Question

我有三个表：log，activity和加入表（many2many）log_activity（log_id和activity_id +其他信息数据作为列）

我想从log和log_activity删除。

我希望保留特定用户的所有日志，并且只保留其他用户的100行。 这意味着我想要删除与WHERE log.user_id != 1匹配的所有行，但最后100行（ORDER BY log.timestamp DESC）。

我还想从连接表log_activity中删除与删除的日志相关的所有条目。 <{1}}表格应该不。

我认为activity在这种情况下没有帮助..

有人能够提出高效的解决方案吗？

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受到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做，但它还没有成功：

db.delete(TABLE_NAME, whereClause , whereArgs);

现在为android部分：

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
);

输出：

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());

这意味着它自己的删除操作很好（我还检查了正确的行被删除，这解决了第一个问题！），但 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个日志之外的所有日志，您可以执行以下操作：

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
);

我不确定这是多么高效，也许有人可以改进它。

Answer 2

如果您有权访问数据库（创建触发器），那么SQL DELETE CASCADE是不是一个选项？

ALTER TABLE log DROP CONSTRAINT aa
ALTER TABLE log ADD CONSTRAIN (FOREIGN KEY (log_id) REFERENCES log_activity ON DELETE CASCADE CONSTRAINT ab)

然后使用您想要的任何子句运行您的普通JDBC删除语句。

Answer 3

根据关系列设置log_id和activity_id等列。

如果它们是not_null，那么你需要根据where子句删除连接表中的行，然后删除日志和活动表中的行。

如果可以为空，则首先在连接表中将log_id和activity_id值设置为null。然后删除日志和活动表。

Answer 4

通过在log_activity表上使用外键来解决问题，如下所示：

FOREIGN KEY(log_id) REFERENCES log(_id) ON DELETE CASCADE

和这样的删除声明：

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);

不要忘记启用外键：

db.execSQL("PRAGMA foreign_keys = ON;");

我遇到的问题是模拟器没有级联log_activities ..但是在它工作的设备上。感谢其他回复者，他们给了我一些提示。

再次查看我的问题以获取更多详细信息。

SQL删除一个表+一个连接表？

4 个答案: