A field is referred to as null when no data entry has been made to it:

• Saying that a field is null doesn't mean that it contains 0 because 0 is a value
• Saying that a field is null doesn't mean that it is empty. A field being empty could mean that the user had deleted its content or that the field itself would not accept what the user was trying to enter into that field, but an empty field may have a value

A field is referred to as null if there is no way of determining the value of its content (in reality, the computer, that is, the operating system, has its own internal mechanism of verifying the value of a field) or its value is simply unknown. As you can imagine, it is not a good idea to have a null field in your table. As a database developer, it is your responsibility to always know with certainty the value held by each field of your table. A field is referred to as required if the user must provide a value for it before moving to another record. In other words, the field cannot be left empty during data entry.

We continue with our university database. To run its many services, the school has a managing team that consists of a president, deans, and other employees.

In this new version of the database, University3, we will create a table for employees. The table will be stored in the Academics schema.

1. Start the computer and log in
2. Launch Microsoft SQL Server and click Connect
3. In the Object Explorer, right-click the Database node and click New Database...
4. Set the Database Name to University3
5. Click OK
6. In the Object Explorer, expand the University3 node and expand its Security node
7. Right-click Schemas and click New Schema...
8. Set the name to Academics
9. Click OK
10. Under University3, right-click Tables and click New Table...
11. While the table is displaying, in the Properties window, click Schema, then click the arrow of its combo box and select Academics
12. Create the following columns for the table:
     

    Column Name	Data Type
    EmployeeNumber	nvarchar(20)
    FirstName	nvarchar(25)
    MiddleName	nvarchar(25)
    LastName	nvarchar(25)
    EmailAddress	nvarchar(50)
    Username	nvarchar(30)
    Password	nvarchar(24)

13. To save the table, on the Standard toolbar, click the Save button
14. Set the name to Employees
15. Click OK

To solve the problem of null and required fields, the SQL proposes two options: allow or not allow null values on a field. To make sure the user always fills out a certain field before moving to the next field, that is, to require the value, if you are visually creating the table:

• In the top section of the table, clear the Allow Nulls check box for the field. On the other hand, if the value of a field is not particularly important, for example if you don't intend to involve that value in an algebraic operation, check its Allow Nulls check box
• In the top section of the table, click the column. In the bottom section of the table, click the arrow of the combo box of Allow Nulls and select Yes

1. In the top section, click EmployeeNumber and click its corresponding Allow Nulls check box to clear it
2. Complete the table as follows:

   Column Name	Allow Nulls
   EmployeeNumber	Unchecked
   FirstName	Checked
   MiddleName	Checked
   LastName	Unchecked
   EmailAddress	Checked
   Username	Checked
   Password	Checked

3. Save the table

If creating a table using SQL, to specify that it can allow null values, type NULL (remember that the SQL is not case-sensitive) on the right side of the column definition. To specify that the values of the column are required, on the right side, type NOT NULL. If you don't specify NULL or NOT NULL, the column will be created as NULL by default. Here are examples:

CREATE TABLE Persons
(
    FirstName nvarchar(20) NULL,
    LastName nvarchar(20) NOT NULL,
    Gender smallint
);
GO

If the table was already created and it holds some values (already), you cannot set the Allow Nulls option on columns that don't have values.

After specifying that a column would NOT allow NULL, if the user tries creating a record but omits to create a value for the column, an error would display. Here is an example:

This error message box indicates that the user attempted to submit null for a column that requires a value. To cancel the action, you can press Esc.

If you specify that a column will allow null, during data entry, if you don't have a value for that column, you can leave its placeholder empty:

CREATE TABLE Persons
(
    FirstName nvarchar(20) NULL,
    LastName nvarchar(20) NOT NULL,
    Gender smallint
);
GO

INSERT Persons(FirstName, LastName) -- The Gender is left out
VALUES(N'Martin', N'Binam');
GO
INSERT Persons(LastName, Gender) -- The FirstName is left out
VALUES(N'Howley', 2);
GO
INSERT Persons(LastName) -- The FirstName and the Gender are left out
VALUES(N'Crouch');
GO

When performing data entry, if the table has columns that allow nulls and whenever you don't have a value for a null column, you should provide a list of the columns as seen in the above examples. For the columns that are not included in the list, the database engine would automatically set their values to NULL. As an alternative, Transact-SQL allows you to keep the list of columns or to use any list of columns, including columns that allow null. Then, in the placeholder of a column, specify its value as NULL. Here are examples:

INSERT Persons -- All columns are used
VALUES(N'Alex', N'Hough', NULL);
GO
INSERT Persons(LastName, Gender, FirstName) -- The Gender will be set to null
VALUES(N'Kousseusseu', NULL, N'Tchipseu');
GO
INSERT Persons -- All columns are used
VALUES(NULL, N'Beltram', NULL),
      (NULL, N'Ohari', 1),
      (N'Jamrah', N'Belhassen', NULL);
GO

Sometimes most records under a certain column may hold the same value although just a few would be different. For example, if a school is using a database to register its students, all of them are more likely to be from the same state. In such a case, you can assist the user by automatically providing a value for that column. The user would then simply accept the value and change it only in the rare cases where the value happens to be different. To assist the user with this common value, you create what is referred to as a default value.

