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## The dynamic interface

To use the dynamic interface, use the `results` class. `results` is an in-memory representation of a resultset. We can depict it like this (this is actually a simplified representation, since some statements may return more than one resultset).

We can see that `results::rows` returns a matrix-like object, containing the retrieved rows. This section is dedicated on diving deeper on how to use these objects.

#### Rows and fields

This matrix-like structure is composed of variant-like objects called fields. Field objects are capable of representing any value retrieved from MySQL.

This library defines the following classes to work with rows and fields:

`field`

  The smallest unit of data. A single "cell" in a MySQL table. This is an owning, variant-like type.

`field_view`

  Like `field`, but non-owning.

`row`

  An owning, `vector`-like collection of fields.

`row_view`

  Like `row`, but non-owning.

`rows`

  An owning, matrix-like collection of fields. Represents several rows of the same size in an optimized way.

`rows_view`

  Like `rows`, but non-owning.

`results::rows` returns a `rows_view` object. The memory for the rows is owned by the `results` object. Indexing the returned view also returns view objects:

    // Populate a results object
    results result;
    conn.execute("SELECT 'Hello world'", result);

    // results::rows() returns a rows_view. The underlying memory is owned by the results object
    rows_view all_rows = result.rows();

    // Indexing a rows_view yields a row_view. The underlying memory is owned by the results object
    row_view first_row = all_rows.at(0);

    // Indexing a row_view yields a field_view. The underlying memory is owned by the results object
    field_view first_field = first_row.at(0);  // Contains the string "Hello world"

Views behave similarly to `std::string_view`. You must make sure that you don't use a view after the storage it points to has gone out of scope. In this case, you must not use any of the views after the `results` object has gone out of scope.

As it happens with `std::string_view`, you can take ownership of a view using its owning counterpart:

    // You may use all_rows_owning after result has gone out of scope
    rows all_rows_owning{all_rows};

    // You may use first_row_owning after result has gone out of scope
    row first_row_owning{first_row};

    // You may use first_field_owning after result has gone out of scope
    field first_field_owning{first_field};

#### Using fields

`field` and `field_view` are specialized variant-like types that can hold any type you may find in a MySQL table. Once you obtain a field, you can access its contents using the following functions:

- You can query a field's type by using `kind`, which returns a `field_kind` enum.
- You can query whether a field contains a certain type with `field::is_xxx`.
- You can get the underlying value with `field::as_xxx` and `field::get_xxx`. The `as_xxx` functions are checked (they will throw an exception if the actual type doesn't match), while the `get_xxx` are unchecked (they result in undefined behavior on type mismatch).
- You can stream fields and compare them for equality.

For example:

    results result;
    conn.execute("SELECT 'abc', 42", result);

    // Obtain a field's underlying value using the is_xxx and get_xxx accessors
    field_view f = result.rows().at(0).at(0);  // f points to the string "abc"
    if (f.is_string())
    {
        // we know it's a string, unchecked access
        string_view s = f.get_string();
        std::cout << s << std::endl;  // Use the string as required
    }
    else
    {
        // Oops, something went wrong - schema mismatch?
    }

    // Alternative: use the as_xxx accessor
    f = result.rows().at(0).at(1);
    std::int64_t value = f.as_int64();  // Checked access. Throws if f doesn't contain an int
    std::cout << value << std::endl;    // Use the int as required

`NULL` values are represented as field objects having `kind() == field_kind::null`. You can check whether a value is `NULL` or not using `is_null`. This is how `NULL`s are typically handled:

    results result;

    // Create some test data
    conn.execute(
        R"%(
            CREATE TEMPORARY TABLE products (
                id VARCHAR(50) PRIMARY KEY,
                description VARCHAR(256)
            )
        )%",
        result
    );
    conn.execute("INSERT INTO products VALUES ('PTT', 'Potatoes'), ('CAR', NULL)", result);

    // Retrieve the data. Note that some fields are NULL
    conn.execute("SELECT id, description FROM products", result);

    for (row_view r : result.rows())
    {
        field_view description_fv = r.at(1);
        if (description_fv.is_null())
        {
            // Handle the NULL value
            // Note: description_fv.is_string() will return false here; NULL is represented as a separate
            // type
            std::cout << "No description for product_id " << r.at(0) << std::endl;
        }
        else
        {
            // Handle the non-NULL case. Get the underlying value and use it as you want
            // If there is any schema mismatch (and description was not defined as VARCHAR), this will
            // throw
            string_view description = description_fv.as_string();

