Tài liệu Create, Modify, and Delete Tables docx

6.6 Create, Modify, and Delete Tables It is common in database applications to programmatically create, modify, and delete tables. How do I do this using T-SQL? Technique To perform these tasks, you will use the CREATE TABLE, ALTER TABLE, and DROP TABLE T-SQL statements. With these statements, you can handle any requirements that your application might have. Look at these statements one at a time. Creating a Table Using CREATE TABLE With the CREATE TABLE statement, not only can you specify columns and their data types, but you also can specify indexes, check constraints, and other table level properties. For this How-To, you will be use the following T-SQL statement: CREATE TABLE ListsExample ( ListID smallint IDENTITY(1, 1) PRIMARY KEY CLUSTERED, LastName varchar(50) NOT NULL, FirstName varchar(50) NOT NULL, Age smallint , DateEntered datetime NOT NULL DEFAULT GetDate() CHECK (DateEntered <= GetDate()) ) This statement shows how to perform a number of different tasks: 1. Specify the name of the table: 2. CREATE TABLE ListsExample 3. Create an Identity column, which is the primary key: 4. ListID smallint IDENTITY(1, 1) PRIMARY KEY CLUSTERED, 5. Create fields that can't be NULL, and using different data types: 6. LastName varchar(50) NOT NULL, 7. FirstName varchar(50) NOT NULL, 8. Age smallint , 9. Create a field with the Default Value set, and with a Check Constraint specified. 10. DateEntered datetime NOT NULL DEFAULT GetDate() 11. CHECK (DateEntered <= GetDate()) Modifying a Table Using ALTER TABLE The example T-SQL statement used for modifying the table in this How-To is pretty simple in that it adds a column and removes (drops) a column: ALTER TABLE ListsExample ADD MyNewColumn varchar(30) ALTER TABLE ListsExample DROP COLUMN Age You can perform quite a few other tasks with this statement. You can even see by the syntax displayed here that you can handle many tasks, including dropping constraints. ALTER TABLE table { [ ALTER COLUMN column_name { new_data_type [ ( precision [ , scale ] ) ] [ COLLATE < collation_name > ] [ NULL | NOT NULL ] | {ADD | DROP } ROWGUIDCOL } ] | ADD { [ < column_definition > ] | column_name AS computed_column_expression } [ , n ] | [ WITH CHECK | WITH NOCHECK ] ADD { < table_constraint > } [ , n ] | DROP { [ CONSTRAINT ] constraint_name | COLUMN column } [ , n ] | { CHECK | NOCHECK } CONSTRAINT { ALL | constraint_name [ , n ] } | { ENABLE | DISABLE } TRIGGER { ALL | trigger_name [ , n ] } } You can do even more. Look at the Books Online for SQL Server to see complete coverage of this statement. Deleting a Table Using the DROP TABLE Statement This statement is the easiest, and it's a one liner: DROP TABLE ListsExample However, you need to keep some things in mind when you are trying to drop a table: • You can't use the DROP TABLE statement when the table is used in a relationship and is referenced in the FOREIGN KEY constraint. You will need to drop the other table or the constraint. • You will need to be the administrator or owner of the table to be able to use the DROP TABLE statement. • You can't use the DROP TABLE statement on system tables. Steps Open and run the Visual Basic .NET-Chapter 6 solution. From the main form, click on the button with the caption How-To 6.6 (see Figure 6.7). 1. Create a Windows Form. Then place the controls listed in Table 6.6 with the following properties set, as displayed in Figures 6.7. Table 6.6. Control Property Settings for This How-To Object Property Setting Label Text SQL Statement to Create a Table Label Name lblCreateTable Button Name btnCreateTable Text Create Table Label Text SQL Statement to Modify a Table Label Name lblModifyTable Button Name btnModifyTable Text Modify Table Label Text SQL Statement to Delete a Table Label Name lblDeleteTable Button Name btnDeleteTable Text Delete Table 2. Add the code in Listing 6.11 to the Load event of the form. (Double-click on the form to bring up the code.) This routine creates the SQL statements for all three tasks and assigns them to the appropriate