Toán tử so sánh trong SQL

Bảng theo dõi bảng SINHVIEN có các bản ghi sau:

SQL> SELECT * FROM SINHVIEN;
+----+---------+-----+-----------+--------+
| ID |  TEN    | TUOI|  KHOAHOC  | HOCPHI |
+----+---------+-----+-----------+---------+
|  1 | Hoang   |  21 | CNTT      | 4       |
|  2 | Viet    |  19 | DTVT      | 3.0     |
|  3 | Thanh   |  18 | KTDN      |  4      |
|  4 | Nhan    |  19 | CK        |   4.5   |
|  5 | Huong   |  20 | TCNH      |   5     |
+----+---------+-----+-----------+---------+
5 rows in set (0.00 sec)

Dưới đây là ví dụ sử dụng toán tử so sánh trong SQL:

SQL> SELECT * FROM SINHVIEN WHERE HOCPHI > 4;
+----+----------+-----+---------+----------+
| ID | TEN     | TUOI | KHOAHOC | HOCPHI   |
+----+----------+-----+---------+----------+
|  4 | Nhan    |  19 | CK        |   4.5   |
|  5 | Huong   |  20 | TCNH      |   5     |
+----+---------+-----+-----------+---------+
2 rows in set (0.00 sec)
SQL>  SELECT * FROM SINHVIEN WHERE HOCPHI = 4;
+----+---------+-----+-----------+---------+
| ID | TEN    | TUOI | KHOAHOC   | HOCPHI  |
+----+---------+-----+-----------+---------+
|  1 | Hoang   |  21 | CNTT      | 4       |
|  3 | Thanh   |  18 | KTDN      |  4      |
+----+---------+-----+-----------+---------+
2 rows in set (0.00 sec)

SQL>  SELECT * FROM SINHVIEN WHERE HOCPHI != 4;
+----+----------+-----+---------+----------+
| ID | TEN     | TUOI | KHOAHOC | HOCPHI   |
+----+----------+-----+---------+----------+
|  2 | Viet    |  19 | DTVT      | 3.0     |
|  4 | Nhan    |  19 | CK        |   4.5   |
|  5 | Huong   |  20 | TCNH      |   5     |
+----+----------+-----+---------+----------+
3 rows in set (0.00 sec)

SQL> SELECT * FROM SINHVIEN WHERE HOCPHI <> 4;
+----+----------+-----+---------+----------+
| ID | TEN     | TUOI | KHOAHOC | HOCPHI   |
+----+----------+-----+---------+----------+
|  2 | Viet    |  19 | DTVT      | 3.0     |
|  4 | Nhan    |  19 | CK        |   4.5   |
|  5 | Huong   |  20 | TCNH      |   5     |
+----+----------+-----+---------+----------+
3 rows in set (0.00 sec)

SQL> SELECT * FROM SINHVIEN WHERE HOCPHI >= 4.5;
+----+----------+-----+---------+----------+
| ID | TEN     | TUOI | KHOAHOC | HOCPHI   |
+----+----------+-----+---------+----------+
|  4 | Nhan    |  19 | CK        |   4.5   |
|  5 | Huong   |  20 | TCNH      |   5     |
+----+---------+-----+-----------+---------+
2 rows in set (0.00 sec)