refactor: replace env vars in example with hardcoded constants

basic_query.cpp now uses named constants (k_host, k_port, k_database,
k_user, k_password) at the top of the file instead of reading five
environment variables at runtime. The config_from_env() helper and its
early-exit error path are removed. Users adjust the constants directly
before building.

Also commit the in-progress sql.hpp change (on_duplicate_key_update now
returns insert_into_upsert_builder<T> with build_sql() instead of a
raw std::string) and update the corresponding unit test to call
.build_sql(), keeping it consistent with the rest of the builder API.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Pierre-Luc Gagné

examples/basic_query.cpp

@@ -12,21 +12,26 @@
 //   cmake --preset release && cmake --build build -j$(nproc)
 //   ./build/bin/examples/example_basic_query
 //
-// Set these environment variables to point at a real MySQL instance:
-//   DS_MYSQL_TEST_HOST       (e.g. 127.0.0.1)
-//   DS_MYSQL_TEST_PORT       (e.g. 3307)
-//   DS_MYSQL_TEST_DATABASE   (e.g. ds_mysql_example)
-//   DS_MYSQL_TEST_USER
-//   DS_MYSQL_TEST_PASSWORD
-
-#include <cstdlib>
-#include <iostream>
+// Edit the connection constants below to match your MySQL instance.
+
 #include <optional>
 #include <print>
 #include <string>
 
 #include "ds_mysql.hpp"
 
+// ===================================================================
+// Connection configuration — adjust to match your MySQL instance.
+// ===================================================================
+
+namespace {
+    constexpr auto k_host     = "127.0.0.1";
+    constexpr auto k_database = "ds_mysql_example";
+    constexpr auto k_user     = "root";
+    constexpr auto k_password = "";
+    constexpr unsigned int k_port = 3306;
+}
+
 // ===================================================================
 // Define a typed table struct.
 //
@@ -62,60 +67,24 @@ struct product {
     created_at  created_at_;
 };
 
-// ===================================================================
-// Read connection config from environment variables.
-// ===================================================================
-
-[[nodiscard]] std::optional<ds_mysql::mysql_config> config_from_env() {
-    auto get = [](char const* name, char const* fallback = "") -> std::string {
-        char const* v = std::getenv(name);
-        return v ? v : fallback;
-    };
-
-    const std::string host     = get("DS_MYSQL_TEST_HOST");
-    const std::string database = get("DS_MYSQL_TEST_DATABASE", "ds_mysql_example");
-    const std::string user     = get("DS_MYSQL_TEST_USER");
-    const std::string password = get("DS_MYSQL_TEST_PASSWORD");
-
-    if (host.empty() || user.empty()) {
-        return std::nullopt;
-    }
-
-    const unsigned int port = [&] {
-        char const* p = std::getenv("DS_MYSQL_TEST_PORT");
-        return p ? static_cast<unsigned int>(std::stoul(p))
-                 : ds_mysql::default_mysql_port.to_unsigned_int();
-    }();
-
-    return ds_mysql::mysql_config{
-        ds_mysql::host_name{host},
-        ds_mysql::database_name{database},
-        ds_mysql::auth_credentials{ds_mysql::user_name{user}, ds_mysql::user_password{password}},
-        ds_mysql::port_number{port},
-    };
-}
-
 // ===================================================================
 // Main
 // ===================================================================
 
 int main() {
-    const auto config = config_from_env();
-    if (!config) {
-        std::println(stderr,
-            "Set DS_MYSQL_TEST_HOST, DS_MYSQL_TEST_USER, DS_MYSQL_TEST_PASSWORD "
-            "to run this example.");
-        return 1;
-    }
-
     // --- Connect ---
-    auto db_result = ds_mysql::mysql_database::connect(*config);
+    auto db_result = ds_mysql::mysql_database::connect(ds_mysql::mysql_config{
+        ds_mysql::host_name{k_host},
+        ds_mysql::database_name{k_database},
+        ds_mysql::auth_credentials{ds_mysql::user_name{k_user}, ds_mysql::user_password{k_password}},
+        ds_mysql::port_number{k_port},
+    });
     if (!db_result) {
         std::println(stderr, "Connection failed: {}", db_result.error());
         return 1;
     }
     auto& db = *db_result;
-    std::println("Connected to {}:{}", config->host().to_string(), config->port().to_unsigned_int());
+    std::println("Connected to {}:{}", k_host, k_port);
 
     // --- Create table ---
     if (auto r = db.execute(ds_mysql::create_table<product>().if_not_exists()); !r) {

lib/include/ds_mysql/sql.hpp

@@ -1817,6 +1817,27 @@ class describe_builder {
 // ---------------------------------------------------------------
 // insert_into_values_builder / insert_into_builder
 // ---------------------------------------------------------------
+template <typename T>
+class insert_into_upsert_builder {
+public:
+    insert_into_upsert_builder(std::string insert_sql, std::string update_clause)
+        : insert_sql_(std::move(insert_sql)), update_clause_(std::move(update_clause)) {
+    }
+
+    [[nodiscard]] std::string build_sql() const {
+        std::string sql;
+        sql.reserve(insert_sql_.size() + 26 + update_clause_.size());
+        sql += insert_sql_;
+        sql += " ON DUPLICATE KEY UPDATE ";
+        sql += update_clause_;
+        return sql;
+    }
+
+private:
+    std::string insert_sql_;
+    std::string update_clause_;
+};
+
 template <typename T>
 class insert_into_values_builder {
 public:
@@ -1828,9 +1849,8 @@ class insert_into_values_builder {
     // Each argument must be a FieldOf<T> column_field instance carrying the new value.
     template <FieldOf<T>... Cols>
         requires(sizeof...(Cols) > 0)
-    [[nodiscard]] std::string on_duplicate_key_update(Cols const&... assignments) const {
+    [[nodiscard]] insert_into_upsert_builder<T> on_duplicate_key_update(Cols const&... assignments) const {
         std::ostringstream ss;
-        ss << build_sql() << " ON DUPLICATE KEY UPDATE ";
         bool first = true;
         (
             [&](auto const& field) {
@@ -1842,7 +1862,7 @@ class insert_into_values_builder {
                 first = false;
             }(assignments),
             ...);
-        return ss.str();
+        return insert_into_upsert_builder<T>{build_sql(), ss.str()};
     }
 
     [[nodiscard]] std::string build_sql() const {

tests/unit/test_dml.cpp

@@ -322,7 +322,7 @@ suite<"DML"> dml_suite = [] {
         row.ticker_ = "AAPL";
         row.instrument_ = "Stock";
         auto const sql =
-            insert_into<asset>().values(row).on_duplicate_key_update(asset::ticker{"AAPL"}, asset::instrument{"Stock"});
+            insert_into<asset>().values(row).on_duplicate_key_update(asset::ticker{"AAPL"}, asset::instrument{"Stock"}).build_sql();
         expect(sql ==
                "INSERT INTO asset (id, exchange_id, ticker, instrument, name, sector, currency, created_date, "
                "last_updated_date) "