Question

 StringBuffer sql = new StringBuffer("{ call ? := mailmerge_package.getLetters(?, ?, ?)}");

I know it’s like an sql statement but theres no such thing as ‘call’ in SQL.

Can someone explain to me what it means and how does it come to be understood by Java

EDIT:

import oracle.jdbc.driver.OracleTypes;
//omitted code
CallableStatement cs = null;
        ResultSet rs = null;
     StringBuffer sql = new StringBuffer("{ call ? := mailmerge_package.getLetters(?, ?, ?)}");
 try {
            cs = conn.prepareCall(sql.toString());
            cs.registerOutParameter(1, OracleTypes.CURSOR);
            DAOUtils.setLong(cs, 3, checklistAnsMastId);
            DAOUtils.setLong(cs, 2, workEntityId);
            cs.setLong(4, patientId);
            DAOUtils.setLong(cs, 5, encounterId);
            cs.setString(6, encounterType);
            cs.execute();
            rs = (ResultSet)cs.getObject(1);

Answer 1

{call <<procedure name>>}

is a SQL escape sequence. Basically, since different databases have different syntax for how to call a user-defined procedure and different databases have different built-in functions for various things like common date/ time functions, JDBC drivers implement a number of escape sequences where the driver translates a generic specification (i.e. {call <<procedure name>>}) and expands that to be the database-specific syntax. There are various other escape sequences for things like outer joins, date literals, and string functions that can be useful if you’re trying to write database agnostic code.

FYI, these escape sequences were originally defined in the ODBC API and then adopted by JDBC so you may find more documentation related to ODBC than JDBC.