2.18 Open SQL and ODBC
The alternative Open SQL interface can be used with ODBC. This section provides some specific details and examples.
2.18.1 Placeholder Syntax
The placeholder syntax for ODBC is:
?
Here is an example:
INSERT INTO PICNICTABLE VALUES( ? )
The use of placeholders in OpenSQL statements is not a portable feature. The different access modules use different placeholder syntax depending on their underlying vendor implementation.
2.18.2 RWDBDataCallback and Open SQL
The data callback classes can be used with ODBC to insert and fetch data. These classes are useful for very large data that may not fit in memory for a single database call and are described in
Section 15.3, “The Data Callback Classes,” in the
DB Interface Module User’s Guide.
ODBC drivers vary in their capabilities. Some drivers do not support data callbacks used for columns in the select list before the last bound column. There may also be a restriction on the number of rows that can be fetched at one time from a result set, so that calls to RWDBOSql::fetch() may return only one row. Check the documentation for the SQL_GETDATA_EXTENSIONS supported by the ODBC driver for more information.
Multiple rows can be inserted in one call to
RWDBOSql::execute(). The number of rows is determined by the
entries parameter passed to the constructor of a data callback class or an
RWDBTBuffer instance. Please see
Chapter 15, “Using the Open SQL Classes,” in the
DB Interface Module User’s Guide for information about array input.
2.18.2.1 Piece Size Setting
The DB Access Module for ODBC sets the piece size for inserting or fetching data to 32768. This value represents bytes in the case of binary data, and code units in the case of character data.
2.18.2.2 RWDBDataCallback::getLength()
When inserting data using data callbacks, some databases need to be provided the total length of the inserted value, which requires overriding the RWDBDataCallback::getLength() method.
For ODBC, some drivers require the total length of the data value prior to inserting the value when using data callbacks. Either check the ODBC driver documentation for the SQL_NEED_LONG_DATA_LEN option, or call the function SQLGetInfo() with the SQL_NEED_LONG_DATA_LEN option; if it returns “Y“, the method must be overridden in the derived classes.
2.18.2.3 Stored Procedures and RWDBDataCallback
Stored procedure execution is not supported with the data callback classes.
2.18.3 Stored Procedures and Open SQL
Standard ODBC CALL syntax can be used when executing stored procedures with
RWDBOSql. Please see an ODBC API reference for additional information regarding the ODBC CALL syntax.
RWDBOSql myOpenSqlObject("{ CALL myStoredProc(?, ?) }", RWDBOSql::Procedure);
In this example, myStoredProc is the name of the stored procedure, and ? characters are the placeholders for the parameters to be passed to the stored procedure. Note the provided hint RWDBOSql::Procedure.
RWDBOSql myOpenSqlObject("{ ? = CALL myStoredProc(?, ?) }", RWDBOSql::Procedure);
This example uses an additional placeholder to receive the return value from the stored procedure as an OUT parameter.
Please see
Section 2.18.5.3 for an example that uses
RWDBOSql with stored procedures.
Stored procedure execution is not supported with the data callback classes.
2.18.3.1 Sequence of Fetching Result Sets and Out Parameters
Any restrictions on the sequence of fetching OUT parameters and result sets will be imposed by the underlying database ODBC driver. Some database drivers return the OUT parameters only after the result sets are returned. To be on the safe side, call fetchReturnParams() only after completely processing all the result sets.
2.18.4 Executing Cancel in Open SQL
Method
RWDBOSql::cancel() can be used either to cancel the current result set or to cancel all the pending result sets of the current execution. This method should be called only after the
RWDBOSql object is executed.
Method
RWDBOSql::cancel(RWDBOSql::Current) cancels the current result set and moves the
RWDBOSql to the next available result set, if any.
Method RWDBOSql::cancel(RWDBOSql::All) cancels all the pending result sets. No subsequent results will be available from the current execution after this call has been made.
2.18.5 Open SQL Examples
These examples illustrate how an
RWDBOSql object can be used to
All statements are based on an employee table
emp. The examples assume an error handler is associated with the connection in use, and don’t check for any errors after calls to the
RWDBOSql object.
The structure of the table emp is:
empno INT NOT NULL
ename VARCHAR(10) NOT NULL
posn VARCHAR(9) NOT NULL
mgr INT NULL
sal NUMERIC(7,2) NOT NULL
dept INT NOT NULL
2.18.5.1 An Open SQL Insert Example
This example shows how to execute an
INSERT statement using
RWDBOSql to insert data in the
emp table.
const size_t NUM_OF_ROWS = 14; //1
RWDBTBuffer<int> empnoBuff(NUM_OF_ROWS), mgrBuff(NUM_OF_ROWS),
deptBuff(NUM_OF_ROWS);
RWDBTBuffer<RWCString> enameBuff(NUM_OF_ROWS), posnBuff(NUM_OF_ROWS);
RWDBTBuffer<RWDecimalPortable> salBuff(NUM_OF_ROWS); //2
empnoBuff[0] = 1;
enameBuff[0] = "ABC XYZ";
posnBuff[0] = "ADMIN";
mgrBuff[0] = 6;
salBuff[0] = "58000.00";
deptBuff[0] = 4; //3
// Populate rest of the rows
RWDBOSql openSql("INSERT INTO emp VALUES(?, ?, ?, ?, ?, ?)",
RWDBOSql::NonQuery); //4
openSql << empnoBuff << enameBuff << posnBuff
<< mgrBuff << salBuff << deptBuff; //5
openSql.execute(cn); //6
long rowsInserted = openSql.rowsAffected(); //7
std::cout << (openSql.isValid() ? "Data insertion successful." //8
: "Data insertion failed.") << std::endl;
if (rowsInserted >= 0) {
std::cout << "Inserted " << rowsInserted << " rows." << std::endl;
}
2.18.5.2 Open SQL Query Examples
These examples illustrate how an
RWDBOSql object can be used to execute
SELECT queries and retrieve their results. The
SELECT query is executed on table
emp and retrieves all columns of the table for employees in a particular department.
