added UNION clause

Author: transistive

src/Clauses/UnionClause.php

@@ -0,0 +1,56 @@
+<?php
+
+namespace WikibaseSolutions\CypherDSL\Clauses;
+
+/**
+ * This class represents the union clause.
+ *
+ * @see https://neo4j.com/docs/cypher-manual/current/clauses/union/
+ */
+class UnionClause extends Clause
+{
+    /** @var bool Whether the union should include all results or remove the duplicates instead. */
+    private bool $all;
+
+    /**
+     * @param bool $all Whether the union should include all results or remove the duplicates instead.
+     */
+    public function __construct(bool $all = false)
+    {
+        $this->all = $all;
+    }
+
+    /**
+     * Returns whether the union includes all results or removes the duplicates instead.
+     *
+     * @return bool
+     */
+    public function includesAll(): bool
+    {
+        return $this->all;
+    }
+
+    /**
+     * @inheritDoc
+     */
+    public function canBeEmpty(): bool
+    {
+        return true;
+    }
+
+    /**
+     * @inheritDoc
+     */
+    protected function getSubject(): string
+    {
+        return 'UNION';
+    }
+
+    /**
+     * @inheritDoc
+     */
+    protected function getClause(): string
+    {
+        return $this->all ? 'ALL' : '';
+    }
+}