Java 如何在Sqlite中为一个表赋予优先级

我有两张桌子。在这些表中，我的站点id可能在其中一个或两个表中。如果我的stop id在B中不存在，它应该从A中选取。如果它在A中不存在，它应该从B中选取。如果stop id在两个表中都存在，那么我想优先考虑B并从B中检索行。下面的查询总是从B中返回值。您能帮我解决这个问题吗

String selectQuery = "SELECT " 
    + stop_id + "," 
    + name + ","
    + latitude + ","
    + longitude + ","

    +"1 as "+priority
    +" FROM "+ A

    +" UNION SELECT "
    + stop_id + "," 
    + name + ","
    + latitude + ","
    + longitude + ","
    +"2 as "+priority
    +" FROM "+ B
    +" WHERE " + stop_id + " =? "
    +" ORDER BY "+priority
    +" LIMIT 1 ";


    Cursor cursor = db.rawQuery(selectQuery, new String[] {id});

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在您的

UNION ALL

中，您应该将

WHERE

子句应用于两个

SELECT

s；当前代码仅将条件应用于

B

侧，而

A

侧返回所有行<代码>限制1最后隐藏了这个问题

字符串串联不便于查看，但您应该在查询中传递两次

id

，并使用两个

？

占位符：

SELECT stop_id, name, latitude, longitude, 1 as priority
FROM A
WHERE stop_id = ?
UNION ALL
SELECT stop_id, name, latitude, longitude, 2 as priority
FROM B
WHERE stop_id = ?
ORDER BY priority DESC
LIMIT 1

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我刚想出来，是来告诉你的，但谢谢你的回答。=）