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comm.Parameters.AddWithValue("@Name", name) |> ignore use adapter = new SqlDataAdapter(comm) let table = new DataTable() adapter.Fill(table) |> ignore table We can execute the stored procedure as follows to find the employees with the last name Smith: > for row in (GetEmployeesByLastName "Smith").Rows do printfn "row = %O, %O" (row.Item("FirstName")) (row.Item("LastName"));; row = Joe, Smith row = Eve, Smith val it : unit = ()
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You saw in 14 how data tables can be visualized in web applications. The return value of GetEmployeesByLastName from the previous section is a DataTable. These objects can also be directly bound to a Windows Forms data grid, a visual data control that supports the DataSource property and that can display data in a tabular format. Windows Forms controls were discussed in 11. open System.Windows.Forms let emps = GetEmployeesByLastName "Smith" let grid = new DataGrid(Width=300, Height=200, DataSource=emps) let form = new Form(Visible=true, TopMost=true) form.Controls.Add(grid) Figure 15-1 shows what you will see when you run this code.
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Figure 15-1. Calling the GetEmployeesByLastName stored procedure
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CHAPTER 15 WORKING WITH DATA
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Stored procedures can also perform deletions or updates (executed via the ExecuteNonQuery() method of the command object) or return data through out parameters. These can be defined using the OUTPUT keyword after a single parameter definition in the stored procedure. When calling the stored procedure, the out parameter has to have its direction set to ParameterDirection.Output, and after executing the stored procedure, its return value can be read using the Value property of the given parameter.
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Working with Databases in Visual Studio
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Many of the database tasks you saw earlier in this chapter can be easily performed using the built-in capabilities of Visual Studio. It also gives you good tools for working with stored procedures and views, building SQL queries, or designing entity models.
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Creating a Database
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Assuming you have a version of SQL Server installed, you can create a new SQL Server database in Visual Studio s Server Explorer (Ctrl+Alt+S, or View Server Explorer) by right-clicking Data Connections and selecting the Create New SQL Server Database menu item. In the screen shown in Figure 15-2, you can configure your connection details and specify the name of the new SQL Server database.
Figure 15-2. Creating a new Microsoft SQL Server database Besides creating a new database, this also creates a Server Explorer connection to the new database, which you can use in Visual Studio to browse, create, manipulate, or delete tables, triggers, views, stored procedures, functions, and other database artifacts.
Visual Data Modeling: Adding Relationships
Various tools exist to assist application architects in designing their data layer. On a logical level, entity-relationship (ER) models provide a visual representation of data tables and their relationships, showing table fields, primary and foreign keys, and various constraints. Visual
C HA PTER 15 WO RK ING WI T H D A TA
Studio simplifies the design process and supports visual modeling, so let s take a brief look at how you can exploit its capabilities. In the previous section, you used Visual Studio to create an SQL Server database called company, and earlier you saw how to create a table to store data about employees. We now want to extend this database with a new table that stores addresses and link the existing Employees table to it to enable us to store an optional address record for each employee. This means we allow multiple employees to live at the same address, but not multiple addresses for a given employee. You can start by creating the Addresses table by right-clicking Add New Table from the Tables list item in Server Explorer Data Connections under the company database. You should add the columns shown in Figure 15-3.
Figure 15-3. The Addresses table in the designer mode Note that we designated AddID as a non-null primary key (right-click, select Set Primary Key, and clear the Allow Nulls flag). Also, under the Column Properties you should set the Is Identity property to Yes; this will take care of automatically incrementing the primary key value when inserting new records. Next, create a new database diagram and add/drag your existing two tables, Employees and Addresses, onto it. Before you can add a relationship between these two tables, you must create a new nullable field in the Employees table to store the address foreign key; you should call it AddressID. Now you are ready to link the two tables. First, right-click the Employees table, and select Relationships. Next, click Add to add a new foreign key relationship, name it FK_Employee_Address under Identity, and then click the ellipsis icon next to Tables and Columns Specification under General to configure the relationship. The foreign key table (the table that stores references to rows in another table) is Employees, and this is grayed out since you started by adding a relationship to the Employees table, but you can select the foreign key field to be AddressID. Then select the primary key table to be Addresses, with the field AddID. This will link the two tables by storing unique address IDs in the AddressID field of the Employees table records, giving you a one-to-many relationship between addresses and employees. Figure 15-4 shows our table design canvas after we are done. Similarly, linking two primary keys will yield a one-to-one relationship.
Note Linking tables via explicit relationships and controlling the nullability of the foreign key columns
gives you fine control of referential integrity. For instance, you will run into a foreign key violation if you try to remove a record that is linked to another by a foreign key constraint. On the other hand, storing raw primary key values without an explicit constraint in related tables will lose these benefits.
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