Checking NULL values from Database using POCO

Question

I'm making a query using POCO::Data with ODBC Connector, and I need to check for NULL values from code. In the documentation, the Recordset object is supposed to have the function isNull, but the code I download here does not contains such a method (version 1.4.6p4 right now).

How can I check then if a value is NULL with POCO library? I'm trying to create a JSON string with the data retrieved from database.

My actual code looks like this:

Session session(bdName, conn);
Statement select(session);  

select << sSQL;  
select.execute();

RecordSet rs(select);
bool more = rs.moveFirst();
std::size_t cols = rs.columnCount();

sResult = "{\"rowsField\":[";

while (more) {
    if (sResult.back() != '[') sResult += ','; // Not first time
    sResult += "{\"columnsField\":[";

    for (std::size_t col = 0; col < cols; ++col) {
        std::string cName = rs.columnName (col);
        std::string tName = getPocoTypeName(rs.columnType(col));
        std::string val = "";

        if (!rs[col] || rs.value(col).isEmpty()) 
            val = "NULL"; // DOES NOT WORK
        else
            val = rs[col].convert<std::string>();

        if (col != 0) sResult += ',';
        sResult += "\n{\"nameField\":\"" + cName + '\"';
        sResult += ",\"typeField\":\"" + tName + '\"';
        sResult += ",\"valueField\":\"" + val + '\"';

        sResult += "}"; // each JSON column/value
    }
    sResult += "]}\n"; // columnsField (one per row)

    more = rs.moveNext();
}

riot_starter · Accepted Answer · 2014-10-22 20:42:50Z

1

Use Nullable<std::string> val = std::string("");

isNull is a member of Nullable, so you will be able to check if the return value is null..

answered Oct 22, 2014 at 20:42

riot_starter

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Comments

Antonio · Accepted Answer · 2016-07-07 15:58:49Z

I know this is an old question but I have had issues with it. I am also formatting as JSON strings. You can use Poco::Recordset::IsNull(ColumnName) to check if a given column name's value is NULL or not.

The below code will replace NULLS with the empty string to prevent throwing errors:

Poco::Data::Statement statement(session);
statement << query;
statement.execute();
Poco::Data::RecordSet record_set(statement);

bool more = record_set.moveFirst();

while (more)
{
    std::map<std::string, std::string> row;

    for (std::size_t i = 0; i < record_set.columnCount(); ++i)
    {
         std::string val = "";
         if(!record_set.isNull(record_set.columnName(i)))  
             val = record_set[i].convert<std::string>(); 
           // print results for testing
         cout << "->" << record_set.columnName(i) << ":{" << val << "}";
    }

    ret.push_back(row);
    more = record_set.moveNext();
}

Tiit Ülejõe · Accepted Answer · 2020-07-02 13:38:08Z

0

Use record_set[col].isEmpty() for checking NULL.

edited Jul 2, 2020 at 13:38

answered Apr 21, 2020 at 9:07

Tiit Ülejõe

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