The Records of a Database
A table is an object that holds the information of a database. Because a table is the central part of a database, the information it holds must be meticulously organized. To better manage its information, data of a table is arranged in a series of fields called cells. Once a table contains information, you can review it using either SQL Server Management Studio or an external application.
The tables of a database display in the Object Explorer under their database node. To open a table, you can right-click it and click click Open Table.
Data Navigation consists of displaying and viewing data. Because information of a database is stored in tables, your primary means of viewing data consists of opening a table in a view that displays its information.
When a table displays its records, you navigate through its fields using the mouse or the keyboard. With the mouse, to get to any cell, you can just click it. To navigate through records using the keyboard, you can press:
As you are probably aware already, columns are used to organize data by categories. Each column has a series of fields under the column header. One of the actual purposes of a table is to display data that is available for each field under a particular column. Data entry consists of providing the necessary values of the fields of a table. Data is entered into a field and every time this is done, the database creates a row of data. This row is called a record. This means that entering data also self-creates rows.
There are four main ways you can perform data entry for a Microsoft SQL Server table:
Probably the easiest and fastest way to enter data into a table is by using SQL Server Management Studio. Of course, you must first open the desired table from an available database. In the Object Explorer, after expanding the Databases and the Tables nodes, open a table for data entry. If the table does not contain data, it would appear with one empty row. If some records were entered already, their rows would show and the table would provide an empty row at the end, expecting a new record.
To perform data entry on a table, you can click in a field. Each column has a title, called a caption, on top. This gray section on top is called a column header. In SQL Server, it displays the actual name of the column. You refer to the column header to know what kind of data should/must go in a field under a particular column. This is why you should design your columns meticulously. After identifying a column, you can type a value. Except for text-based columns, a field can accept or reject a value if the value does not conform to the data type that was set for the column. This means that in some circumstances, you may have to provide some or more explicit information to the user.
To perform data entry using SQL:
In the SQL, data entry is performed using the INSERT combined with the VALUES keywords. 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 in the table. This is done with the following syntax:
INSERT INTO TableName VALUES(Column1, Column2, Column_n)
The TableName factor must be a valid name of an existing table in the database you are using. If the name is wrong, the SQL interpreter would simply consider that the table you are referring to doesn't exist. Consequently, you would receive an error.
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 BIT data type, you must specify one of its values as 0 or 1.
If the column is a numeric type, you should pay attention to the number you type. If the column was configured to receive an integer (int, bigint, smallint), you should provide a valid natural number without the decimal separator.
If the column is for a decimal number (float, real, decimal, 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, make sure you provide a valid date.
If the data type of a column is a string type, you should include its entry between single quotes. For example, a shelf number can be specified as 'HHR-604' and a middle initial can be given as 'D'.
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. With this subsequent list in mind, enter the value of each field in its correct position.
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.
The adjacent data entry we have performed requires that you know the position of each column. The SQL provides an alternative that allows you to perform data entry using the name of a column instead of its position. This allows you to provide the values of columns in an order of your choice. We have just seen a few examples where the values of some of the fields were not available during data entry. Instead of remembering to type 0 or NULL for such fields or leaving empty quotes for a field, you can use the fields' names to specify the fields whose data you want to provide.
To perform data entry in an order of your choice, you must provide your list of the fields of the table. You can either use all columns or provide a list of the same columns but in your own order. In the same way, you don't have to provide data for all fields, just those you want, in the order you want.
During data entry, users of your database will face fields that expect data. Sometimes, for one reason or another, data will not be available for a particular field. An example would be an MI (middle initial) field: some people have a middle initial, some others either don't have it or would not (or cannot) provide it. This aspect can occur for any field of your table. Therefore, you should think of a way to deal with it.
A field is referred to as null when no data entry has been made to it:
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.
To solve the problem of null and required fields, Microsoft SQL Server proposes one of two options: allow or not allow null values on a field. For a typical table, there are pieces of information that the user should make sure to enter; otherwise, the data entry would not be validated. 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, 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.
If creating a table using SQL, to specify that it can allow null values, type NULL on the right side of the column. 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. Here are examples:
CREATE TABLE Persons ( FirstName varchar(20) NULL, LastName varchar(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.
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 happen 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 following the rules of the column's data type:
To specify the default value in a SQL statement, when creating the column, before the semi-colon or the closing parenthesis of the last column, assign the desired value to the DEFAULT keyword. Here are examples:
CREATE TABLE Employees ( FullName VARCHAR(50), Address VARCHAR(80), City VARCHAR(40), State VARCHAR(40) DEFAULT = 'NSW', PostalCode VARCHAR(4) DEFAULT = '2000', Country VARCHAR(20) DEFAULT = 'Australia' ); GO
After creating 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.
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:
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.
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.
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
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.
