Open SQL Query Examples

SourcePro : DB Access Module for DB2 CLI User’s Guide : Technical Information : Open SQL and DB2 CLI : Open SQL Examples : 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 the table emp to retrieve all columns of the table for employees in a particular department.

The first example assumes knowledge of the structure of table emp (Open SQL Examples ) 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;

}

//1 Creates an RWDBOSql object with the supplied SQL query and the hint RWDBOSql::Query indicating that the SQL is a SELECT statement. The “?” in the SQL statement is a placeholder for the department number whose employees are to be selected. The placeholder needs to be bound before executing the RWDBOSql.

//2 Creates an RWDBTBuffer object to bind to the placeholder. This object is templatized on type int, as the dept column is of server type INT. The buffer will have a single entry of type int.

//3 Uses the index operator to access the single entry from the buffer created in //2, assigning it the department number for which the records will be queried.

//4 Binds the buffer to the RWDBOSql object. The SQL query has only one placeholder, hence only one buffer needs to be bound. The RWDBOSql object is now ready for execution.

//5 Executes the RWDBOSql object on a supplied RWDBConnection object.

//6 Defines a constant for the number of rows to fetch at a time from the query result set.

//7 Creates RWDBTBuffer objects to be used in fetching the query result set, with one object per column of the result set. The objects are templatized on the expected server types of the result columns as mapped to SourcePro types. (See the type mapping in Datatypes.) Each buffer is created with the number of entries equal to the number of rows to be fetched at a time.

//8 Binds the output buffers to the first RWDBMultiRow object in the RWDBOSql. As this is the first (and only) result set from the execution of the RWDBOSql, the buffers are bound to the first RWDBMultiRow object accessed using the index operator on RWDBOSql.

//9 Fetches the result into the bound buffers with each call to fetch() . The call rowsFetched() returns the number of rows fetched. The call to fetch() is repeated until all rows in the result set are fetched, indicated when rowsFetched() returns 0.

//10 Uses the index operator on the bound output buffers to access the fetched data.

//11 Uses the isNull() method on the bound output buffer mgrBuff to determine if a particular entry in the buffer is NULL.

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.

//1 Fetches the schema of the available result set using the schema() method on the RWDBOSql object. (You can use the RWDBSchema API to browse through the contents of the result set schema.) Here, we use the returned schema directly to create an RWDBMultiRow that creates buffers matching the datatypes of the result set columns. Each buffer will be created with ROWS_TO_FETCH entries.

//2 Assigns the created RWDBMultiRow to the RWDBOSql for use in fetching the first result set, by assigning to the 0th position in the RWDBOSql's result sets.

//3 Fetches rows into the buffers in the RWDBMultiRow with the fetch() call. The number of rows fetched is returned by rowsFetched(). This is repeated until all rows in the result set are fetched, indicated when rowsFetched() returns 0.

//4 Finds the number of buffers in the RWDBMultiRow using method numberOfColumns().

//5 Accesses each row’s data inside the RWDBMultiRow buffers as an RWDBRow object by calling the index operator on RWDBMultiRow and passing in the row number needed. Each value in the RWDBRow instance can then be accessed as an RWDBValue instance using the index operator on RWDBRow and passing in the column number needed.

//6 The RWDBValue API can be used to determine the type of the value, to check if it is NULL, or to process it in other ways