MySQL supported types

The uMySQL driver supports various std:: types for both input and output directions and some userver-specific types.
The driver also supports aggregates of supported types as input or output parameters:
whenever there is a row-wise mapping to/from T, T is expected to be an SimpleAggregate, that is an aggregate type without base classes, const fields, references, or C arrays.

The following table shows how types are mapped from C++ to MySQL types.

C++ to MySQL Mapping

Input C++ types	Corresponding MySQLType
std::uint8_t/std::int8_t	TINYINT
std::uint16_t/std::int16_t	SMALLINT
std::uint32_t/std::int32_t	INT
std::uint64_t/std::int64_t	BIGINT
float	FLOAT
double	DOUBLE
std::string	TEXT
std::string_view	TEXT
const char *	TEXT
std::chrono::system_clock::timepoint	TIMESTAMP
storages::mysql::Date	DATE
storages::mysql::DateTime	DATETIME
formats::json::Value	TEXT
decimal64::Decimal<Prec, Policy>	DECIMAL

The driver also supports std::optional<T> for every supported type, and that maps to either NULL or exactly as T does, depending on whether is optional engaged or not.

Mapping MySQL types to C++ types is a more complicated matter:

• First, the driver forbids mapping signed to unsigned or vice versa - an attempt to do so will abort in debug build and throw in release (this behavior is referred as UINVARIANT later), this check happens before actual extraction.
  
• Second, the driver forbids mapping NULLABLE column into non-optional C++ type, an attempt to do so will UINVARIANT, this check happens before actual extraction. However, other combinations of optional/non-optional<->NULL/NOT NULL are supported:

Is column NULLABLE	Is type optional	behavior
true	true	allowed
true	false	UINVARIANT
false	true	allowed
false	false	allowed

• Third, an attempt to parse DECIMAL will throw at extraction stage if precision/rounding policy of corresponding decimal::Decimal64<Prec, Policy> is violated.
• Fourth, an attempt to parse DATE or DATETIME not within TIMEPOINT range into std::chrono::system_clock::timepoint will throw at extraction stage. The decision was made to allow this conversion, since it might be very convenient to use, but in general this conversion is narrowing.
• Fifth, general type mismatch will UINVARIANT, however widening numeric conversions are allowed, this check happens before actual extraction. You can see to what C++ types MySQL types can map in the following table:

MySQL to C++ mapping

Output C++ type	Allowed MySQL types
std::uint8_t	TINYINT UNSIGNED NOT NULL
std::int8_t	TINYINT NOT NULL
std::optional<std::uint8_t>	TINYINT UNSIGNED NOT NULL, TINYINT UNSIGNED
std::optional<std::int8_t>	TINYINT NOT NULL, TINYINT
std::uint16_t/std::int16_t/optional	SMALLINT + all types above, with respect to signed/unsigned and NULL/NOT NULL
std::uint32_t/std::int32_t/optional	INT, INT24 + all types above, with respect to signed/unsigned and NULL/NOT NULL
std::uint64_t/std::int64_t/optional	BIGINT + all types above, with respect to signed/unsigned and NULL/NOT NULL
float	FLOAT NOT NULL
std::optional<float>	FLOAT NOT NULL, FLOAT
double	DOUBLE NOT NULL, FLOAT NOT NULL
std::optional<double>	DOUBLE, DOUBLE NOT NULL, FLOAT, FLOAT NOT NULL
std::string/optional	CHAR, BINARY, VARCHAR, VARBINARY, TINYBLOB, TINYTEXT, BLOB, TEXT, MEDIUMBLOB, MEDIUMTEXT, LONGBLOB, LONGTEXT, with respect to NULL/NOT NULL
std::chrono::system_clock::timepoint/optional	TIMEPOINT, DATETIME, DATE, with respect to NULL/NOT NULL
storages::mysql::Date + optional	DATE, with respect to NULL/NOT NULL
storages::mysql::DateTime +optional	DATETIME, with respect to NULL/NOT NULL
formats::json::Value + optional	JSON, with respect to NULL/NOT NULL
decimal64::Decimal<Prec, Policy> + optional	DECIMAL, with respect to NULL/NOT NULL

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