Understanding NULL and Three-Valued Logic in SQL Server
1. Definition of NULL in the database world:
NULL refers to the absence of a value or unknown value. It is not equivalent to zero or an empty string.
Instead, NULL represents the lack of a defined value for a specific attribute of an entity. When a field in a table has a NULL value, it means that the information is missing or unknown.
This is a common occurrence in the database world, and SQL Server treats NULL uniquely to accommodate this.
2. Result of logical expression involving NULL:
When a logical expression involves a NULL value, the result is neither TRUE nor FALSE but UNKNOWN.
This is because, in 3VL, binary logic ceases to be sufficient to capture all possible truth values of a logical expression in the presence of NULL values. For example, consider a simple logical expression, (A OR B) where A is TRUE, and B is NULL.
The result of this expression would be UNKNOWN rather than TRUE or FALSE. Thus logical expressions involving NULL values are called tri-state or three-valued.
3. Comparisons with NULL:
Comparisons involving NULL values are also handled differently in SQL Server. Any comparison operator (<,>,=) involving NULL values will result in an outcome of UNKNOWN.
Thus, to compare a value with NULL, we use the IS NULL or IS NOT NULL operator.
4. Usage of IS NULL operator:
The IS NULL operator compares a value with a NULL value.
If the value is NULL, the expression returns TRUE, and if the value is not NULL, the expression returns FALSE.
5. Usage of IS NOT NULL operator:
The IS NOT NULL operator evaluates a value and returns TRUE if the value is not NULL, and FALSE if the value is NULL.
6. Use of IS NULL and IS NOT NULL Operators in SQL Server:
To better understand the usage of IS NULL and IS NOT NULL operators in SQL Server, let us consider a sample database with a customers table. Overview of customers table from sample database:
The customers table has four columns, namely, ID, Name, Age, and Phone.
The Phone column has some missing or unknown values.
6.1. Usage of WHERE clause with phone = NULL:
If we use the WHERE clause to select customers whose phone number is unknown as follows,
SELECT * FROM customers WHERE phone = NULL;6.2. Evaluation of WHERE clause expression:
We would expect to retrieve all clients with phone numbers as NULL.
However, this is not the case as the expression will evaluate to UNKNOWN. As mentioned earlier, any comparison involving NULL values will give an UNKNOWN outcome.
Therefore, we cannot use the equals (=) operator to compare NULL values.
6.3. Usage of IS NULL operator with WHERE clause:
Instead, we use the IS NULL operator to retrieve all clients whose phone number is unknown as follows,
SELECT * FROM customers WHERE phone IS NULL;The expression will now evaluate to TRUE if the phone number is unknown, and we will retrieve all relevant customers.
6.4. Usage of IS NOT NULL operator with WHERE clause:
To obtain customers with defined phone numbers, we use the IS NOT NULL operator as follows,
SELECT * FROM customers WHERE phone IS NOT NULL;This will now retrieve all clients whose phone number is not unknown.
Conclusion:
In summary, SQL Server treats NULL values differently, as it represents an unknown value or the lack of a defined value for an entity’s attribute.
When NULL values are involved in logical expressions or comparisons, the outcome is UNKNOWN. The IS NULL and IS NOT NULL operators are used to deal with NULL values effectively.
The WHERE clause together with these operators help to filter and retrieve data from the database table as per our preferences. Understanding these concepts is essential in SQL Server to prevent errors, unexpected outcomes, and to obtain more accurate and meaningful data from the database table.
Summary of NULL and Three-Valued Logic in SQL Server:
As discussed earlier, NULL is a special value that represents an unknown or missing value in a database system.
It is not equivalent to zero, empty string, or any other value. Additionally, 3VL is a logical system that extends the traditional binary (true/false) logic to accommodate the UNKNOWN value resulting from NULL.
3VL provides three possible truth values: TRUE, FALSE, and UNKNOWN, meaning that a logical expression’s outcome can be one of these three values. In SQL Server, NULL is implemented as a binary code, and the values TRUE and FALSE are represented as a single bit each.
When a bit is undefined, it represents an UNKNOWN value. SQL Server uses this representation to handle missing or unknown values in a database and provide a mechanism for handling NULL values in expressions.
Usage of IS NULL and IS NOT NULL operators to test for NULL values:
In SQL Server, the IS NULL and IS NOT NULL operators are used to test for NULL values in the database. Let us explore these operators in more detail.
1. The IS NULL Operator:
The IS NULL operator tests if a value is NULL. If the value is NULL, the expression will return TRUE.
If the value is not NULL, the expression will return FALSE. The following query returns all records from a customer’s table where the phone is NULL:
SELECT * FROM customers WHERE phone IS NULL;This query retrieves all records from the customer’s table where the phone field is NULL.
2. The IS NOT NULL Operator:
The IS NOT NULL operator is used to test if a value is not NULL. If the value is not NULL, the expression will return TRUE.
If the value is NULL, the expression will return FALSE. The following query returns all records from a customer’s table where the phone is not NULL:
SELECT * FROM customers WHERE phone IS NOT NULL;This query retrieves all records from the customer’s table where the phone field is not NULL.
NULL Values in Comparisons:
As we have already seen, comparisons containing NULL values result in an UNKNOWN outcome. Therefore, we cannot use the equals (=) operator to compare NULL values.
Instead, we use the IS NULL and IS NOT NULL operators as discussed above to test for NULL values.
Conclusion:
NULL and Three-Valued Logic (3VL) are fundamental concepts in the database world, and understanding them is essential.
SQL Server manages NULL values differently, and any comparison of NULL values results in UNKNOWN. Therefore, we use the IS NULL and IS NOT NULL operators to handle NULL values effectively.
These operators help to filter and retrieve data from the database table as per our preferences and improve the quality and accuracy of our data. By utilizing these operators and understanding these concepts, we can prevent errors, obtain meaningful data from the database table, and ensure the accuracy of our results.
In conclusion, understanding NULL and Three-Valued Logic in SQL Server is essential for managing and manipulating data effectively. NULL represents an unknown or missing value, and logical expressions involving NULL values result in UNKNOWN.
To handle NULL values, we use the IS NULL and IS NOT NULL operators, which help to filter and retrieve data from the database table as per our preferences. By utilizing these operators, we can prevent errors, obtain meaningful data, and ensure the accuracy of our results.
The topic may seem complex, but with a sound understanding of these concepts, we can work more effectively in SQL Server and improve the quality of our data.