Question

private static final String QUERY = "SELECT * FROM " + TABLE_SONG_DETAILS + " WHERE " + TABLE_SONG_DETAILS + "." + "artist" + "=? ORDER BY track ?";
private PreparedStatement queryAllSongsInfo = conn.prepareStatement(QUERY);

// the user inputs the artist_name and ORDER
queryAllSongsInfo.setString(1, artist_name);
if (order == ORDER_BY_DESC) {
    queryAllSongsInfo.setString(2, "DESC");
} else {
    queryAllSongsInfo.setString(2, "ASC");
}

显示错误：SQL错误或缺少数据库(near “?”: syntax error) 如果我只包括第一个占位符，那么它工作正常。

queryAllSongsInfo.setString(1, artist_name);

为什么我不能使用多个占位符？为什么第二个占位符不考虑用户的第二个输入？

Answer 1

不使用时：

queryAllSongsInfo.setString(2, "DESC");

这会将DESC或ASC个关键字放在两个引号之间，例如ORDER BY track 'DESC'，这是不正确的。

而是直接使用查询连接，例如：

String QUERY = "SELECT * FROM "+TABLE_SONG_DETAILS +" WHERE "+TABLE_SONG_DETAILS+"."+"artist"+"=? ORDER BY track ";
if(order==ORDER_BY_DESC) {
    QUERY += "DESC";
}else {
    QUERY += "ASC";
}
PreparedStatement queryAllSongsInfo = conn.prepareStatement(QUERY);
queryAllSongsInfo.setString(1, artist_name);

Answer 2

您只能将占位符用于列值。您不能将它们用于表名，列名或（在本示例中尝试过）保留字。

您可以创建两个SQL字符串，一个用于升序，另一个用于降序：

private static final String QUERY_ASC = "SELECT * FROM "+TABLE_SONG_DETAILS +" WHERE "+TABLE_SONG_DETAILS+"."+"artist"+"=? ORDER BY track ASC";
private static final String QUERY_DESC = "SELECT * FROM "+TABLE_SONG_DETAILS +" WHERE "+TABLE_SONG_DETAILS+"."+"artist"+"=? ORDER BY track DESC";

private PreparedStatement queryAllSongsInfo = conn.prepareStatement(order==ORDER_BY_DESC?QUERY_DESC:QUERY_ASC);

// the user inputs the artist_name and ORDER
queryAllSongsInfo.setString(1, artist_name);

SQL错误或缺少数据库（接近“？”：语法错误）

2 个答案: