Question

I have three tables: log, activity and the jointable (many2many) log_activity (with log_id and activity_id + additional info data as columns).

I want to delete from log and log_activity.

I want to keep all logs from a specific user and only keep 100 rows from other users.
That means I want to delete all rows that match a WHERE log.user_id != 1, but the last 100 (ORDER BY log.timestamp DESC).

I also want to delete from the jointable log_activity all entries that are related to the logs which get deleted. The activity table should not be touched.

I think that db.delete(TABLE_NAME, whereClause , whereArgs); is not helpful in this case..

So is someone able to come up with an efficient solution?

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UPDATE UPDATE UPDATE UPDATE UPDATE UPDATE UPDATE UPDATE UPDATE

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Inspired by the answers of Jacob Eggers and plafond and by further research I am trying like this now, but it does not work yet:

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
);

Now for the android part:

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());

output:

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

This means the DELETE operation it self is fine (I also checked that the correct rows are deleted which solves the first issue!!), but ON DELETE CASCADE does not work… why?

Answer 1

Solved the problem by using a foreign key on the log_activity table like this:

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

and a delete statement like this:

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);

Don’t forget to enable foreign keys:

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

and I had the issue that the emulator did not cascade the log_activities.. but on a device it works. Thanks to the other answerers who gave me some hints.

See my question again for more details.