The following query selects details of all those employees who have not been given a bonus. This implies that the bonus column will be blank.

Question

Accepted Answer

> [!TLDR]
> "The bonus column is blank" means the column contains **NULL**. NULL is not zero and not an empty string — it is the *absence* of a value, and it can only be tested with the `IS NULL` operator:
> ```sql
> SELECT * FROM EMPLOYEE WHERE Bonus IS NULL;
> ```

## Concept understanding — what NULL really is

| Value in the column | Meaning | Test that finds it |
|---|---|---|
| `0` | a bonus of zero rupees was given | `Bonus = 0` |
| `''` (empty string) | an empty text value was stored | `Bonus = ''` |
| **`NULL`** | **no bonus value exists at all — unknown / not applicable** | **`Bonus IS NULL`** |

The three are genuinely different. An employee with `Bonus = 0` *was* processed and awarded nothing; an employee with `Bonus = NULL` was never assigned a bonus at all. The question says the column is *blank*, which is the NULL case.

> [!IMPORTANT]
> SQL uses **three-valued logic**: TRUE, FALSE and UNKNOWN. Any arithmetic or comparison involving NULL yields **UNKNOWN**, and a `WHERE` clause only keeps rows for which the condition is **TRUE**. So `Bonus = NULL` is UNKNOWN for every single row — including the NULL ones — and the query returns nothing at all. It is not an error; it silently returns an empty set, which is far more dangerous.

## The real EMPLOYEE table (Table 9.8)

| EmpNo | Ename | Salary | Bonus | DeptId |
|---|---|---|---|---|
| 101 | Aaliya | 10000 | 234 | D02 |
| 102 | Kritika | 60000 | 123 | D01 |
| 103 | Shabbir | 45000 | 566 | D01 |
| 104 | Gurpreet | 19000 | 565 | D04 |
| 105 | Joseph | 34000 | 875 | D03 |
| 106 | Sanya | 48000 | 695 | D02 |
| 107 | Vergese | 15000 | **NULL** | D01 |
| 108 | Nachaobi | 29000 | **NULL** | D05 |
| 109 | Daribha | 42000 | **NULL** | D04 |
| 110 | Tanya | 50000 | 467 | D05 |

## The query

```sql
mysql> SELECT * FROM EMPLOYEE
    -> WHERE Bonus IS NULL;
```

**Output**

Value in the column	Meaning	Test that finds it
`0`	a bonus of zero rupees was given	`Bonus = 0`
`''` (empty string)	an empty text value was stored	`Bonus = ''`
`NULL`	no bonus value exists at all — unknown / not applicable	`Bonus IS NULL`

Q.The following query selects details of all those employees who have not been given a bonus. This implies that the bonus column will be blank.

Concept understanding — Set Difference

Concept understanding — what NULL really is

The real EMPLOYEE table (Table 9.8)

The query

Row-by-row evaluation

EmpNo	Ename	Salary	Bonus	DeptId
101	Aaliya	10000	234	D02
102	Kritika	60000	123	D01
103	Shabbir	45000	566	D01
104	Gurpreet	19000	565	D04
105	Joseph	34000	875	D03
106	Sanya	48000	695	D02
107	Vergese	15000	NULL	D01
108	Nachaobi	29000	NULL	D05
109	Daribha	42000	NULL	D04
110	Tanya	50000	467	D05

Employee	Bonus	`Bonus IS NULL`	Selected?
Aaliya	234	FALSE	no
Kritika	123	FALSE	no
Shabbir	566	FALSE	no
Gurpreet	565	FALSE	no
Joseph	875	FALSE	no
Sanya	695	FALSE	no
Vergese	NULL	TRUE	yes

Q.The following query selects details of all those employees who have not been given a bonus. This implies that the bonus column will be blank.

Concept understanding — what NULL really is

The real EMPLOYEE table (Table 9.8)

The query

Row-by-row evaluation

🎓Unlock everything free for 14 days

Unlock everything free for 14 days