JDBC 的 MySQL 语法错误

Question

我正在尝试使用 JDBC 在“已投票”列中从 MySQL 中读取一个值。该语法应该搜索另一个名为“用户名”的列（它是主键并且是唯一的）并返回“已投票”的相应值。 这是方法的代码，后跟错误代码。

    boolean hasVoted(int userid){
    String q = "SELECT has_voted from voters"+
                "WHERE Username ="+ "'" + userid +"'";
    try{
        Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/demo","abhi", "abhi");
        Statement s = conn.createStatement();

        ResultSet r = s.executeQuery(q);
        if(r.next()){
            int b = r.getInt("has_voted");
            if(b==0){
                return(false);
            }
            else{
                return(true);
            }
        }
        return(false);

    }
    catch(Exception e){

        e.printStackTrace();
        return(false);

}

}

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 '='2'' at line 1
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
at java.lang.reflect.Constructor.newInstance(Unknown Source)
at com.mysql.jdbc.Util.handleNewInstance(Util.java:377)
at com.mysql.jdbc.Util.getInstance(Util.java:360)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:978)
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3887)
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3823)
at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:2435)
at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2582)
at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2526)
at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2484)
at com.mysql.jdbc.StatementImpl.executeQuery(StatementImpl.java:1446)
at DistrictDatabase.hasVoted(DistrictDatabase.java:242)
at DistrictDatabase.initDatabase(DistrictDatabase.java:64)
at Server.main(Server.java:46)

Answer 1

像这样在“voters”和“where”之间添加空格

 String q = "SELECT has_voted from voters"+
                " WHERE Username ="+ "'" + userid +"' ;";