            // Use description as required
            std::cout << "product_id " << r.at(0) << ": " << description << std::endl;
        }
    }

#### MySQL to C++ type mappings

Every MySQL type is mapped to a single C++ type. The following table shows these mappings:

`field_kind` | C++ type | MySQL types | `is` accessor | `as` accessor | `get` accessor
--- | --- | --- | --- | --- | ---
`int64` | `std::int64_t` | `TINYINT`, `SMALLINT`, `MEDIUMINT`, `INT`, `BIGINT` | `is_int64` | `as_int64` | `get_int64`
`uint64` | `std::uint64_t` | Unsigned `TINYINT`, `SMALLINT`, `MEDIUMINT`, `INT`, `BIGINT`, `YEAR`, `BIT` | `is_uint64` | `as_uint64` | `get_uint64`
`string` | `string_view` for `field_view` `std::string` for `field` | `CHAR`, `VARCHAR`, `TEXT` (all sizes), `ENUM`, `SET`, `DECIMAL`, `NUMERIC`, `JSON` | `is_string` | `as_string` | `get_string`
`blob` | `blob_view` for `field_view` `blob` for `field` | `BINARY`, `VARBINARY`, `BLOB` (all sizes), `GEOMETRY` | `is_blob` | `as_blob` | `get_blob`
`float_` | `float` | `FLOAT` | `is_float` | `as_float` | `get_float`
`double_` | `double` | `DOUBLE` | `is_double` | `as_double` | `get_double`
`date` | `date` | `DATE` | `is_date` | `as_date` | `get_date`
`datetime` | `datetime` | `DATETIME`, `TIMESTAMP` | `is_datetime` | `as_datetime` | `get_datetime`
`time` | `time` | `TIME` | `is_time` | `as_time` | `get_time`
`null` |  | Any of the above, when they're `NULL` | `is_null` |  |

No character set conversion is applied on strings. They are provided as the server sends them. If you've run `async_set_character_set`, strings will be encoded according to the passed character set. For details, see this section.

#### The field class

`field_view` is to `field` what `std::string_view` is to `std::string`.

`field_view`s are cheap to create and to copy, as they are small objects and don't perform any memory allocations. They are also immutable. On the other hand, `field`s may be more expensive to create and copy, as they may perform memory allocations. `field`s are mutable.

`field` and `field_view` use the same underlying types for scalars. For strings and blobs, `field` uses the owning types `std::string` and `blob`, while `field_view` uses the reference types `string_view` and `blob_view`.

`field` accessors return references, which allow you to mutate the underlying object:

    field f("my_string");            // constructs a field that owns the string "my_string"
    std::string& s = f.as_string();  // s points into f's storage
    s.push_back('2');                // f now holds "my_string2"

You can also mutate a `field` using the assignment operator. This allows you to also change the underlying type of a `field`:

    field f("my_string");  // constructs a field that owns the string "my_string"
    f = 42;                // destroys "my_string" and stores the value 42 as an int64

#### Multi-resultset and multi-function operations

You can use both with the dynamic interface. Please refer to the sections on multi-resultset operations and multi-function operations for more information.

#### MySQL to C++ type mapping reference

The following table reflects mapping from database types to C++ types. The range column shows the range of values that MySQL admits for that type. This library guarantees that any field retrieved from the database honors that range. The `column_type` column shows what `metadata::type` would return for a column of that type.

