Not like not like not like

Dear lazyweb,

I’m possibly being stupid, but can someone explain the differences between these two queries?

mysql> select count(*) from Table
   where blobid is null or not blobid like '%-%';
+----------+
| count(*) |
+----------+
| 15262487 |
+----------+
1 row in set (25 min 4.18 sec)

mysql> select count(*) from Table
   where blobid is null or blobid not like '%-%';
+----------+
| count(*) |
+----------+
| 20044216 |
+----------+
1 row in set (24 min 54.06 sec)

For reference:

mysql> select count(*) from Table where blobid is null;
+----------+
| count(*) |
+----------+
| 15262127 |
+----------+
1 row in set (24 min 7.15 sec)

Update: It turns out that the former was doing (not blobid) like ‘%-%’ which turns out to not do anything sensible:

mysql> select not blobid from Table limit 10;
+------------+
| not blobid |
+------------+
|          0 |
|       NULL |
|          1 |
|          0 |
|          0 |
|          0 |
|          1 |
|       NULL |
|          1 |
|          0 |
+------------+
10 rows in set (0.02 sec)

mysql> select blobid from Table limit 10;
+-------------------------------------------------------------------+
| blobid                                                            |
+-------------------------------------------------------------------+
| 5cd1237469cc4b52ca094e215156c582-9ef460ac4134c600a4d2382c4b0acee7 |
| NULL                                                              |
| d20cb4037f8f9ab1de5de264660f005c-2c34209dcfb39251cf7c16bb6754bbd2 |
| 845a8d06719d8bad521455a8dd47745c-095d9a0831433c92cd269e14e717b3a9 |
| 9580ed23f34dd68d35da82f7b2a293d6-bf39df7509d977a1de767340536ebe80 |
| 06c9521472cdac02a2d4b2a18f8bec0f-0a8a28d3b63df54860055f1d1de92969 |
| ed3cd0dd9b55f76db7544eeb64f3cfa0-80a6a3eb6d73c0a58f88b7c332866d5c |
| NULL                                                              |
| b339f6545651fbfa49fa500b7845c4ce-6defb5ffc188b8f72f1aa10bbd5c6bec |
| 642075963d6f69bb11c35a110dd07c2c-8db54ac2d2accae7fa4a22db1d6caae9 |
+-------------------------------------------------------------------+
10 rows in set (0.00 sec)

The documentation says Logical NOT. Evaluates to 1 if the operand is 0, to 0 if the operand is non-zero, and NOT NULL returns NULL. but doesn’t describe the behaviour of NOT ‘string’. It would appear that a string starting with a number returns 0 and a string starting with a letter returns 1. Either way, neither has a hyphen in.