Understanding SQL’s NOT EQUAL TO Operator in SQL Server 2016

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The NOT EQUAL TO operator, denoted by != or <=>, can be a source of confusion when used with the = operator. In this article, we will delve into the subtleties of how these operators interact and explore alternative solutions to achieve your desired result.

The Confusion: OR vs AND Behavior

When using the NOT EQUAL TO operator in SQL Server 2016, it can sometimes behave like an OR operator instead of an AND operator. This can lead to unexpected results, especially when combined with other operators or used in more complex queries.

Example Query

The following example query demonstrates this behavior:

SELECT COUNT(*) 
FROM table1 
WHERE year != '2017/2018' AND SOURCE != 'CSV'

As expected, the query returns 30 rows instead of 70. This is because SQL Server interprets the != operator as an OR condition, allowing rows where either year or SOURCE does not match the specified value.

Alternative Solution: De Morgan’s Laws

To achieve the desired result, we can use De Morgan’s laws to rephrase the query:

WHERE NOT (year = '2017/2018' AND SOURCE = 'CSV')

De Morgan’s laws state that NOT (A AND B) is equivalent to (NOT A) OR (NOT B). Applying this law to our query, we get:

(Year != ‘2017/2018’) OR (SOURCE != ‘CSV’)

This rephrased query will return the correct result of 70 rows.

Visualizing the Behavior: A Table Example

To better understand the behavior of the NOT EQUAL TO operator, let’s examine a table example:

| Year | SOURCE | Result |
| --- | --- | --- |
| 2018 | Ford | 0    |
| 2018 | Honda | 0    |
| ... | ...   | ...   |

In this table, we have 4 rows where year equals ‘2018’, and for each of these rows, there is a corresponding row with SOURCE equal to either ‘Ford’ or ‘Honda’. This results in a total of 6 rows where both conditions are true.

Counting the Inverse: A Silver BMW Example

If we want to count all the cars that are NOT red AND NOT BMW, we cannot simply add up the number of cars that do not meet either condition. Instead, we must count the number of cars that do not meet both conditions, as shown in the table below:

| Year | SOURCE | Result |
| --- | --- | --- |
| Red  | Ford   | 0    |
| Red  | Honda  | 1    |
| ...  | ...    | ...   |
Silver| BMW     | 1    |

As we can see, there are 7 cars that meet the condition of NOT being red AND NOT being a BMW.

Conclusion

In conclusion, the NOT EQUAL TO operator in SQL Server 2016 can sometimes behave like an OR operator instead of an AND operator. By applying De Morgan’s laws and rephrasing our query, we can achieve the desired result. Additionally, visualizing the behavior using table examples can help us better understand how these operators interact.

Last modified on 2023-11-26