java - 准备好的语句，字符串卡在引号中

Question

我一直试图弄清楚为什么我准备好的语句在字符串之前和之后放置 ' 。当我尝试将此代码用于 Postgressql 数据库服务器时，出现语法错误。这是因为该程序处理代码如下:

假设我按如下方式调用此方法:

selectStringQuery("username", "users", "id", 1);

然后程序返回以下准备好的语句:

SELECT 'username' FROM 'users' WHERE 'id' = 1;

运行程序时出现以下错误:

org.postgresql.util.PSQLException: ERROR: syntax error at or near "$2"
Position: 16

它应该产生这样的结果:

SELECT username FROM users WHERE id = 1;

有人可以告诉我我在这里缺少什么吗？

 * Executes a select query.
 * @param selectFieldName
 * @param tableName
 * @param conditionFieldName
 * @param conditionValue
 * @return 
 */public String selectStringQuery(String selectFieldName, String tableName, String conditionFieldName, int conditionValue){
    try {
        // *** Start execution of query ***
        query = "SELECT ? FROM ? WHERE ? = ?;";
        preparedStatement = prepareStatement(query);
        preparedStatement.setString(1, selectFieldName);
        preparedStatement.setString(2, tableName);
        preparedStatement.setString(3, conditionFieldName);
        preparedStatement.setInt(4, conditionValue);
        System.out.println(preparedStatement);
        resultSet = preparedStatement.executeQuery();
        // *** End execution of query ***

        // *** Start validity checks ***       
        if(!resultSet.next()){
            System.out.println("Query did not return any results.");
            return null;
        }
        // *** End validity checks ***

        // *** Start process query results ***
        String result = resultSet.getString(selectFieldName);
        return result;
        // *** End process query results ***
    } catch (Exception ex) {
        System.out.println(ex);
        return null;
    }
}

Answer 1

尝试在查询字符串中连接表名和列名。

query = "SELECT "+selectFieldName+" FROM "+tableName+" WHERE "+conditionFieldName+" = ?;";

我认为您不能将表名和列名作为参数传递。参数将被视为列值，并将 '' 添加到其中