você pode usar uma consulta como esta:
INSERT INTO autoinc (cid,info,customer)
SELECT
COALESCE(max(cid),0) +1
, 'A Customer 1'
, 12345
FROM autoinc
WHERE customer = 12345;
amostra
mysql> SELECT * from autoinc;
Empty set (0,00 sec)
mysql> INSERT INTO autoinc (cid,info,customer)
-> SELECT
-> COALESCE(max(cid),0) +1
-> , 'A Customer 1'
-> , 12345
-> FROM autoinc
-> WHERE customer = 12345;
Query OK, 1 row affected (0,00 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> SELECT * from autoinc;
+----+------+--------------+----------+
| id | cid | info | customer |
+----+------+--------------+----------+
| 1 | 1 | A Customer 1 | 12345 |
+----+------+--------------+----------+
1 row in set (0,00 sec)
mysql> INSERT INTO autoinc (cid,info,customer)
-> SELECT
-> COALESCE(max(cid),0) +1
-> , 'A Customer 1'
-> , 12345
-> FROM autoinc
-> WHERE customer = 12345;
Query OK, 1 row affected (0,00 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> SELECT * from autoinc;
+----+------+--------------+----------+
| id | cid | info | customer |
+----+------+--------------+----------+
| 1 | 1 | A Customer 1 | 12345 |
| 2 | 2 | A Customer 1 | 12345 |
+----+------+--------------+----------+
2 rows in set (0,00 sec)
mysql> INSERT INTO autoinc (cid,info,customer)
-> SELECT
-> COALESCE(max(cid),0) +1
-> , 'B Customer 2'
-> , 9876
-> FROM autoinc
-> WHERE customer = 9876;
Query OK, 1 row affected (0,00 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> SELECT * from autoinc;
+----+------+--------------+----------+
| id | cid | info | customer |
+----+------+--------------+----------+
| 1 | 1 | A Customer 1 | 12345 |
| 2 | 2 | A Customer 1 | 12345 |
| 3 | 1 | B Customer 2 | 9876 |
+----+------+--------------+----------+
3 rows in set (0,00 sec)
mysql> INSERT INTO autoinc (cid,info,customer)
-> SELECT
-> COALESCE(max(cid),0) +1
-> , 'A Customer 1'
-> , 12345
-> FROM autoinc
-> WHERE customer = 12345;
Query OK, 1 row affected (0,00 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> SELECT * from autoinc;
+----+------+--------------+----------+
| id | cid | info | customer |
+----+------+--------------+----------+
| 1 | 1 | A Customer 1 | 12345 |
| 2 | 2 | A Customer 1 | 12345 |
| 3 | 1 | B Customer 2 | 9876 |
| 4 | 3 | A Customer 1 | 12345 |
+----+------+--------------+----------+
4 rows in set (0,00 sec)
mysql>
