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You should ensure that the new row doesn t violate the constraints defined for the table. For example, you must provide a value for all non-nullable columns and set a unique value for the primary key and for all the keys whose Unique property is True. Here s an example that adds a row to the Employees table defined previously. (Notice that it doesn t set the primary key because you ve defined an auto-incrementing column.)
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Get a reference to the Employees table.
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Dim dtEmp As DataTable = ds.Tables( Employees )
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Create a new row with the same schema.
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Dim dr As DataRow = dtEmp.NewRow()
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Set all the columns.
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dr( FirstName ) = Joe"
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dr( LastName ) = Doe"
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dr( BirthDate ) = #1/15/1955#
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dr( HomeAddress ) = 1234 A Street"
dr( City ) = Los Angeles"
dr( DeptId ) = 1
Add to the Rows collection.
dtEmp.Rows.Add(dr)
When adding a large number of rows, you can optimize the performance of your code by using the LoadDataRow method (which takes an array of the values to be assigned) and bracketing your code in the DataTable s BeginLoadData and EndLoadData methods. (These methods temporarily disable and then reenable notifications, index maintenance, and constraints while loading data.) For example, suppose you want to import data into the Employees table from a semicolon-delimited file structured as follows:
Andrew";"Fuller";2/19/1952;"908 W. Capital Way";"Tacoma" "Janet";"Leverling";8/30/1963;"722 Moss Bay Blvd.";"Kirkland
(Notice that the third field should follow the data format in use in your system.) Here s how you can solve the problem with a concise routine that s also as efficient as possible:
Open the file, and read its contents. Dim sr As New StreamReader( employees.dat ) Dim fileText As String = sr.ReadToEnd() sr.Close() This regular expression defines a row of named elements. Dim re As New Regex( _ ""( <fname>[^""]+)"";""( <lname>[^""]+)"";( <bdate>[^;]+); _ & ""( <addr>[^""]+)"";""( <city>[^""]+)"" ) Turn off index maintenance and constraints.
dtEmp.BeginLoadData()
Repeat for each match (that is, each line in the file).
For Each ma As Match In re.Matches(fileText)
Load all fields in one operation. Dim values() As Object = {ma.Groups( fname ).Value, _ ma.Groups( lname ).Value, ma.Groups( bdate ).Value, _ ma.Groups( addr ).Value, ma.Groups( city ).Value} dtEmp.LoadDataRow(values, True)
Part V:
Database Applications
Next
Turn on index maintenance and constraints.
dtEmp.EndLoadData()
The syntax of the regular expression used to parse the file is maybe the most complex part of this code, but it s simpler than you might imagine. The purpose of this regular expression is to define the structure of each line in the data file and assign a distinct name to each group of characters delimited by semicolons and (in some cases) enclosed in double quotation marks. The meaning of the pattern becomes clearer if you get rid of the repeated double quotation marks that you see inside the string itself and split the expression according to each of the fields referenced:
( <fname>[^"]+)"; "( <lname>[^"]+)"; ( <bdate>[^;]+); "( <addr>[^"]+)"; "( <city>[^"]+)
Thanks to the groups defined in the regular expression, you can then reference each field by its name when you create the array of values. Be aware that the LoadDataRow method adds a new row only if the key field isn t already in the table. If you re passing the primary key as a value and the table already contains a record with that key, the LoadDataRow method replaces the existing row with the new values. (This is the same behavior that you have when filling the DataTable with data coming from a database via a DataAdapter.) For this reason, it s impor tant that you correctly set the DataTable s primary key to avoid duplicate keys. The BeginLoadData and EndLoadData methods are also useful for performing changes in a DataTable that would result in a temporary violation of the referential integrity rules or other constraints such as the uniqueness of a column for example, when you have to exchange the primary keys of two rows in a table.
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