T sql when is not null

In other words, the index that is automatically created when you define a UNIQUE constraint will guarantee that no two rows in that table can have the same value for the columns participating in that index, with the ability to insert only one unique NULL value to these columns, if the column allows NULL. Let us create a small table with two columns, ID and Name.

The first two records will be inserted successfully, with no constraint preventing duplicate values of the Name column. The constraint in SQL creation will fail, due to having duplicate values of that column in the table, as shown in the error message below:. You can also see the automatically created index that is used to guarantee the column values uniqueness.

Right-click on the needed table and choose Design. Entity integrity ensures that each row in the table is a uniquely identifiable entity. Assume that we have the below simple table with two columns; the ID and Name. You will see that the first record will be inserted successfully as both the ID and Name values are valid. Checking the inserted values, you will see that only the first record is inserted successfully as below:.

Feedback will be sent to Microsoft: By pressing the submit button, your feedback will be used to improve Microsoft products and services. Privacy policy. Skip to main content. This browser is no longer supported. Download Microsoft Edge More info. Contents Exit focus mode. Is this page helpful? Yes No. Any additional feedback? If a field in a table is optional, it is possible to insert a new record or update a record without adding a value to this field.

Then, the field will be saved with a NULL value. A field with a NULL value is one that has been left blank during record creation!

Select all records from the Customers where the PostalCode column is empty. We just launched W3Schools videos.