Determine which senders sent themselves a message is to use a self-join

    
mysql>
mysql> CREATE TABLE mail
    -> (
    ->  t               DATETIME,       # when message was sent
    ->  senderUser      CHAR(8),        # sender (source user and host)
    ->  senderHost      CHAR(20),
    ->  recipientUser   CHAR(8),        # recipient (destination user and host)
    ->  recipientHost   CHAR(20),
    ->  size    BIGINT,         # message size in bytes
    ->  INDEX   (t)
    -> );
Query OK, 0 rows affected (0.00 sec)

mysql>
mysql> INSERT INTO mail (t,senderHost,senderUser,recipientHost,recipientUser,size)
    ->  VALUES
    ->          ('2010-05-11 10:15:08','saturn','barb','mars','tricia',58274),
    ->          ('2010-05-12 12:48:13','mars','tricia','venus','gene',194925),
    ->          ('2010-05-12 15:02:49','mars','phil','saturn','phil',1048),
    ->          ('2010-05-13 13:59:18','saturn','barb','venus','tricia',271),
    ->          ('2010-05-14 09:31:37','venus','gene','mars','barb',2291),
    ->          ('2010-05-14 11:52:17','mars','phil','saturn','tricia',5781),
    ->          ('2010-05-14 14:42:21','venus','barb','venus','barb',98151),
    ->          ('2010-05-14 17:03:01','saturn','tricia','venus','phil',2394482),
    ->          ('2010-05-15 07:17:48','mars','gene','saturn','gene',3824),
    ->          ('2010-05-15 08:50:57','venus','phil','venus','phil',978),
    ->          ('2010-05-15 10:25:52','mars','gene','saturn','tricia',998532),
    ->          ('2010-05-15 17:35:31','saturn','gene','mars','gene',3856),
    ->          ('2010-05-16 09:00:28','venus','gene','mars','barb',613),
    ->          ('2010-05-16 23:04:19','venus','phil','venus','barb',10294),
    ->          ('2010-05-17 12:49:23','mars','phil','saturn','tricia',873),
    ->          ('2010-05-19 22:21:51','saturn','gene','venus','gene',23992);
Query OK, 16 rows affected (0.00 sec)
Records: 16  Duplicates: 0  Warnings: 0

mysql>
mysql> SELECT DISTINCT m1.senderUser, m2.recipientUser
    -> FROM mail AS m1, mail AS m2
    -> WHERE m1.senderUser = m2.senderUser AND m2.recipientUser = m1.senderUser;
+------------+---------------+
| senderUser | recipientUser |
+------------+---------------+
| phil       | phil          |
| barb       | barb          |
| gene       | gene          |
+------------+---------------+
3 rows in set (0.00 sec)

mysql>
mysql> drop table mail;
Query OK, 0 rows affected (0.00 sec)

Related examples in the same category

1.	The self-join can be extended to display the number of days elapsed at each date
2.	To calculate cumulative distance in kilometers at each stage, use a self-join
3.	Eliminating Duplicates from a Self-Join Result

Determine which senders sent themselves a message is to use a self-join : Self Join « Join « SQL / MySQL

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