diff -r 1574965c7dc7 -r 57cfb94ab99f src/main/java/de/uapcore/lightpit/dao/Functions.java
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/main/java/de/uapcore/lightpit/dao/Functions.java	Thu May 14 22:48:01 2020 +0200
@@ -0,0 +1,62 @@
+/*
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS HEADER.
+ *
+ * Copyright 2018 Mike Becker. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ *   1. Redistributions of source code must retain the above copyright
+ *      notice, this list of conditions and the following disclaimer.
+ *
+ *   2. Redistributions in binary form must reproduce the above copyright
+ *      notice, this list of conditions and the following disclaimer in the
+ *      documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+package de.uapcore.lightpit.dao;
+
+import java.sql.PreparedStatement;
+import java.sql.SQLException;
+import java.sql.Types;
+import java.util.Optional;
+import java.util.function.Function;
+
+/**
+ * Some DAO utilities.
+ */
+public final class Functions {
+
+    public static void setStringOrNull(PreparedStatement stmt, int index, String str) throws SQLException {
+        if (str == null || str.isBlank()) {
+            stmt.setNull(index, Types.VARCHAR);
+        } else {
+            stmt.setString(index, str);
+        }
+    }
+
+    public static <T> void setForeignKeyOrNull(PreparedStatement stmt, int index, T instance, Function<? super T, Integer> keyGetter) throws SQLException {
+        Integer key = Optional.ofNullable(instance).map(keyGetter).orElse(null);
+        if (key == null) {
+            stmt.setNull(index, Types.INTEGER);
+        } else {
+            stmt.setInt(index, key);
+        }
+    }
+
+    private Functions() {
+
+    }
+}