2.3 Datatypes

Data fetched from a database into a DBTools.h++ application may be stored internally by DBTools.h++ in instances of the RWDBValue class. Most applications can ignore RWDBValue and fetch data directly into C++ variables. If you do need to use it, Table 1 shows how Informix datatypes are mapped into RWDBValues, and lists the C++ class or datatype appropriate for each Informix datatype.

Table 1 -- Type mapping when retrieving data

Informix Datatype	RWDBValue::ValueType	C++ Datatype
`BOOLEAN`	UnsignedChar	unsigned char
`BYTE`	`Blob`	*RWDBBlob*
`CHAR(n)`	`String`	*RWCString*
`DATE`	`Date`	*RWDate*
`DATETIME`	`DateTime`	*RWDBDateTime*
`DECIMAL(p,s)`	`Decimal`	*RWDecimalPortable*
`DOUBLE PRECISION`	`Double`	`double`
`FLOAT`	`Double`	`double`
`INTEGER`	`Long`	`long int`
`INTERVAL`	`Duration`	*RWDBDuration*
`MONEY(p,s)`	`Decimal`	*RWDecimalPortable*
NUMERIC(p,s)	Decimal	*RWDecimalPortable*
`SERIAL`	`Long`	`long int`
`SMALLFLOAT`	`Float`	`float`
`SMALLINT`	`Short`	`short int`
`TEXT`	`String`	*RWCString*
`VARCHAR`	`String`	*RWCString*

When an application uses the DBTools.h++ Informix Access Library to create a table, RWDBValue::ValueTypes are used to define the table's schema. These RWDBValue::ValueTypes must be mapped to a type name specific to the Informix server. Here is how that is done.

First, an Informix ODBC Driver SQL type associated with RWDBValue::ValueType must be found. Table 2 shows how DBTools.h++ chooses an SQL type to associate with a particular RWDBValue::ValueType:

Table 2 -- Type mapping when creating tables

RWDBValue::ValueType	Informix ODBC Driver SQL Datatypes
`Blob`	If `length`>255 or `length` = 0, `SQL_LONGVARBINARY;` otherwise, `SQL_VARBINARY`
`Char`, `UnsignedChar`	`SQL_TINYINT`
`Date`	`SQL_TYPE_DATE`
`DateTime`	`SQL_TYPE_TIMESTAMP`
`Decimal`	`SQL_DECIMAL`
`Double`	`SQL_DOUBLE`
`Duration`	`SQL_INTERVAL_SECOND`
`Float`	`SQL_REAL`
`Int`, `UnsignedInt`	`SQL_SMALLINT`
`Long`, `UnsignedLong`	`SQL_INTEGER`
`Short`, `UnsignedShort`	`SQL_SMALLINT`
`String`	If `length` is > 254 or `length` = 0, `SQL_LONGVARCHAR;` otherwise, `SQL_VARCHAR`

In Table 2, the column on the right holds an Informix ODBC Driver SQL type to associate with an RWDBValue::ValueType.

After the type is selected, the Informix ODBC Driver function SQLGetTypeInfo() is used to find the type name associated with the chosen SQL type. The type name is then used by the DBTools.h++ Informix Access Library to create tables.

2.3.1 Restrictions on the Use of Datatypes

Neither DBTools.h++ nor the ODBC specification impose restrictions on the use of most datatypes. Check for specific restrictions in the documentation provided with your Informix ODBC Driver. Table 3 outlines the known restrictions associated with each DBTools.h++ type.

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

RWDBValue::ValueType	Restrictions
`Blob`	If the underlying datatype of the blob is `CHAR`, the blob cannot contain `null(0)` bytes. Typically, blobs can be used in all expressions; however, if the underlying datatype of the blob is `BYTE`, the Informix server imposes restrictions on use. See the Informix Guide to SQL, Reference.
`Char, UnsignedChar`	No restrictions.
`Date`	No restrictions.
`DateTime`	No restrictions.
`Decimal`	No restrictions.
`Double`	No restrictions.
`Duration`	No restrictions.
`Float`	No restrictions.
`Int, UnsignedInt`	No restrictions.
`Long, UnsignedLong`	No restrictions.
`MBString`	This datatype is not supported.
`String`	Typically strings can be used in all expressions; however, if the underlying datatype of the string is `TEXT`, the Informix server imposes restrictions on use. See the Informix Guide to SQL, Reference.
`Short, UnsignedShort`	No restrictions.
`WString`	This datatype is not supported.