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Introduction to Records

 

Records Fundamentals

 

Introduction

A table holds the records of a database. Here is an example of a table creation:

SQL> CREATE TABLE Employees
  2  (
  3     EmployeeNumber char(6),
  4     FirstName nvarchar2(20),
  5     LastName nvarchar2(20),
  6     HourlySalary number(6, 2)
  7  );

Table created.

SQL>

After creating it, you can populate it with records.

Introduction to Data Entry

To perform data entry, you use a Data Definition Language (DDL) command known as INSERT. This DDL command is combined with VALUES. The primary statement uses the following syntax:

INSERT TableName VALUES(Column1, Column2, Column_n);

Alternatively, or to be more precise, you can use the INTO keyword between the INSERT keyword and the TableName factor to specify that you are entering data into the table. This is done with the following syntax:

INSERT INTO TableName VALUES(Column1, Column2, Column_n)

The TableName must be a valid name of an existing table.

The VALUES keyword indicates that you are ready to list the values of the columns. The values of the columns must be included in parentheses.

If the column is a BOOLEAN data type, you must specify one of its values as TRUE or FALSE.

If the column is a numeric type, provide a valid natural number without the decimal separator.

If the column is for a decimal number (float, real, numeric), you can type the value with its character separator (the period for US English).

If the column was created for a date data type, provide a valid date in single-quotes.

If the data type of a column is a string type, include its value in single quotes.

Adjacent Data Entry

The most common technique of performing data entry requires that you know the sequence of fields of the table in which you want to enter data. Here is an example:

SQL> INSERT INTO Employees
  2  VALUES('65-804', 'Jeannette', 'Collins', 24.50);

1 row created.

In the same way, you can add as many records as you want.

During data entry on adjacent fields, if you don't have a value for a numeric field, you should type 0 as its value. For a string field whose data you don't have and cannot provide, type two single-quotes '' to specify an empty field.

Random Data Entry

Adjacent data entry requires that you know the position of each column. An alternative is to provide the values of columns in an order of your choice. To do this, after the name of the table, provide your list of columns. Then, provide their corresponding values in the parentheses after VALUES. Here is an example:

SQL> INSERT INTO Employees(LastName, EmployeeNumber)
  2  VALUES('Tandem', '82-680');

1 row created.

With this technique, you choose the columns you want and provide values only for those ones.

 

 

 
 

 

Assistance with Data Entry: Identity Columns

 

Introduction

One of the goals of a good table is to be able to uniquely identity each record. In most cases, the database engine should not confuse two records. Consider the following table:

Category Item Name Size Unit Price
Women Long-sleeve jersey dress Large 39.95
Boys Iron-Free Pleated Khaki Pants S 39.95
Men Striped long-sleeve shirt Large 59.60
Women Long-sleeve jersey dress Large 45.95
Girls Shoulder handbag   45.00
Women Continental skirt Petite 39.95

Imagine that you want to change the value of an item named Long-sleeve jersey dress. Because you must find the item programmatically, you can start looking for an item with that name. This table happens to have two items with that name. You may then decide to look for an item using its category. In the Category column, there are too many items named Women. In the same way, there are too many records that have a Large value in the Size column, same thing problem in the Unit Price column. This means that you don't have a good criterion you can use to isolate the record whose Item Name is Long-sleeve shirt.

To solve the problem of uniquely identifying a record, you can create a particular column whose main purpose is to distinguish one record from another. To assist you with this, the SQL allows you to create a column whose data type is an integer type but the user doesn't have to enter data for that column. A value would automatically be entered into the field when a new record is created. This type of column is called an identity column.

You cannot create an identity column one an existing table, only on a new table.

Visually Creating an Identity Column

To create an identity column, if you are visually working in the design view of the table, in the top section, specify the name of the column. By tradition, the name of this column resembles that of the table but in singular. Also, by habit, the name of the column ends with _id, Id, or ID.

