Added support for UNION and UNION ALL (#96)

docs/README.md

@@ -29,6 +29,7 @@ Welcome to the sqlgen documentation. This guide provides detailed information ab
 - [sqlgen::inner_join, sqlgen::left_join, sqlgen::right_join, sqlgen::full_join](joins.md) - How to join different tables
 - [sqlgen::insert](insert.md) - How to insert data within transactions
 - [sqlgen::select_from](select_from.md) - How to read data from a database using more complex queries
+- [sqlgen::unite and sqlgen::unite_all](unite.md) - How to combine results from multiple SELECT statements
 - [sqlgen::update](update.md) - How to update data in a table
 
 ## Other Operations

docs/unite.md

@@ -0,0 +1,80 @@
+# `unite` and `unite_all`
+
+The `unite` and `unite_all` functions allow you to combine the results of multiple `SELECT` statements into a single result set.
+
+## `unite`
+
+The `unite` function corresponds to the SQL `UNION` operator. It combines the result sets of two or more `SELECT` statements and removes duplicate rows.
+
+### Example
+
+```cpp
+struct User1 {
+  std::string name;
+  int age;
+};
+
+struct User2 {
+  std::string name;
+  int age;
+};
+
+const auto s1 = sqlgen::select_from<User1>("name"_c, "age"_c);
+const auto s2 = sqlgen::select_from<User2>("name"_c, "age"_c);
+
+const auto united = sqlgen::unite<std::vector<User1>>(s1, s2);
+```
+
+## `unite_all`
+
+The `unite_all` function corresponds to the SQL `UNION ALL` operator. It combines the result sets of two or more `SELECT` statements, including all duplicate rows.
+
+### Example
+
+```cpp
+struct User1 {
+  std::string name;
+  int age;
+};
+
+struct User2 {
+  std::string name;
+  int age;
+};
+
+const auto s1 = sqlgen::select_from<User1>("name"_c, "age"_c);
+const auto s2 = sqlgen::select_from<User2>("name"_c, "age"_c);
+
+const auto united = sqlgen::unite_all<std::vector<User1>>(s1, s2);
+```
+
+## Nesting in `SELECT` statements
+
+You can use the result of a `unite` or `unite_all` operation as a subquery in a `SELECT` statement.
+
+### Example
+
+```cpp
+const auto united = sqlgen::unite<std::vector<User1>>(s1, s2);
+
+const auto sel = sqlgen::select_from(united.as("u"), "name"_c, "age"_c);
+```
+
+## Nesting in `JOIN` statements
+
+You can also use the result of a `unite` or `unite_all` operation as a subquery in a `JOIN` statement.
+
+### Example
+
+```cpp
+struct Login {
+  int id;
+  std::string username;
+};
+
+const auto united = sqlgen::unite<std::vector<User1>>(s1, s2);
+
+const auto sel = select_from<Login, "t1">("id"_t1, "username"_t1) |
+                 inner_join<"t2">(united, "username"_t1 == "name"_t2) |
+                 where("id"_t1 == 1) | to<std::vector<Login>>;
+```