label for display. Listing 6.11 frmHowTo6_6.vb: Loading the SQL Statements into the Appropriate Labels Private Sub frmHowTo6_6_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load ' Build the SQL String for Creating a Table Dim strSQL As String strSQL = "CREATE TABLE ListsExample" & vbCrLf strSQL &= "(" & vbCrLf strSQL &= " ListID smallint IDENTITY(1, 1) " & _ "PRIMARY KEY CLUSTERED," & vbCrLf strSQL &= " LastName varchar(50) NOT NULL, " & vbCrLf strSQL &= " FirstName varchar(50) NOT NULL, " & vbCrLf strSQL &= " Age smallint , " & vbCrLf strSQL &= " DateEntered datetime NOT NULL DEFAULT GetDate() " & vbCrLf strSQL &= " CHECK (DateEntered <= GetDate())" & vbCrLf strSQL &= ")" Me.lblCreateTable.Text = strSQL strSQL = "ALTER TABLE ListsExample ADD MyNewColumn varchar(30) " & vbCrLf strSQL &= "ALTER TABLE ListsExample DROP COLUMN Age" Me.lblModifyTable.Text = strSQL strSQL = "DROP TABLE ListsExample" Me.lblDeleteTable.Text = strSQL End Sub 3. Add the code in Listing 6.12 to the Click event of the btnCreateTable button. This routine calls the function PerformTask(), passing the text in the lblCreateTable label. PerformTask() is described in the next step. If you perform the task successfully, then a message box is displayed letting you know that all went as it should have. Listing 6.12 frmHowTo6_6.vb: Calling PerformTask() from the btnCreateTable Click Event Private Sub btnCreateTable_Click(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles btnCreateTable.Click If PerformTask(Me.lblCreateTable.Text) Then MessageBox.Show("Table Created Successfully, " & _ _Look for ListsExample " & "Table in Northwind Database in Server Explorer", _ "Action Performed") End If End Sub 4. In the class module of the form created for this How-To, create the code displayed in Listing 6.13 for the PerformTask() function. This code creates a Connection object. Next, create a Command object that is based on the string passed in strSQL. Open the connection and execute the command. Notice that the execution of the command has been wrapped in the Try Catch End Try code block to make sure the command is executed correctly; if it's not, a message is displayed. Listing 6.13 frmHowTo6_6.vb: Executing the SQL Statement Passed in Using strSQL Function PerformTask(ByVal strSQL As String) As Boolean Dim cnn As New OleDb.OleDbConnection(BuildCnnStr("(local)", "Northwind")) Dim cmdAction As New OleDb.OleDbCommand(strSQL) cmdAction.Connection = cnn cnn.Open() PerformTask = True Try cmdAction.ExecuteNonQuery() Catch excp As Exception MessageBox.Show(excp.Message, "Error with Action") PerformTask = False End Try cnn.Close() End Function 5. Add the code snippets in Listing 6.14 to the appropriate Click events for btnModifyTable and btnDeleteTable to the Load event of the form. Listing 6.14 frmHowTo6_6.vb: Calling PerformTask from btnModifyTable and btnDeleteTable Click Events Private Sub btnModifyTable_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnModifyTable.Click If PerformTask(Me.lblModifyTable.Text) Then MessageBox.Show("Table Modified Successfully, " & _ Look for ListsExample " & "Table in Northwind Database in Server Explorer", _ "Action Performed") End If End Sub Private Sub btnDeleteTable_Click(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles btnDeleteTable.Click If PerformTask(Me.lblDeleteTable.Text) Then MessageBox.Show("Table Deleted Successfully, " & _ "Look for ListsExample " & "Table in Northwind Database in Server Explorer", _ "Action Performed") End If End Sub Figure 6.7. A common problem with inner joins is retrieving multiple records when you just want to see one per occurrence. Comments You don't have to add, modify, and delete tables manually. Just make sure that you back up your data before performing these tasks. . 6.6 Create, Modify, and Delete Tables It is common in database applications to programmatically create, modify, and delete tables. How do. Modify Table Label Text SQL Statement to Delete a Table Label Name lblDeleteTable Button Name btnDeleteTable Text Delete Table 2. Add the code in Listing