java - SQL从一个表中删除+一个可连接的表？

Question

我有三个表:log、activity 和可连接的 (many2many) log_activity(带有 log_id 和activity_id + 作为列的附加信息数据)。

我想从log 和log_activity 中删除。

我想保留特定用户的所有日志，而只保留其他用户的 100 行。这意味着我想删除所有匹配 WHERE log.user_id != 1 的行，但最后 100 行(ORDER BY log.timestamp DESC)。

我还想从可连接的 log_activity 中删除与被删除的日志相关的所有条目。 activity 表应该不被触及。

我认为 db.delete(TABLE_NAME, whereClause , whereArgs); 在这种情况下没有帮助..

那么有人能够想出一个有效的解决方案吗？

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受 Jacob Eggers 和 plafond 的回答以及进一步研究的启发，我现在正在尝试这样，但它还不起作用:

CREATE TABLE IF NOT EXISTS log ( 
    _id INTEGER PRIMARY KEY AUTOINCREMENT,
    user_id INTEGER NOT NULL,
    timestamp LONG NOT NULL
);

CREATE TABLE IF NOT EXISTS log_activity ( 
    _id INTEGER PRIMARY KEY AUTOINCREMENT,
    log_id INTEGER NOT NULL,
    activity_id INTEGER NOT NULL,
    points INTEGER NOT NULL,
    FOREIGN KEY(log_id) REFERENCES log(_id) ON DELETE CASCADE,
    FOREIGN KEY(activity_id) REFERENCES activity(_id) ON DELETE CASCADE
);

现在是安卓部分:

SQLiteDatabase db = openHelper.getWritableDatabase();
db.execSQL("PRAGMA foreign_keys = ON;");
db.execSQL(CREATE_LOG); // see sql above
db.execSQL(CREATE_ACTIVITY); // not shown here, but like the sql-creates above
db.execSQL(CREATE_LOG_ACTIVITY); // see sql above

// ... insert some data ...
INSERT INTO "log" VALUES(1,1,1307797289000);
INSERT INTO "log" VALUES(2,1,1307710289000);
INSERT INTO "log" VALUES(3,2,1308089465000);
INSERT INTO "log" VALUES(4,2,1308079465000);

INSERT INTO "log_activity" VALUES(1,1,1,1);
INSERT INTO "log_activity" VALUES(2,1,2,2);
INSERT INTO "log_activity" VALUES(3,2,1,1);
INSERT INTO "log_activity" VALUES(4,2,2,2);
INSERT INTO "log_activity" VALUES(5,3,1,1);
INSERT INTO "log_activity" VALUES(6,3,2,2);
INSERT INTO "log_activity" VALUES(7,4,1,1);
INSERT INTO "log_activity" VALUES(8,4,2,2);

// check count of logs
Cursor c = db.query(false, "log", null, null, null, null, null, "_id asc", null);
android.util.Log.d("TEST", "log count before: "+c.getCount());

// check count of log_activities
Cursor c2 = db.query(false, "log_activity", null, null, null, null, null, "_id asc", null);
android.util.Log.d("TEST", "la count before: "+c2.getCount());

// delete some log-rows
long userId = 1;
int keepXLogsOfOthers = 1;
String del = "DELETE FROM log" +
                " WHERE user_id != " + userId +
                "  AND log._id NOT IN (" +
                "    SELECT _id" +
                "    FROM (" +
                "      SELECT _id" +
                "      FROM log" +
                "      WHERE user_id != " + userId +
                "      ORDER BY timestamp DESC" +
                "      LIMIT " + keepXLogsOfOthers +
                "    ) logs_of_others_to_keep" +
                ");";
db.execSql(del);

// check count of logs
Cursor c3 = db.query(false, "log", null, null, null, null, null, "_id asc", null);
android.util.Log.d("TEST", "log count after: "+c3.getCount());

// check count of log_activities
Cursor c4 = db.query(false, "log_activity", null, null, null, null, null, "_id asc", null);
android.util.Log.d("TEST", "la count after: "+c4.getCount());

输出:

06-16 10:40:01.748: DEBUG/TEST(451): log count before: 4
06-16 10:40:01.748: DEBUG/TEST(451): la count before: 8
06-16 10:40:01.828: DEBUG/TEST(451): log count after: 3
06-16 10:40:01.838: DEBUG/TEST(451): la count after: 8

这意味着它自己的 DELETE 操作没问题(我还检查了是否删除了正确的行，这解决了第一个问题!!)，但是 ON DELETE CASCADE 不起作用...为什么？

Answer 1

您可以创建一个触发器来自动执行此操作。

CREATE TRIGGER [delete_log_joins]
BEFORE DELETE
ON [log]
FOR EACH ROW
BEGIN
DELETE FROM log_activity WHERE log_activity.log_id = old.id;
END

要选择删除除最新的 100 条日志之外的所有日志，您可以执行如下操作:

delete * from log where log.id not in (
  select id
  from (
    select l.id
    from log l
    where l.id in (
      select top 100 l2.id
      from log l2
      where l2.user_id = l.user_id
      order by log.timestamp desc
    )
  ) the_tops
);

我不确定它的性能如何，也许有人可以改进它。