After specifying the name of the column, set its data type to an integer-based type. Usually, the data type used is int. In the bottom section, click and expand the Identity Specification property. The first action you should take is to set its (Is Identity) property from No to Yes.

Once you have set the value of the (Is Identity) property to Yes, the first time the user performs data entry, the value of the first record would be set to 1. This characteristic is controlled by the Identity Seed property. If you want the count to start to a value other than 1, specify it on this property.

After the (Is Identity) property has been set to Yes, the SQL interpreter would increment the value of each new record by 1, which is the default. This means that the first record would have a value of 1, the second would have a value of 2, and so on. This aspect is controlled by the Identity Increment property. If you want to increment by more than that, you can change the value of the Identity Increment property.

Practical LearningPractical Learning: Creating an Identity Column

  1. In the Object Explorer, under WorldStatistics, right-click Tables and click New Table...
  2. Set the name of the column to ContinentID and press Tab
  3. Set its data type to int and press F6.
    In the lower section of the table, expand Identity Specification and double-click (Is Identity) to set its value to Yes
  4. Complete the table as follows:
     
    Column Name Data Type Allow Nulls
    ContinentID    
    Continent varchar(80) Unchecked
    Area bigint  
    Population bigint  
  5. Save the table as Continents

Creating an Identity Column Using SQL

If you are programmatically creating a column, to indicate that it would be used as an identity column after its name and data type, type identity followed by parentheses. Between the parentheses, enter the seed value, followed by a comma, followed by the increment value. Here is an example:

CREATE TABLE StoreItems(
ItemID int IDENTITY(1, 1) NOT NULL, 
Category varchar(50),
[Item Name] varchar(100) NOT NULL,
Size varchar(20),
[Unit Price] money);
GO

Practical LearningPractical Learning: Creating an Identity Column Using SQL

  1. Display the PowerShell window
  2. Type the following:
     
    USE CeilInn1;
    GO
  3. To create a table with an identity column, type the following and press Enter after each line:
     
    DROP TABLE Rooms;
    GO
    
    CREATE TABLE Rooms (
        RoomID int identity(1, 1) NOT NULL,
        RoomNumber nvarchar(10),
        RoomType nvarchar(20) default N'Bedroom',
        BedType nvarchar(40) default N'Queen',
        Rate money default 75.85,
        Available bit default 0
    );
    GO
  4. To perform data entry on the new table, type the following and press Enter at the end:
     
    INSERT INTO Rooms(RoomNumber) VALUES(104);
    GO
  5. To add another record to the new table, type the following:
     
    INSERT INTO Rooms(RoomNumber, BedType, Rate, Available)
               VALUES(105, N'King', 85.75, 1),
    		 (106, N'King', 85.75, 1)
    GO
  6. To add another record, type the following:
     
    INSERT INTO Rooms(RoomNumber, Available) VALUES(107, 1)
    GO
  7. To add another record, type the following:
     
    INSERT INTO Rooms(RoomNumber, BedType, Rate) VALUES(108, N'King', 85.75)
    GO
  8. To add another record, type the following:
     
    INSERT INTO Rooms(RoomNumber, Available) VALUES(109, 1)
    GO
  9. To add one more record, type the following:
     
    INSERT INTO Rooms(RoomNumber, RoomType, BedType, Rate, Available)
    	   VALUES(110, N'Conference', N'', 450.00, 1)
    GO
  10. Return to Microsoft SQL Server Management Studio

Assistance with Data Entry: The Uniqueness of Records

 

Introduction

One of the primary concerns of records is their uniqueness. In a professional database, you usually want to make sure that each record on a table is unique. 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.

Creating a Uniqueness Rule

To assist you with creating a columns whose values will be distinguishable, Transact-SQL provides the UNIQUE operator. 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.

