PrimaryKey usage with Many Fields
When Microsoft Access Tables are designed, or Tables in any database systems for that matter, especially the master table, the first thing we consider is to maintain uniqueness of at least one field contents for easier retrieval of information. For example: Employee Codes in Employees Table in the Northwind.mdb sample database. Each employee is identified with a unique code number, but who will check for the uniqueness of the numbers entered into the employee-code field. We can validate User inputs for uniqueness with VBA programs before storing them into the table. But, this is already built into the database systems and used in the form of PrimeryKey Index. This not only prevents from entering duplicate values into this field but also arranges the data in Ascending/Descending order.
We can use this field values in programs to find information very quickly. Let us write a small VBA Routine to see how this is used in programs.
Public Function PrimaryKey_Example1(ByVal EmpCode As Integer) Dim db As Database, rst As Recordset Set db = CurrentDb Set rst = db.OpenRecordset("Employees", dbOpenTable) rst.Index = "PrimaryKey" 'activate the Index on Employee Code rst.Seek "=", EmpCode 'we are looking for the record of Employee Code provided 'Let us test whether the search for Employee Code was successfull or not If Not rst.NoMatch Then MsgBox "EC: " & rst![ID] & " - " & rst![First Name] & " " & rst![last name] Else MsgBox "EC: " & EmpCode & " Not found!" End If Set rst = Nothing Set db = Nothing End Function
The statement rst.Index = "PrimaryKey" activates the Index with the name PrimaryKey. Under PrimaryKey Index there can be more than one Field. One Table can have several Indexes in the Indexes Group with different combination of fields as well. If one field alone cannot maintain uniqueness of information then we can add other related fields under the required Index Group. We can activate the required Index depending on which way we would like to get the data organized before proceeding with processing steps.
It is not necessary that the field contents should be always numbers only instead it can be any value like FirstName or LastName fields or both or any combination of field types like text, date, number etc. except Memo field. You can give any suitable name for the Index in the Index Name column.
The Recordset’s Seek() method is used for search operations on the Table. One of the Indexes should be active before the Seek() operation can be executed.
In the above example the rst.Seek "=", Empcode statement checks for the Employee Code passed to the Index_Example1() function. The statement look for the exact match ("=" ).
When the Index is defined with more than one field then search keys must be separated with commas in the Seek() method. Let us modify the above program to provide multiple values for the Index Keys in the Seek() method, assuming that FirstName and LastName fields are the members of MyIndex.
Public Function PrimaryKey_Example2(ByVal strFirstName As String, ByVal strLastName As String) Dim db As Database, rst As Recordset Set db = CurrentDb Set rst = db.OpenRecordset("Employees", dbOpenTable) rst.Index = "MyIndex" 'activate the Index 'Search for the Employee record rst.Seek "=", strFirstName, strLastName 'Let us test whether the search for the Employee was successfull or not If Not rst.NoMatch Then MsgBox "Employee: " & rst![ID] & " - " & rst![First Name] & " " & rst![last name] Else MsgBox "Employee: " & strFirstName & " " & strLastName & " Not found!" End If Set rst = Nothing Set db = Nothing End Function
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