You can create a default value of a column when creating a table. To specify the default value of a column, in the top section, click the column. In the bottom section, click Default Value or Binding, type the desired value.

When specifying the default value of a column, you must follow rules that depend on the column's data type:

• Text-based (char, varchar, varchar(max), and their variants):Enter the value in single-quotes
• Numeric-based: Enter the value as a number but following the rules of the data type. Here is an example:

  

  Remember to following the rules of the data type. For example, if you enter a value higher than 255 for a tinyint, you would receive an error
• Bit: Enter 0 if the value is false or any number if the value is true

1. In the top section, click Password
2. In the bottom section, click Default Value or Binding
3. Type N'Password1'
4. Save the table

To specify the default value in a SQL statement, when creating the column, after specifying the other pieces of information of the column, type DEFAULT followed by an empty space and followed by the desired value. Here are examples:

CREATE TABLE Employees
(
    FullName NVARCHAR(50),
    Address NVARCHAR(80),
    City NVARCHAR(40),
    State NVARCHAR(40) DEFAULT L'NSW',
    PostalCode NVARCHAR(4) DEFAULT L'2000',
    Country NVARCHAR(20) DEFAULT L'Australia'
);
GO

When performing data entry on the table, the user does not have to provide a value for a column that has a default. If the user does not provide the value, the default would be used when the record is saved.

If the user provides a value for a column that has a default value and then deletes the value, the default value rule would not apply anymore: The field would simply become empty.

During programmatic data entry, if you don't list a column that has a default value, its default value would be assigned to the column. On the other hand, if you add such a column in the list of an INSERT statement, you must provide a value or give an empty value. Fortunately, Transact-SQL makes it possible to force the default value for the column. To do this, in the placeholder of the value of the column, use the DEFAULT keyword. Here is an example:

USE Exercise;
GO
CREATE TABLE Employees
(
    EmployeeNumber int,
    FirstName nvarchar(20),
    LastName nvarchar(20),
    HourlySalary money default 12.50
);
GO

INSERT INTO Employees
VALUES(28404, N'Amadou', N'Sulleyman', 18.85),
      (82948, N'Frank', N'Arndt', DEFAULT),
      (27749, N'Marc', N'Engolo', 14.50);
GO

In this example, the second record would receive the default value, which is 12.50. In the same way, you can apply the default keyword for each column whose default value was indicated when the table was created.

One of the primary concerns of records is their uniqueness. In a database, you usually want to make sure that each record on a table can be uniquely identified. Microsoft SQL Server provides many means of taking care of this. These include the identity column, the primary key, and the indexes. We will review these issues in later lessons. Still, one way to do this is to apply a uniqueness rule on a column.

To visually specify that each value of a column must be unique, in the Table window, right-click the desired column and click Indexes/Keys...

In the Indexes/Keys dialog box, click Add. On the right side, set the Is Unique field to Yes:

After doing this, click Close.

1. In the top section of the table, right-click EmployeeNumber and click Indexes/Keys...
2. In the Indexes/Keys dialog box, click Add
3. On the right side, double-click Is Unique to change its value from No to Yes

   

4. Click Close
5. Save the table

To assist you with creating a columns whose values will be distinguishable, Transact-SQL provides the UNIQUE keyword. To apply it on a column, after the data type, type UNIQUE. Here is an example:

USE Exercise;
GO
CREATE TABLE Students
(
    StudentNumber int UNIQUE,
    FirstName nvarchar(50),
    LastName nvarchar(50) NOT NULL
);
GO

When a column has been marked as unique, during data entry, the user must provide a unique value for each new record created. If an existing value is assigned to the column, this would produce an error:

USE Exercise;
GO
CREATE TABLE Students
(
    StudentNumber int UNIQUE,
    FirstName nvarchar(50),
    LastName nvarchar(50) NOT NULL
);
GO

INSERT INTO Students
VALUES(24880, N'John', N'Scheels'),
      (92846, N'Rénée', N'Almonds'),
      (47196, N'Peter', N'Sansen'),
      (92846, N'Daly', N'Camara'),
      (36904, N'Peter', N'Sansen');
GO

By the time the fourth record is entered, since it uses a student number that exists already, the database engine would produce an error:

Msg 2627, Level 14, State 1, Line 2
Violation of UNIQUE KEY constraint 'UQ__Students__DD81BF6C145C0A3F'. 
Cannot insert duplicate key in object 'dbo.Students'.
The statement has been terminated.