java - 如何使用准备好的语句从数据库中选择实体

Question

在尝试添加可能具有相同 key 的学生之前，我正在尝试检查数据库中是否存在某个 key 。我有一个名为 DataBaseHandler 的类，它具有操作数据库的函数，如果给定的 id 存在，则 StudentAlreadyExist 函数返回 true，如果不存在，则返回 false。当我尝试运行该应用程序时，它给了我这个错误

com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '?' at line 1
*******

这是我的代码片段，用于测试学生是否存在

//check if the student exist
public boolean studentAlreadyExists(String id) throws SQLException{
   //first connect to the database;
   connectToDataBase(); 

   selectSQL = "select name from student where idNumber like ?";
   prstmt = con.prepareStatement(selectSQL);
   prstmt.setString(1, id);

   ResultSet rs = prstmt.executeQuery(selectSQL);
   String dbname = null;
   while(rs.next()){

       dbname = rs.getString("name");
   }

    return dbname != null;
}

Answer 1

你之前已经准备好了，你不必再次传递queryString作为参数。

API: https://docs.oracle.com/javase/7/docs/api/java/sql/PreparedStatement.html#executeQuery()

应该看起来像:

public boolean studentAlreadyExists(String id) throws SQLException{
   connectToDataBase(); 

   selectSQL = "select name from student where idNumber like ?";
   prstmt = con.prepareStatement();
   prstmt.setString(1, id);

   /* Executes the SQL query in this PreparedStatement object and returns the ResultSet object generated by the query.*/
   ResultSet rs = prstmt.executeQuery();
   String dbname = null;
   while(rs.next()){

       dbname = rs.getString("name");
   }

    return dbname != null;
}