mysql> select version();
+-----------+
| version() |
+-----------+
| 5.0.51b   | 
+-----------+
1 row in set (0.00 sec)

mysql> use test;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> drop table if exists foo;
Query OK, 0 rows affected (0.00 sec)

mysql> create table foo(i decimal(23,0) primary key);
Query OK, 0 rows affected (0.02 sec)

mysql> insert into foo values(6700000000000000498);
Query OK, 1 row affected (0.00 sec)

mysql> insert into foo values(6700000000000000499);
Query OK, 1 row affected (0.00 sec)

mysql> insert into foo values(6700000000000000500);
Query OK, 1 row affected (0.00 sec)

mysql> insert into foo values(6700000000000000501);
Query OK, 1 row affected (0.00 sec)

mysql> insert into foo values(6700000000000000502);
Query OK, 1 row affected (0.00 sec)

mysql> select i from foo where i=6700000000000000498;
+---------------------+
| i                   |
+---------------------+
| 6700000000000000498 | 
+---------------------+
1 row in set (0.00 sec)

mysql> select i from foo where i='6700000000000000498';
+---------------------+
| i                   |
+---------------------+
| 6700000000000000498 | 
+---------------------+
1 row in set (0.00 sec)

mysql> select i from foo where i=6700000000000000499;
+---------------------+
| i                   |
+---------------------+
| 6700000000000000499 | 
+---------------------+
1 row in set (0.00 sec)

mysql> select i from foo where i='6700000000000000499';
+---------------------+
| i                   |
+---------------------+
| 6700000000000000499 | 
+---------------------+
1 row in set (0.00 sec)

mysql> select i from foo where i=6700000000000000500;
+---------------------+
| i                   |
+---------------------+
| 6700000000000000500 | 
+---------------------+
1 row in set (0.00 sec)

mysql> select i from foo where i='6700000000000000500';
Empty set (0.00 sec)

mysql> select i from foo where i=6700000000000000501;
+---------------------+
| i                   |
+---------------------+
| 6700000000000000501 | 
+---------------------+
1 row in set (0.00 sec)

mysql> select i from foo where i='6700000000000000501';
Empty set (0.00 sec)

mysql> select i from foo where i=6700000000000000502;
+---------------------+
| i                   |
+---------------------+
| 6700000000000000502 | 
+---------------------+
1 row in set (0.00 sec)

mysql> select i from foo where i='6700000000000000502';
Empty set (0.00 sec)