Null Values.ppt briefing about null values in SQL. Very helpful if you are learning SQL
- 1. Null Values: Disallowing Null Values, Comparisons Using Null Values Dr. Shalini Gambhir
- 2. • In SQL, you can disallow null values in a column by using the NOT NULL constraint when creating or altering a table. This constraint ensures that every row in the table must have a value for that column and that value cannot be NULL.
- 3. Here's an example of creating a table with the NOT NULL constraint: • CREATE TABLE Employees ( • EmployeeID INT PRIMARY KEY, • FirstName VARCHAR(50) NOT NULL, • LastName VARCHAR(50) NOT NULL, • Department VARCHAR(50) • ); • In this example, both FirstName and LastName columns are specified as NOT NULL, meaning that whenever a new record is inserted into the Employees table, these columns must have a value, and NULL values are not allowed.
- 4. • If you try to insert a record without providing a value for a column with the NOT NULL constraint, you'll receive an error. For instance: • INSERT INTO Employees (EmployeeID, FirstName, LastName) VALUES (1, 'John', NULL); • This would result in an error because LastName is declared as NOT NULL, and a NULL value is being attempted to be inserted.
- 5. • To allow NULL values again, you would need to alter the table and remove the NOT NULL constraint: • ALTER TABLE Employees • ALTER COLUMN LastName VARCHAR(50) NULL; • Now, LastName column will allow NULL values. • In summary, using the NOT NULL constraint ensures data integrity by disallowing NULL values in the specified column, thereby requiring that each row has a valid value for that column.
- 6. Comparisons Using Null Values • When performing comparisons involving null values in SQL, it's essential to understand how null values behave. Null represents a missing or unknown value in SQL. Comparisons involving null values often yield results that may not be immediately intuitive due to the unknown nature of null.
- 7. • Here are some common behaviors when comparing null values in SQL: 1. Equality Comparisons: Comparing a null value to another value, including another null value, using the equality operator (=) results in an unknown or null outcome. This is because the value of the null is unknown. SELECT * FROM Employees WHERE FirstName = NULL; This query doesn't return any rows, even if there are records with a null value for the FirstName column. Instead, you should use the IS NULL or IS NOT NULL operators to check for null values: SELECT * FROM Employees WHERE FirstName IS NULL;
- 8. 2. Inequality Comparisons: Similarly, comparing a null value to another value using inequality operators (<>, !=, >, <, >=, <=) also results in an unknown outcome. • SELECT * FROM Employees WHERE FirstName <> NULL; • This query also doesn't return any rows. Instead, you should use the IS NULL or IS NOT NULL operators to check for null values.
- 9. 3. Aggregate Functions: Aggregate functions such as COUNT, SUM, AVG, etc., generally ignore null values unless explicitly specified otherwise. SELECT COUNT(FirstName) FROM Employees; This query counts the non-null values in the FirstName column. To count all values, including nulls, you could use: SELECT COUNT(*) FROM Employees;
- 10. • Sorting: Null values are often treated differently when sorting data. Depending on the database system, null values might be sorted either at the beginning or end of a result set. You can use the ORDER BY clause with the NULLS FIRST or NULLS LAST option to specify how nulls should be ordered. • SELECT * FROM Employees ORDER BY FirstName NULLS FIRST; • his query sorts the result set by the FirstName column, with null values appearing first. • Overall, handling null values properly is crucial for obtaining accurate query results in SQL. Understanding how nulls behave in comparisons and utilizing appropriate operators like IS NULL or IS NOT NULL helps ensure correct query logic.