参数化内部联接

Question

我有两个名为mytest和class的表，我需要做的是通过两个表中都存在的列ID将它们连接起来，但是问题是我需要对firstName列进行参数化才能连接所选的firstName，而不是与fristName列具有相同ID的所有列，这是我得到的要点，但是我有一个错误

    SELECT mytest.firstName,class.name 
    FROM mytest 
    INNER JOIN class ON mytest.id=class.id 
    where fristName=?

错误

> Servlet.service() for servlet [com.web.studentregister] in context with path [/MySystemApp] threw exception [com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near '?' at line 1] with root cause
com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near '?' at line 1

我还必须提到我正在servlet应用程序中使用此查询，这也是与此查询交互的代码

数据库类

public List <studentrgister> showcourse(String names) throws Exception {
        List<studentrgister> students = new ArrayList<>();
         String connectionURL = "jdbc:mysql://localhost:3306/web_student_tracker";
          System.out.println("loding the driver");
          Statement s=null;
        studentrgister mystudent=null;
        Connection myConn = null;
        myConn= DriverManager.getConnection(connectionURL, "webstudent", "webstudent"); 
      System.out.println("username and password is correect");

        PreparedStatement myStmt=null;
        ResultSet myRs = null;
        String  name=names;
try {
      Class.forName("com.mysql.jdbc.Driver");
          System.out.println("driver is loaded");
    String sql="SELECT mytest.firstName,class.name FROM mytest INNER JOIN class ON mytest.id=class.id where mytest.firstName=? " ;
    s =myConn.createStatement();
    s.executeQuery(sql);
    myRs=s.getResultSet();
    if (myRs.next()) {
        String classname = myRs.getString("name");
        int numbername=myRs.getInt("firstName");
        mystudent = new studentrgister(classname,numbername);
        students.add(mystudent);
        }
        else {
            throw new Exception("Could not find student id: " );
        }           




        return students;
}
        finally {
            close(myConn,myStmt,null);

        }

    }

servlet类

private void seecourse(HttpServletRequest request, HttpServletResponse response) throws Exception {
        String names=request.getParameter("names");
        student_db_util.showcourse(names);
        List<studentrgister> student=student_db_util.showcourse(names);
        request.setAttribute("mycourse", student);
        RequestDispatcher rd=request.getRequestDispatcher("showcourse.jsp");  
        rd.forward(request,response); 
    }

我发送firstName参数的jsp页面

<form action="studentregister" method="get">
                        <input type="hidden" name="command" value="pass" />

<table>
<c:forEach var="tempstudent" items="${studentlist}">
<c:url var="seecourse" value="studentregister">
            <c:param name="command" value="seecourse"/>
            <c:param name="names" value="${tempstudent.name}"/>
            </c:url>
                        <td >${tempstudent.email}</td>
                                                <td >${tempstudent.name}</td>
                                                <a href="${seecourse}"
                        >

                        seecourse</a>   
</c:forEach>

</table>
</form>

Answer 1

如果要在SQL语句中使用参数，则需要使用PreparedStatement

。例如：