Reading Tables: Class RWDBReader
Before we look at the other types of tables available in the DB Interface Module, let’s look at how the data in a table is read. The next example program shows how to read information from a table using the class
RWDBReader. The database table it references is the
purchase table, which is part of the tutorials for SourcePro DB.
NOTE: You must install and compile the tutorials when you install the SourcePro DB in order to run this example. See Installing and Building Your SourcePro Products and Building Your Applications for more information.
To run this example, change the parameters for the RWDBManager::database() call to fit your system. To use this code, the <ver> placeholder in any library names would need to be replaced with the actual digits representing the library version number.
Example 2 – Reading a database table
// Example Program 2 - Reading a database table
#include <rw/rstream.h>
#include <rw/db/db.h>
int
main()
RWDBDatabase myDbase = RWDBManager::database(
"libctl<ver>12d.so", // DB Access Module name
"SYBASE100", // server name
"user", // user name
"pwd", // password
"DEMO_DB" // database name
);
RWDBConnection myConnection = myDbase.connection();
RWDBTable purchases = myDbase.table("purchase");
RWDBReader rdr = purchases.reader(myConnection);
int videoID;
int supplierID;
int purchaseOrderNumber;
RWDecimalPortable pricePerUnit;
int quantity;
while (rdr()) {
RWDateTime date;
rdr >> videoID >> supplierID >> purchaseOrderNumber //1
>> pricePerUnit >> quantity >> date;
std::cout << videoID << "\t" << supplierID << "\t"
<< purchaseOrderNumber << "\t" << pricePerUnit << "\t"
<< quantity << "\t" << date.asString() << std::endl;
}
return 0;
}
RWDBReader reads tabular data, providing sequential access to a table's rows, and random access to fields within a row.
While traversing a table's rows, a reader acts much like an iterator. When a reader is first obtained from a table, it is positioned before the first row of the table. The C++ function call operator() is used to advance the reader to the next row. When there are no more rows, it returns 0.
Within a row, the reader is used with the extraction operator >>, which extracts values from the reader into program variables. Because indexing by column name and column number is also supported, we could have written //1 of our example like this:
rdr["videoID"] >> videoID;
rdr["supplierID"] >> supplierID;
// etc.
If we were concerned about NULL values in certain columns, we could have used indicator variables with our reader as a way to detect them:
RWDBNullIndicator nullVideo, nullSupplier;
int videoID;
int supplierID;
int purchaseOrderNumber;
RWDecimalPortable pricePerUnit;
int quantity;
RWDateTime date;
while (rdr()) {
rdr >> nullVideo >> videoID >> nullSupplier >> supplierID
>> purchaseOrderNumber >> pricePerUnit >> quantity >> date;
if (nullVideo || nullSupplier) {
std::cout << "Missing Information!" << std::endl;
}
else {
std::cout << videoID << "\t" << supplierID << "\t"
<< purchaseOrderNumber << "\t" << pricePerUnit << "\t"
<< quantity << "\t" << date.asString() << std::endl;
}
}
