(1)

mysql> select name,salary from employee where salary=NULL or salary<300000;
+----------+--------+
| name     | salary |
+----------+--------+
| 山田太郎 | 250000 |
| 佐藤次郎 | 250000 |
| 鈴木花子 | 250000 |
+----------+--------+
3 rows in set (0.00 sec)

(2)
mysql> select name,birthday from employee where not birthday>'1990-01-01';
+----------+------------+
| 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' and salary>'400000' and dep_id='D001';
Empty set (0.00 sec)