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Finally, the Convert function takes the value to convert and a string that corresponds to the full name of the target type, as in:
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$ + Convert(Quantity, System.String )
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An expression can also refer to a field in another table if a relationship exists between the current table and the other table. When working with relationships, you can use two different syntaxes, depending on whether the other table is the parent table or the child table in the relationship. For example, if there is a relationship named DeptEmp between the Departments and the Employees tables in the DeptId column that they have in common, you can add a calculated column to the Employees table that returns the name of the department, as follows:
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Extend the Employees table with a calculated column that
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returns the name of the department for that employee.
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dtEmp.Columns.Add( Department", GetType(String), Parent(DeptEmp).Name) )
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The following example uses the DeptBoss relationship to extend the Departments table with a calculated field equal to the name of the department s boss:
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dtDept.Columns.Add( BossName", GetType(String), _ Parent(DeptBoss).CompleteName )
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If the current table is the parent table of the relationship and you want to reference a field in the child table, you use Child(relname).fieldname syntax. However, because most relationships are of the one-to-many kind when seen from the perspective of the parent table, in most cases what you really want is to evaluate an aggregate function on the matching rows in the child table. The expression engine supports all the usual aggregation functions, including Count, Sum, Min, Max, Avg (average), StDev (standard deviation), and Var (variance). For example, you can leverage the DeptEmp relationship to extend the Departments table with a calculated column that returns the count of employees in each department:
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Add a calculated column to the Departments table that returns the number of employees for each department. dtDept.Columns.Add( EmployeesCount", GetType(Integer), _ Count(Child(DeptEmp).EmpID) )
Assuming that the Employees table has a Salary column, you can add other calculated fields in the Departments table that evaluate to the minimum, maximum, and average salary for the employees in each department:
dtDept.Columns.Add( MinSalary", GetType(Double), _ Min(Child(DeptEmp).Salary) ) dtDept.Columns.Add( MaxSalary", GetType(Double), _ Max(Child(DeptEmp).Salary) ) dtDept.Columns.Add( AvgSalary", GetType(Double), _ Avg(Child(DeptEmp).Salary) )
Part V:
Database Applications
You often use aggregate functions together with the DataTable s Compute method, which offers a simple method to extract data from all or a subset of the rows in a DataTable object. This method takes an expression and a filter that specifies the rows that are used to compute the expression:
Evaluate the average salary of all employees.
Debug.WriteLine(dtEmp.Compute( Avg(Salary)", Nothing))
Evaluate the average salary for employees in a given department.
Debug.WriteLine(dtEmp.Compute( Avg(Salary)", DeptId = 2 ))
Enforcing Constraints
The DataTable object supports the creation of constraints, where a constraint is a con dition that must be met when you add or modify a row in the table. Two different types of constraints are supported: unique constraints and foreign-key constraints. A unique constraint mandates that all the values in a column or a collection of columns must be unique in other words, you can t have two rows that contain the same value for the column or combination of columns specified in the constraint. In the majority of cases, the constraint affects only one column. This is the case with the EmpId field in the Employees table. You can enforce this type of constraint by simply setting the column s Unique property to True when you create the column:
The DeptId field must be unique.
Dim dcDeptId As DataColumn = dtDept.Columns.Add( DeptId", GetType(Integer))
dcDeptId.Unique = True
In more complex cases, you have multiple columns whose combination must be unique. For example, let s say that the combination of FirstName and LastName col umns must be unique. (In other words, you can t have two people with the same name.) To enforce this type of constraint, you must create a UniqueConstraint object, pass an array of columns to its constructor, and add the constraint to the table s Con straints collection:
Prepare the array of involved DataColumn objects. Dim cols() As DataColumn = { dtEmp.Columns( LastName ), _ dtEmp.Columns( FirstName ) } Create the UniqueConstraint object, assigning it a name. Dim uc As New UniqueConstraint( UniqueName", cols) Add it to the table s Constraints collection. dtEmp.Constraints.Add(uc)
Or you can do everything with a single (but less readable) statement:
dtEmp.Constraints.Add(New UniqueConstraint(New DataColumn() _ {dtEmp.Columns( LastName ), dtEmp.Columns( FirstName )}))
22:
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