Entering multiple entries in SQL where the count increases by one
As I'd suggested on comments, this is a trigger in action BEFORE INSERT to autogenerate a thing_id with the UID given.

TRIGGER:

USE `test`;

DELIMITER $$

DROP TRIGGER IF EXISTS test.Things_BEFORE_INSERT$$
USE `test`$$
CREATE DEFINER = CURRENT_USER TRIGGER `test`.`Things_BEFORE_INSERT` BEFORE INSERT ON `Things` FOR EACH ROW
BEGIN
SET NEW.thing_id=IFNULL((SELECT MAX(thing_id)+1 FROM test.Things WHERE UID=NEW.UID),1); -- If NULL set it to 1 as first thing_id for this UID
END
$$
DELIMITER ;


Note: Change the database and table to yours.

Test:

mysql&gt; SELECT * FROM test.Things;
Empty set (0.00 sec)

mysql&gt; 
mysql&gt; USE `test`;
Database changed
mysql&gt; 
mysql&gt; DELIMITER $$
mysql&gt; 
mysql&gt; DROP TRIGGER IF EXISTS test.Things_BEFORE_INSERT$$
Query OK, 0 rows affected (0.00 sec)

mysql&gt; USE `test`$$
Database changed
mysql&gt; CREATE DEFINER = CURRENT_USER TRIGGER `test`.`Things_BEFORE_INSERT` BEFORE INSERT ON `Things` FOR EACH ROW
    -&gt; BEGIN
    -&gt; SET NEW.thing_id=IFNULL((SELECT MAX(thing_id)+1 FROM test.Things WHERE UID=NEW.UID),1); -- If NULL set it to 1 as first thing_id for this UID
    -&gt; END
    -&gt; $$
Query OK, 0 rows affected (0.12 sec)

mysql&gt; DELIMITER ;
mysql&gt; 
mysql&gt; SET @UID='71420150404',@thing1='First Thing.',@thing2='Second Thing.',@thing3='Third Thing.';
Query OK, 0 rows affected (0.00 sec)

mysql&gt; 
mysql&gt; INSERT INTO test.Things(UID,thing) 
    -&gt; VALUES (@UID,@thing1),(@UID,@thing2),(@UID,@thing3);
Query OK, 3 rows affected, 1 warning (0.09 sec)
Records: 3  Duplicates: 0  Warnings: 1

mysql&gt; 
mysql&gt; SELECT * FROM test.Things;
+-------------+----------+---------------+------------+
| UID         | thing_id | thing         | thing_date |
+-------------+----------+---------------+------------+
| 71420150404 |  0000001 | First Thing.  | NULL       |
| 71420150404 |  0000002 | Second Thing. | NULL       |
| 71420150404 |  0000003 | Third Thing.  | NULL       |
+-------------+----------+---------------+------------+
3 rows in set (0.00 sec)

mysql&gt;