const OR NULLの結果

gistfile1.txt

- const OR NULLは
  - constが真の時に短絡評価っぽい感じで真
  - constが偽の時に "偽とNULLの和集合" と評価されて不定(= NULL)

```
mysql57 3> SELECT 1 /* means TRUE */ OR NULL, 0 /* meanse FALSE */ OR NULL;
+------------+------------+
| 1  OR NULL | 0  OR NULL |
+------------+------------+
|          1 |       NULL |
+------------+------------+
1 row in set (0.00 sec)
```

- `真 IS NULL` は偽
- `偽 IS NULL` は偽
- `NULL IS NULL` は真

よって↓になります

```
mysql57 3> SELECT (1 IS NULL) OR NULL, (0 IS NULL) OR NULL, (NULL IS NULL) OR NULL;
+---------------------+---------------------+------------------------+
| (1 IS NULL) OR NULL | (0 IS NULL) OR NULL | (NULL IS NULL) OR NULL |
+---------------------+---------------------+------------------------+
|                NULL |                NULL |                      1 |
+---------------------+---------------------+------------------------+
1 row in set (0.00 sec)
```