Added support for UNION and UNION ALL (#96)

tests/sqlite/test_union.cpp

@@ -0,0 +1,66 @@
+#include <gtest/gtest.h>
+
+#include <rfl.hpp>
+#include <rfl/json.hpp>
+#include <sqlgen.hpp>
+#include <sqlgen/sqlite.hpp>
+#include <vector>
+
+#include "sqlgen/sqlite/to_sql.hpp"
+
+namespace test_union {
+
+struct User1 {
+  std::string name;
+  int age;
+};
+
+struct User2 {
+  std::string name;
+  int age;
+};
+
+struct User3 {
+  int age;
+  std::string name;
+};
+
+TEST(sqlite, test_union) {
+  using namespace sqlgen::literals;
+
+  const auto conn = sqlgen::sqlite::connect();
+
+  const auto user1 = User1{.name = "John", .age = 30};
+  sqlgen::write(conn, user1);
+
+  const auto user2 = User2{.name = "Jane", .age = 25};
+  sqlgen::write(conn, user2);
+
+  const auto user3 = User3{.age = 40, .name = "Joe"};
+  sqlgen::write(conn, user3);
+
+  const auto s1 = sqlgen::select_from<User1>("name"_c, "age"_c);
+  const auto s2 = sqlgen::select_from<User2>("name"_c, "age"_c);
+  const auto s3 = sqlgen::select_from<User3>("name"_c, "age"_c);
+
+  const auto result = sqlgen::unite<std::vector<User1>>(s1, s2, s3)(conn);
+
+  const auto users = result.value();
+
+  const auto query =
+      sqlgen::sqlite::to_sql(sqlgen::unite<std::vector<User1>>(s1, s2, s3));
+
+  EXPECT_EQ(
+      query,
+      R"(SELECT "name", "age" FROM (SELECT "name", "age" FROM "User1") UNION SELECT "name", "age" FROM (SELECT "name", "age" FROM "User2") UNION SELECT "name", "age" FROM (SELECT "name", "age" FROM "User3"))");
+
+  EXPECT_EQ(users.size(), 3);
+  EXPECT_EQ(users.at(0).name, "Jane");
+  EXPECT_EQ(users.at(0).age, 25);
+  EXPECT_EQ(users.at(1).name, "Joe");
+  EXPECT_EQ(users.at(1).age, 40);
+  EXPECT_EQ(users.at(2).name, "John");
+  EXPECT_EQ(users.at(2).age, 30);
+}
+
+}  // namespace test_union