(1)
mysql> select name, salary from employee where salary = 'NULL' or salary <= 300000;
+----------+--------+
| name     | salary |
+----------+--------+
| 佐藤次郎 | 250000 |
| 鈴木花子 | 250000 |
| 高橋良子 | 300000 |
+----------+--------+
3 rows in set, 1 warning (0.00 sec)

(2)
mysql> select name, birthday from employee where not birthday >19900101;
+----------+------------+
| name     | birthday   |
+----------+------------+
| 田中三郎 | 1975-11-03 |
| 高橋良子 | 1985-11-23 |
| 鈴木良枝 | 1970-05-03 |
| 佐藤健次 | 1980-05-05 |
+----------+------------+
4 rows in set (0.00 sec)

(3)
mysql> select name,salary from employee where salary >= 300000 and salary < 400000;
+----------+--------+
| name     | salary |
+----------+--------+
| 高橋良子 | 300000 |
| 佐藤健次 | 350000 |
+----------+--------+
2 rows in set (0.00 sec)

(4)
mysql> select name,salary from employee where salary between 300000 and 399999;
+----------+--------+
| name     | salary |
+----------+--------+
| 高橋良子 | 300000 |
| 佐藤健次 | 350000 |
+----------+--------+
2 rows in set (0.00 sec)

(5)
mysql> select name,gender,salary,dep_id from employee where gender='男' or salary >=400000 or dep_id = 'D001';
+----------+--------+--------+--------+
| name     | gender | salary | dep_id |
+----------+--------+--------+--------+
| 鈴木太郎 | 男     |   NULL | NULL   |
| 佐藤次郎 | 男     | 250000 | D001   |
| 田中三郎 | 男     | 400000 | D003   |
| 鈴木良枝 | 女     | 450000 | D003   |
| 佐藤健次 | 男     | 350000 | D001   |
+----------+--------+--------+--------+
5 rows in set (0.00 sec)