Added numerous operations like concat(...), coalesce(...), abs(...), etc (#20)

tests/sqlite/test_where_with_nullable_operations.cpp

@@ -0,0 +1,48 @@
+#include <gtest/gtest.h>
+
+#include <optional>
+#include <rfl.hpp>
+#include <rfl/json.hpp>
+#include <sqlgen.hpp>
+#include <sqlgen/sqlite.hpp>
+#include <vector>
+
+namespace test_where_with_nullable_operations {
+
+struct Person {
+  sqlgen::PrimaryKey<uint32_t> id;
+  std::string first_name;
+  std::string last_name;
+  std::optional<int> age;
+};
+
+TEST(sqlite, test_where_with_nullable_operations) {
+  const auto people1 = std::vector<Person>(
+      {Person{
+           .id = 0, .first_name = "Homer", .last_name = "Simpson", .age = 45},
+       Person{.id = 1, .first_name = "Bart", .last_name = "Simpson", .age = 10},
+       Person{.id = 2, .first_name = "Lisa", .last_name = "Simpson", .age = 8},
+       Person{
+           .id = 3, .first_name = "Maggie", .last_name = "Simpson", .age = 0},
+       Person{
+           .id = 4, .first_name = "Hugo", .last_name = "Simpson", .age = 10}});
+
+  const auto conn = sqlgen::sqlite::connect();
+
+  sqlgen::write(conn, people1);
+
+  using namespace sqlgen;
+
+  const auto query = sqlgen::read<std::vector<Person>> |
+                     where("age"_c * 2 + 4 < 40 and "first_name"_c != "Hugo") |
+                     order_by("age"_c);
+
+  const auto people2 = query(conn).value();
+
+  const std::string expected =
+      R"([{"id":3,"first_name":"Maggie","last_name":"Simpson","age":0},{"id":2,"first_name":"Lisa","last_name":"Simpson","age":8},{"id":1,"first_name":"Bart","last_name":"Simpson","age":10}])";
+
+  EXPECT_EQ(rfl::json::write(people2), expected);
+}
+
+}  // namespace test_where_with_nullable_operations