JDBC PreparedStatement似乎忽略了占位符

Question

Given the following code for a PreparedStatement , I use a placeholder (?) to be filled with a String :

String sql = "SELECT SUM(Premium), SUM(AgentComm)\n" +
                "FROM \"?_KLCommissions\" \n" +
                "WHERE AgentCode = \"?\";";

    PreparedStatement preparedStatement = conn.prepareStatement(sql);
    preparedStatement.setString(1, year);
    preparedStatement.setString(2, agent.getAgentNumber());

    ResultSet resultSet = preparedStatement.executeQuery();

When executing the query, however, I get a SQL error: no such table: ?_KLCommissions .

Why is the ? placeholder not being replaced when processing the query?

Answer 1

You've got it within double-quotes, which means it isn't a placeholder - it's just part of the value.

Beyond that, most SQL databases don't let you use parameters for things like table names and column names - only values can be parameterised. So you may well need to build the table name part directly, but then use

WHERE AgentCode = ?

for the agent code - so you'll only have a single parameter to set. So something like:

String tableName = year + "_KLCommissions";
// TODO: Put validation here unless you already have *complete confidence*
// that the value of "year" can't be messed around with.
String sql = "SELECT SUM(Premium), SUM(AgentComm) " +
             "FROM " + tableName + " " +
             "WHERE AgentCode = ?";

PreparedStatement preparedStatement = conn.prepareStatement(sql);
preparedStatement.setString(1, agent.getAgentNumber());
ResultSet resultSet = preparedStatement.executeQuery();

Note the TODO there - you really don't want users to be able to provide an arbitrary value for year , as then they could put arbitrary SQL in your query.