Practical LearningPractical Learning: Applying Uniqueness to a Column

  1. In the PowerShell window, to create a new table that has a uniqueness rule on a column, type the following:
     
    CREATE TABLE Customers (
        CustomerID int identity(1, 1) NOT NULL,
        AccountNumber nchar(10) UNIQUE,
        FullName nvarchar(50)
    );
    GO
  2. To perform data entry on the table, type the following and press Enter at the end of each line:
     
    INSERT INTO Customers(AccountNumber, FullName)
    	       VALUES(395805, N'Ann Zeke'),
    	             (628475, N'Peter Dokta'),
    	             (860042, N'Joan Summs')
    GO
  3. To try adding another record to the table, type the following and press Enter at the end of each line:
     
    USE CeilInn1;
    GO
    
    INSERT INTO Customers(AccountNumber, FullName)
    	       VALUES(628475, N'James Roberts')
    GO
  4. Notice that you receive an error

Assistance With Data Entry: Check Constraints

 

Introduction

When performing data entry, in some columns, even after indicating the types of values you expect the user to provide for a certain column, you may want to restrict a range of values that are allowed. In the same way, you can create a rule that must be respected on a combination of columns before the record can be created. For example, you can ask the database engine to check that at least one of two columns received a value. For example, on a table that holds information about customers, you can ask the database engine to check that, for each record, either the phone number or the email address of the customer is entered.

The ability to verify that one or more rules are respected on a table is called a check constraint. A check constraint is a Boolean operation performed by the SQL interpreter. The interpreter examines a value that has just been provided for a column. If the value is appropriate:

  1. The constraint produces TRUE
  2. The value gets accepted
  3. The value is assigned to the column

If the value is not appropriate:

  1. The constraint produces FALSE
  2. The value gets rejected
  3. The value is not assigned to the column

You create a check constraint at the time you are creating a table.

Visually Creating a Check Constraint

To create a check constraint, when creating a table, right-click anywhere in (even outside) the table and click Check Constraints...

Check Constraints

This would open the Check Constraints dialog box. From that window, you can click Add. Because a constraint is an object, you must provide a name for it. The most important piece of information that a check constraint should hold is the mechanism it would use to check its values. This is provided as an expression. Therefore, to create a constraint, you can click Expression and click its ellipsis button. This would open the Check Constraint Expression dialog box.

To create the expression, first type the name of the column on which the constraint will apply, followed by parentheses. In the parentheses, use the arithmetic and/or SQL operators we studied already. Here is an example that will check that a new value specified for the Student Number is greater than 1000:

Check Constraint Expression

After creating the expression, you can click OK. If the expression is invalid, you would receive an error and given the opportunity to correct it.

You can create as many check constraints as you judge necessary for your table:

Check Constraints

After creating the check constraints, you can click OK.

Programmatically Creating a Check Constraint

To create a check constraint in SQL, first create the column on which the constraint will apply. Before the closing parenthesis of the table definition, use the following formula:

CONSTRAINT name CHECK (expression

The CONSTRAINT and the CHECK keywords are required. As an object, make sure you provide a name for it. Inside the parentheses that follow the CHECK operator, enter the expression that will be applied. Here is an example that will make sure that the hourly salary specified for an employee is greater than 12.50:

CREATE TABLE Employees
(
	[Employee Number] nchar(7),
	[Full Name] varchar(80),
	[Hourly Salary] smallmoney,
	CONSTRAINT CK_HourlySalary CHECK ([Hourly Salary] > 12.50)
);

It is important to understand that a check constraint it neither an expression nor a function. A check constraint contains an expression and may contain a function as part of its definition.

After creating the constraint(s) for a table, in the Object Explorer of Microsoft SQL Server Management Studio, inside the table's node, there is a node named Constraints and, if you expand it, you would see the name of the constraint.

With the constraint(s) in place, during data entry, if the user (or your code) provides an invalid value, an error would display. Here is an example:

An Error From an Invalid Value of Check Constraint

Instead of an expression that uses only the regular operators, you can use a function to assist in the checking process. You can create and use your own function or you can use one of the built-in Transact-SQL functions.

