}