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 7. 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 DateTime ); 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('Annette Berceau', '301-988-4615', GETDATE()); GO INSERT INTO RepairOrders(CustomerPhone, CustomerName, RepairDate) VALUES('(240) 601-3795', 'Paulino Santiago', GETDATE()); GO INSERT INTO RepairOrders(CustomerName, RepairDate, CustomerPhone) VALUES('Alicia Katts', GETDATE(), '(301) 527-3095'); GO INSERT INTO RepairOrders(RepairDate, CustomerPhone, CustomerName) VALUES(GETDATE(), '703-927-4002', '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 satifies your requirements, you can create your own, using the techniques we studied in Lesson 6.
There are various ways you can assist the user with data entry. Besides using a function, you can create an expression using operators such as those we reviewed in lessons 3 and 5. You can create an expression when creating a table, whether in the Table window or using SQL in a query window.
To create an expression when visually creating a table, in the top section, specify the column's name (only the column name is important). In the bottom section, expand the Computed Column Specification field and, in its (Formula) field, enter the desired expression. Here is an example:
You can also create an expression in SQL expression you are using to create a table. To do this, in the placeholder of the column, enter the name of the column, followed by AS, and followed by the desired expression. Here is an example:
CREATE TABLE Circle ( CircleID int identity(1,1) NOT NULL, Radius decimal(8, 3) NOT NULL, Area AS Radius *Radius * PI() ); GO
When performing data entry, you must not provide a value for a column that has an expression; the SQL interpreter would provide the value automatically. Here is an example of entering data for the above Circle table:
INSERT INTO Circle(Radius) VALUES(46.82); GO INSERT INTO Circle(Radius) VALUES(8.15); GO INSERT INTO Circle(Radius) VALUES(122.57); GO
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. To assist you with checking whether a newly entered value fits the desired range, Transact-SQL provides what is referred to as 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:
If the value is not appropriate:
You create a check constraint at the time you are creating a table.
To create a check constraint, when creating a table, right-click anywhere in (even outside) the table and click 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:
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:
After creating the check constraints, you can click OK.
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:
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.
This property allows you to specify that a column with the Identity property set to Yes is used as a ROWGUID column.
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.
Another technique used to get data into one or more tables consists of importing already existing data from another database or from any other recognizable data file. Microsoft SQL Server provides various techniques and means of importing data.
The easiest type of data that can be imported into SQL Server, and which is available on almost all database environments, is the text file. Almost every database environment allows you to import a text file but data from that file must be formatted appropriately. For example, the information stored in the file must define the columns as distinguishable by a character that serves as a separator. This separator can be the single-quote, the double-quote, or any valid character. Data between the quotes is considered as belonging to a distinct field. Besides this information, the database would need to separate information from two different columns. Again, a valid character must be used. Most databases, including Microsoft SQL Server, recognize the comma as such a character. The last piece of information the file must provide is to distinguish each record from another. This is easily taken car of by the end of line of a record. This is also recognized as the carriage return.
These directives can help you manually create a text file that can be imported into Microsoft SQL Server. In practicality, if you want to import data that resides on another database, you can ask that application to create the source of data. Most applications can do that and format the data. That is the case for the data we will use in the next exercise: it is data that resided on a Microsoft Access database and was prepared to be imported in Microsoft SQL Server.
After importing data, you should verify and possibly format it to customize its fields.
One of the simplest operations a user can perform on a table consists of looking for a record. To do this, the user would open the table that contains the records and visually check them, looking for a piece of information, such as a student's last name.
As the database developer, you too can look for a record and there are various techniques you can use. To assist you with this, Transact-SQL provides a function named EXISTS. Its syntax is:
BIT EXISTS(SELECT Something)
This function takes one argument. The argument must be a SELECT statement that would be used to get the value whose existence would be checked. For example, in Lesson 2, we mentioned a system database names databases that contains a record of all databases stored on your server. You can use the EXISTS() function to check the existence of a certain database. The formula you would use is:
IF EXISTS ( SELECT name FROM sys.databases WHERE name = N'DatabaseName' )
In the DatabaseName placeholder, you can enter the name of the database.
Before visually performing some operations on a table, you must first select one or more records. In the Table window, to select one record, position the mouse on the left button of the record and click:
To select a range of records, click the gray button of one of the records, press and hold Shift, then click the gray button of the record at the other extreme.
To select the records in a random fashion, select one record, press and hold Ctrl, then click the gray button of each desired record:
To select all records of a table, click the gray button on the left of the first column:
To visually modify one or more records on a table, first open it (you right-click the table in the Object Explorer and click Open Table) to view its records. Locate the record and the field you want to work on and perform the desired operation.
Record maintenance includes viewing records, looking for one or more records, modifying one or more records, or deleting one or more records.