Practical LearningPractical Learning: Creating a Check Constraint

  1. In the PowerShell window, to create a table that has a check mechanism, type the following:
     
    DROP TABLE Customers;
    GO
    
    CREATE TABLE Customers (
        CustomerID int identity(1, 1) NOT NULL,
        AccountNumber nchar(10) UNIQUE,
        FullName nvarchar(50) NOT NULL,
        PhoneNumber nvarchar(20),
        EmailAddress nvarchar(50),
        CONSTRAINT CK_CustomerContact
    	CHECK ((PhoneNumber IS NOT NULL) OR (EmailAddress IS NOT NULL))
    );
    GO
  2. To add records to the new table, type the following:
     
    INSERT INTO Customers(AccountNumber, FullName,
                          PhoneNumber, EmailAddress)
    VALUES(N'395805', N'Ann Zeke', N'301-128-3506', N'azeke@yahoo.jp'),
          (N'628475', N'Peter Dokta', N'(202) 050-1629', 
           N'pdorka1900@hotmail.com'),
          (N'860042', N'Joan Summs', N'410-114-6820', N'jsummons@emailcity.net');
    GO
  3. To try adding a new record to the table, type the following:
     
    INSERT INTO Customers(AccountNumber, FullName)
    	       VALUES(N'228648', N'James Roberts')
    GO
  4. Notice that you receive an error.
    Close the PowerShell window

Assistance With Data Entry: Using Functions

 

Introduction

You can involve a function during data entry. As an example, you can call a function that returns a value to assign that value to a column. You can first create your own function and use it, or you can use one of the built-in functions.

Using Functions

In order to involve a function with your data entry, you must have and identity one. You can use one of the built-in functions of Transact-SQL. You can check one of the functions we reviewed in Lesson 8. Normally, the best way is to check the online documentation to find out if the assignment you want to perform is already created. Using a built-in function would space you the trouble of getting a function. For example, imagine you have a database named AutoRepairShop and imagine it has a table used to create repair orders for customers:

CREATE TABLE RepairOrders
(
  RepairID int Identity(1,1) NOT NULL,
  CustomerName varchar(50),
  CustomerPhone varchar(20),
  RepairDate datetime2
);
GO

When performing data entry for this table, you can let the user enter the customer name and phone number. On the other hand, you can assist the user by programmatically entering the current date. To do this, you would call the GETDATE() function. Here are examples:

INSERT INTO RepairOrders(CustomerName, CustomerPhone, RepairDate)
	    VALUES(N'Annette Berceau', N'301-988-4615', GETDATE());
GO
INSERT INTO RepairOrders(CustomerPhone, CustomerName, RepairDate)
	    VALUES(N'(240) 601-3795', N'Paulino Santiago', GETDATE());
GO
INSERT INTO RepairOrders(CustomerName, RepairDate, CustomerPhone)
	    VALUES(N'Alicia Katts', GETDATE(), N'(301) 527-3095');
GO
INSERT INTO RepairOrders(RepairDate, CustomerPhone, CustomerName)
	    VALUES(GETDATE(), N'703-927-4002', N'Bertrand Nguyen');
GO

You can also involve the function in an operation, then use the result as the value to assign to a field. You can also call a function that takes one or more arguments; make sure you respect the rules of passing an argument to a function when calling it.

If none of the Transact-SQL built-in functions satisfies your requirements, you can create your own, using the techniques we studied in Lesson 7.

Other Features of Data Entry

 

Is RowGuid

This property allows you to specify that a column with the Identity property set to Yes is used as a ROWGUID column.

Collation

Because different languages use different mechanisms in their alphabetic characters, this can affect the way some sort algorithms or queries are performed on data, you can ask the database to apply a certain language mechanism to the field by changing the Collation property. Otherwise, you should accept the default specified by the table.

To find out what language your server is currently using, in a Query window or from PowerShell, you can type:

SELECT @@LANGUAGE;
GO
 
 
   
 

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