The first example assumes awareness of the structure of table
emp (
Section 2.18.5) and hence the schema of the result set returned by the query.
RWDBOSql openSql("SELECT * FROM emp WHERE dept = ?", RWDBOSql::Query); //1
RWDBTBuffer<int> queryDeptBuff; //2
queryDeptBuff[0] = 3; //3
openSql << queryDeptBuff; //4
openSql.execute(cn); //5
const size_t ROWS_TO_FETCH = 5; //6
RWDBTBuffer<int> empnoBuff(ROWS_TO_FETCH), mgrBuff(ROWS_TO_FETCH),
deptBuff(ROWS_TO_FETCH);
RWDBTBuffer<RWCString> enameBuff(ROWS_TO_FETCH), posnBuff(ROWS_TO_FETCH);
RWDBTBuffer<RWDecimalPortable> salBuff(ROWS_TO_FETCH); //7
openSql[0] >> empnoBuff >> enameBuff >> posnBuff >>
mgrBuff >> salBuff >> deptBuff; //8
long rowsFetched = 0;
while ((rowsFetched = openSql.fetch().rowsFetched()) > 0) { //9
for (size_t i = 0; i < rowsFetched; ++i) {
cout << empnoBuff[i] << "\t" << enameBuff[i] << "\t"
<< posnBuff[i] << "\t"; //10
if (mgrBuff.isNull(i)) { //11
cout << "NULL";
}
else {
cout << mgrBuff[i];
}
cout << "\t" << salBuff[i] << "\t" << deptBuff[i] << endl;
}
}
The second example assumes execution of an ad hoc query for which the schema of the result set is not known beforehand.
RWDBOSql openSql("SELECT * FROM emp WHERE dept = ?", RWDBOSql::Query);
RWDBTBuffer<int> queryDeptBuff;
queryDeptBuff[0] = 3;
openSql << queryDeptBuff;
openSql.execute(cn);
const size_t ROWS_TO_FETCH = 5;
RWDBMultiRow mRow(openSql.schema(), ROWS_TO_FETCH); //1
openSql[0] = mRow; //2
long rowsFetched = 0;
while ((rowsFetched = openSql.fetch().rowsFetched()) > 0) { //3
for (size_t row = 0; row < rowsFetched; ++row) {
for (size_t col = 0; col < mRow.numberOfColumns(); ++col) { //4
RWDBValue val = mRow[row][col]; //5
if ( val.isNull() ) { //6
cout << "NULL" << "\t";
}
else {
cout << val.asString() << "\t";
}
}
cout << endl;
}
}
The steps until
RWDBOSql execution are the same as in the previous example.
2.18.5.3 An Open SQL Stored Procedure Example
This example shows how to invoke a stored procedure using an
RWDBOSql object, fetch result sets, and fetch output parameters. Stored procedure
EmpInDept is assumed to take a department number as an input parameter and provide the count of employees in that department as an output parameter. It produces two result sets: the first returns a list of employee numbers and names for each employee in that department; the second result set returns a single row with three columns containing the minimum, average and maximum salary of employees in that department.
RWDBOSql openSql ("{CALL EmpInDept(?, ?) }", RWDBOSql::Procedure); //1
RWDBTBuffer<int> dept, empCount; //2
empCount.paramType(RWDBColumn::outParameter); //3
openSql << dept << empCount; //4
dept[0] = 2; //5
openSql.execute(cn); //6
const size_t NUMBER_OF_ROWS = 10;
RWDBTBuffer<int> empNums (NUMBER_OF_ROWS); //7
RWDBTBuffer<RWCString> empNames (NUMBER_OF_ROWS); //8
openSql[0] >> empNums >> empNames; //9
long count = 0;
while ((count = openSql.fetch().rowsFetched()) > 0 ) { //10
for (int row = 0; row < count; row++) {
std::cout << "Employee: " << empNums[row] << ", "
<< empNames[row] << std::endl; //11
}
}
RWDBTBuffer<RWDecimalPortable> minBuff, avgBuff, maxBuff; //12
openSql[1] >> minBuff >> avgBuff >> maxBuff; //13
openSql.fetch(); //14
std::cout << "\nMinimum Salary: " << minBuff[0]
<< "\nAverage Salary: " << avgBuff[0]
<< "\nMaximum Salary: " << maxBuff[0]
<< std::endl; //15
openSql.fetchReturnParams(); //16
std::cout << "Number of Employees in dept " << dept[0] << ": "
<< empCount[0] << std::endl; //17