sqlgen/docs/defining_tables.md

Defining tables

In sqlgen, tables are defined using C++ structs. In its simplest form, this can look like this:

struct People {
    std::string first_name;
    std::string last_name;
    uint age;
};

When you then try to write a vector of People, the table will be automatically created, if necessary:

const auto people = std::vector<People>(...);

const auto result = sqlgen::write(conn, people);

In other words, sqlgen::write(...) might call the following SQL statement under-the-hood:

CREATE TABLE IF NOT EXISTS "People"("first_name" TEXT NOT NULL, 
                                    "last_name"  TEXT NOT NULL, 
                                    "age"        INTEGER NOT NULL);

(As with all SQL examples, the exact code will vary between dialects.)

Custom table names

Obviously, it is very convenient to just infer the table name from the name of the struct. But sometimes we don't want that. So we can do something like this:

struct People {
    constexpr static const char* tablename = "PEOPLE";

    std::string first_name;
    std::string last_name;
    uint age;
};

Now, the generated SQL code will look like this:

CREATE TABLE IF NOT EXISTS "PEOPLE"("first_name" TEXT NOT NULL, 
                                    "last_name"  TEXT NOT NULL, 
                                    "age"        INTEGER NOT NULL);

Custom schemata

In a similar manner, you can embed your table in a schema:

struct People {
    constexpr static const char* schema = "my_schema";

    std::string first_name;
    std::string last_name;
    uint age;
};

Now, the generated SQL code will look like this:

CREATE TABLE IF NOT EXISTS "my_schema"."People"("first_name" TEXT NOT NULL, 
                                                "last_name"  TEXT NOT NULL, 
                                                "age"        INTEGER NOT NULL);

Nullable types

As we have seen, all columns are non-nullable by default. But what if we do want nullability?

As with reflect-cpp, std::optional, std::shared_ptr and std::unique_ptr are interpreted as nullable types. So you can create nullable fields as follows:

struct People {
    rfl::PrimaryKey<std::string> first_name;
    std::string last_name;
    std::optional<uint> age;
};

You can induce the NULL value by passing std::nullopt to age and NULL values will be translated to std::nullopt.