MySQL type | `field_kind` | C++ type | Range | `column_type` | Considerations
--- | --- | --- | --- | --- | ---
`TINYINT` | `int64` | `std::int64_t` | `-0x80` to `0x7f` | `tinyint` | 1 byte integer
`TINYINT` `UNSIGNED` | `uint64` | `std::uint64_t` | `0` to `0xff` | `tinyint` | 1 byte integer
`SMALLINT` | `int64` | `std::int64_t` | `-0x8000` to `0x7fff` | `smallint` | 2 byte integer
`SMALLINT` `UNSIGNED` | `uint64` | `std::uint64_t` | `0` to `0xffff` | `smallint` | 2 byte integer
`MEDIUMINT` | `int64` | `std::int64_t` | `-0x800000` to `0x7fffff` | `mediumint` | 3 byte integer
`MEDIUMINT` `UNSIGNED` | `uint64` | `std::uint64_t` | `0` to `0xffffff` | `mediumint` | 3 byte integer
`INT` | `int64` | `std::int64_t` | `-0x80000000` to `0x7fffffff` | `int_` | 4 byte integer
`INT` `UNSIGNED` | `uint64` | `std::uint64_t` | `0` to `0xffffffff` | `int_` | 4 byte integer
`BIGINT` | `int64` | `std::int64_t` | `-0x8000000000000000` to `0x7fffffffffffffff` | `bigint` | 8 byte integer
`BIGINT` `UNSIGNED` | `uint64` | `std::uint64_t` | `0` and `0xffffffffffffffff` | `bigint` | 8 byte integer
`YEAR` | `uint64` | `std::uint64_t` | [`1901`, `2155`], plus zero | `year` | 1 byte integer type used to represent years Zero is often employed to represent invalid year values. We represent zero year as a numeric 0.
`BIT` | `uint64` | `std::uint64_t` | Depends on the bitset width. Max `0` to `0xffffffffffffffff`. | `bit` | A bitset between 1 and 64 bits wide.
`FLOAT` | `float_` | `float` | IEEE 754 `float` range | `float_` | 4 byte floating point type
`DOUBLE` | `double_` | `double` | IEEE 754 `double` range | `double_` | 8 byte floating point type
`DATE` | `date` | `date` | [`min_date`, `max_date`] (some MySQL implementations may allow a narrower range), plus invalid and zero dates (see `ALLOW_INVALID_DATES` and strict SQL mode). | `date` |
`DATETIME` | `datetime` | `datetime` | [`min_datetime`, `max_datetime`] (some MySQL implementations may allow a narrower range), plus invalid and zero datetimes (see `ALLOW_INVALID_DATES` and strict SQL mode). | `datetime` | Time point type without time zone, with a resolution of one microsecond.
`TIMESTAMP` | `datetime` | `datetime` | [`min_datetime`, `max_datetime`] (the actual MySQL supported range is usually narrower, but we don't enforce it in the client), plus zero timestamps (see strict SQL mode). | `timestamp` | Time point type with a resolution of one microsecond.
`TIME` | `time` | `time` | [`min_time`, `max_time`] | `time` | Signed time duration, with a resolution of one microsecond.
`CHAR` | `string` | `string_view` or `std::string` |  | `char_` | Fixed-size character string.
`VARCHAR` | `string` | `string_view` or `std::string` |  | `varchar` | Variable size character string with a maximum size.
`TEXT` (all sizes) | `string` | `string_view` or `std::string` |  | `text` | Variable size character string.
`ENUM` | `string` | `string_view` or `std::string` |  | `enum_` | Character string with a fixed set of possible values (only one possible).
`SET` | `string` | `string_view` or `std::string` |  | `set` | Character string with a fixed set of possible values (many possible).
`JSON` | `string` | `string_view` or `std::string` |  | `json` (MySQL) or `text` (MariaDB) | A serialized JSON value of any type. Note that `metadata::type` is different depending on the DB system. MySQL has a dedicated `JSON` type, while in MariaDB `JSON` is an alias for `LONGTEXT`. JSON values are represented as strings by this library in both cases.
`DECIMAL`/`NUMERIC` | `string` | `string_view` or `std::string` | Depends on the column definition | `decimal` | A fixed precision numeric value. In this case, the string will contain the textual representation of the number (e.g. the string `"20.52"` for `20.52`). This type is mapped to a string to avoid losing precision.
`BINARY` | `blob` | `blob_view` or `blob` |  | `binary` | Fixed-size blob.
`VARBINARY` | `blob` | `blob_view` or `blob` |  | `varbinary` | Variable size blob with a maximum size.
`BLOB` (all sizes) | `blob` | `blob_view` or `blob` |  | `blob` | Variable size blob.
`GEOMETRY` | `blob` | `blob_view` or `blob` |  | `geometry` | Any of the spatial data types. The string contains the binary representation of the geometry type.