SqlNullValue.hpp

// SPDX-License-Identifier: Apache-2.0
 
#pragma once
 
#include "Core.hpp"
 
namespace Lightweight
{
 
/// Helper binder type to indicate NULL values in SQL queries.
///
/// @ingroup DataTypes
struct SqlNullType
{
    /// The underlying SQL NULL indicator value.
    SQLLEN sqlValue = SQL_NULL_DATA;
};
 
/// Used to indicate a NULL value in a SQL query.
///
/// @ingroup DataTypes
constexpr inline auto SqlNullValue = SqlNullType {};
 
template <>
struct SqlDataBinder<SqlNullType>
{
    static LIGHTWEIGHT_FORCE_INLINE SQLRETURN InputParameter(SQLHSTMT stmt,
                                                             SQLUSMALLINT column,
                                                             SqlNullType const& value,
                                                             SqlDataBinderCallback& cb) noexcept
    {
        // This is generally ignored for NULL values, but MS SQL Server requires a non-zero value
        // when the underlying type is e.g. an INT.
 
        // For MS SQL Server, we need to determine the actual SQL type of the column to bind the parameter correctly,
        // because requires a correctly convertible SQL type for NULL values.
        // e.g. MS SQL Server cannot convert a VARCHAR NULL to a binary column type (e.g. BLOB).
        // The type comes from the statement rather than a SQLDescribeParam() here: the driver
        // refuses to describe once any parameter is bound, which a NULL after the first one always is.
        SQLSMALLINT const sqlType = cb.ServerType() == SqlServerType::MICROSOFT_SQL
                                        ? cb.DescribeInputParameterType(column).value_or(SQL_C_CHAR)
                                        : SQLSMALLINT { SQL_C_CHAR };
 
        return SQLBindParameter(stmt,
                                column,
                                SQL_PARAM_INPUT,
                                SQL_C_CHAR,
                                sqlType,
                                10,
                                0,
                                nullptr,
                                0,
                                &const_cast<SqlNullType&>(value).sqlValue);
    }
 
    static LIGHTWEIGHT_FORCE_INLINE std::string_view Inspect(SqlNullType const& /*value*/) noexcept
    {
        return "NULL";
    }
};
 
} // namespace Lightweight