Updating a record consists of changing its value for a particular column. To update a record using SQL:
To support record maintenance operations, the SQL provides the UPDATE keyword that is used to specify the table on which you want to maintain the record(s). The basic formula to use is:
UPDATE TableName SET ColumnName = Expression
With this formula, you must specify the name of the involved table as the TableName factor of our formula. The SET statement allows you to specify a new value, Expression, for the field under the ColumnName column.
Consider the following code to create a new database named VideoCollection and to add a table named Videos to it:
CREATE DATABASE VideoCollection; GO USE VideoCollection; GO CREATE TABLE Videos ( VideoID INT NOT NULL IDENTITY(1,1), VideoTitle varchar(120) NOT NULL, Director varchar(100) NULL, YearReleased SMALLINT, VideoLength varchar(30) NULL, Rating varchar(6) ); GO INSERT INTO Videos(VideoTitle, Director, YearReleased, VideoLength) VALUES('A Few Good Men','Rob Reiner',1992,'138 Minutes'); INSERT INTO Videos(VideoTitle, Director, YearReleased, VideoLength) VALUES('The Silence of the Lambs','Jonathan Demme',1991,'118 Minutes'); INSERT INTO Videos(VideoTitle, Director, VideoLength) VALUES('The Distinguished Gentleman', 'James Groeling', '112 Minutes'); INSERT INTO Videos(VideoTitle, Director, VideoLength) VALUES('The Lady Killers', 'Joel Coen & Ethan Coen', '104 Minutes'); INSERT INTO Videos(VideoTitle, Director, VideoLength) VALUES('Ghosts of Mississippi', 'Rob Reiner', '130 Minutes'); GO
Imagine that, at one time, on a particular table, all records need to receive a new value under one particular column or certain columns. There is no particular way to visually update all records of a table. You can just open the table to view its records, and then change them one at a time.
In SQL, the primary formula of the UPDATE statement as introduced on our formula can be used to update all records. Here is an example:
USE VideoCollection; GO UPDATE Videos SET Rating = 'R'; GO
With this code, all records of the Videos table will have their Rating fields set to a value of R:
Editing a record consists of changing a value in a field. It could be that the field is empty, such as the © Year of the the 'The Lady Killers' video of the following table. It could be that the value is wrong, such as the Director of the the 'The Distinguished Gentleman' video of this table:
To edit a record, first open the table to view its records. Locate the record, the column on which you want to work, and locate the value you want to change, then change it.
In SQL, you must provide a way for the interpreter to locate the record. To do this, you would associate the WHERE operator in an UPDATE statement using the following formula:
UPDATE TableName SET ColumnName = Expression WHERE Condition(s)
The WHERE operator allows you to specify how the particular record involved would be identified. It is very important, in most cases, that the criterion used be able to uniquely identify the record. In the above table, imagine that you ask the interpreter to change the released year to 1996 where the director of the video is Rob Reiner. The UPDATE statement would be written as follows:
UPDATE Videos SET YearReleased = 1996 WHERE Director = 'Rob Reiner';
In the above table, there are at least two videos directed by Rob Reiner. When this statement is executed, all video records whose director is Rob Reiner would be changed, which would compromise existing records that didn't need this change. This is where the identity column becomes valuable. We saw earlier that, when using it with the IDENTITY feature, the interpreter appends a unique value to each record. You can then use that value to identify a particular record because you are certain the value is unique.
Here is an example used to specify the missing copyright year of a particular record:
UPDATE Videos SET YearReleased = 1996 WHERE VideoID = 5; GO
Here is an example used to change the name of the director of a particular video:
UPDATE Videos SET Director = 'Jonathan Lynn' WHERE VideoTitle = 'The Distinguished Gentleman';
If you think all records of a particular table are, or have become, useless, you can clear the whole table, which would still keep its structure. To delete all records from a table, first select all of them, and press Delete. You would receive a warning:
If you still want to delete the records, click Yes. If you change your mind, click No.
Using SQL, to clear a table of all records, use the DELETE operator with the following formula:
When this statement is executed, all records from the TableName factor would be removed from the table. Be careful when doing this because once the records have been deleted, you cannot get them back.
If you find out that a record is not necessary, not anymore, or is misplaced, you can remove it from a table. To remove a record from a table, you can right-click its gray box and click Delete. You can also first select the record and press Delete. You would receive a warning to confirm your intention.
To delete a record using SQL:
In SQL, to delete a record, use the DELETE FROM statement associate the WHERE operator. The formula to follow is:
DELETE FROM TableName WHERE Condition(s)
The TableName factor is used to identify a table whose record(s) would be removed.
The Condition(s) factor allows you to identify a record or a group of records that carries a criterion. Once again, make sure you are precise in your criteria so you would not delete the wrong record(s).
Here is an example used to remove a particular record from the table:
DELETE FROM Videos WHERE VideoTitle = 'The Lady Killers';
Here is an example used to clear the table of all videos:
DELETE FROM Videos;
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