2.3 Datatypes

DBTools.h++ stores data fetched from a database into a DBTools.h++ application in instances of the RWDBValue class. Most applications can ignore RWDBValue and fetch data directly into C++ variables. If you need to use RWDBValue, however, Table 1 shows how SQL Server datatypes are mapped to RWDBValue datatypes, as well as appropriate C++ datatypes.

Table 1 -- Datatype mapping when retrieving data

SQL Server datatype	RWDBValue::ValueType	C++ datatype
`char(n), varchar(n), text`	`String`	*RWCString*
¹nchar(n), nvarchar(n), ntext	WString	*RWWString*
`binary(n), varbinary(n), image`	`Blob`	*RWDBBlob*
`bit, tinyint, smallint`	`Short`	`short int`
`int`	`Long`	`long int`
`float(p)`	`Double`	`double`
`real`	`Float`	`float`
`decimal(p,s), money, smallmoney`	`Decimal`	*RWDecimalPortable*
timestamp	Blob	*RWDBBlob*
`datetime, smalldatetime`	`DateTime`	*RWDBDateTime*

1	Note: SQLServer 6.5 doesn't support the nchar, varchar, and ntext datatypes.

When an application uses DBTools.h++ to create a SQL Server table, an RWDBValue::ValueType is used to define the table's schema. Table 2 shows how DBTools.h++ maps its datatypes to SQL server datatypes when defining a schema

Table 2 -- Datatype mapping when creating tables.

RWDBValue::ValueType	SQL Server datatype
`Blob`	`varbinary` if `length` < 256, otherwise `image` (SQL Server 6.5) `varbinary` if `length` < 8000, otherwise `image` (SQL Server 7.0)
`Char, UnsignedChar`	`tinyint`
`Date, DateTime`	`datetime`
`Decimal`	`decimal(p,s)` using `precision` and `scale` of column.
`Duration`	Not supported by Microsoft SQL Server
`Float`	`real`
`Double`	`float(p)`, using precision of column
`Int, UnsignedInt`	`int`
`Long, UnsignedLong`	`int`
MBString	Not supported by Microsoft SQL Server
`Short, UnsignedShort`	`smallint`
`String`	`varchar` if `length` < 256 bytes, otherwise `text` (SQL Server 6.5) `varchar` if `length` < 8000 bytes, otherwise `text` (SQL Server 7.0)
¹WString	`varchar` if `length` < 256 bytes, otherwise `text` (SQL Server 6.5) `nvarchar` if `length` < 8000 bytes, otherwise `ntext` (SQL Server 7.0)

1	SQL Server 6.5 doesn't support the nchar, nvarchar, or ntext datatypes.

2.3.1 Restrictions on the Use of Datatypes

Microsoft SQL Server places restrictions on the use of certain datatypes. Where possible, DBTools.h++ tries to be flexible about datatypes. However, there are some situations where conversions are not possible. In these cases your application must be aware of the contexts in which certain datatypes can be used. Table 3 outlines the restrictions associated with each DBTools.h++ type.

Table 3 -- Restrictions on the use of DBTools.h++ datatypes

RWDBValue::ValueType	Restrictions
`Blob`	No restrictions.
`Char, UnsignedChar`	No restrictions.
`Date`	No restrictions.
`DateTime`	No restrictions.
`Decimal`	No restrictions.
`Double`	No restrictions.
`Duration`	Not supported by Microsoft SQL Server
`Float`	No restrictions.
`Int, UnsignedInt`	No restrictions.
`Long, UnsignedLong`	No restrictions.
`MBString`	Not supported by Microsoft SQL Server
`Short, UnsignedShort`	No restrictions.
`String, WString`	No restrictions.

2.3.2 Using Identity Columns

Microsoft SQL Server supports identity columns. Identity columns can be used within DBTools.h++, following the restrictions specified by the Microsoft SQL Server documentation. For more information, see the Microsoft SQL Server ODBC driver documentation.

Applications can create tables with identity columns by providing the native SQL statement, then executing it using RWDBConnection::executeSql().

Applications can also acquire the schema of a table that contains an identity column. The schema will return identity columns as RWDBValue::Decimal datatypes. However, there is no way to determine if the column is actually an identity column. Applications must know the identity column of a table to use it correctly.

New rows can be inserted into tables by not providing values for identity columns, effectively allowing the SQL server to provide values. In general, you should only provide values for identity columns if an application requires a specific value. All values must follow the restrictions of the SQL server.

Of course, updating the identity columns is strictly forbidden by the SQL server.