在DataTable中查询应该注意的问题
完成一个查询,返回一个DataTable后,很多时候都想在查询结果中继续搜索。这时可以使用DataTable.Select方法对结果进行再查询。
Select方法有4个重载,我们经常用到的就是DataTable.Select(String)
这个String的参数是查询的限定式。相当于SQL查询语言中的WHERE语句(不含WHERE),其语法符合SQL语言语法。
Select方法的返回的是包含查询到的数据的DataRow,但是这个DataRow只是被查询的DataTable的一个映射,所以DataRow是随着DataTable的行变化而变化的。例如,DataTable的行都被删除了,那么DataRow中的数据同样被删除了(即便是先Select,再删除的)
所以,要想把返回的DataRow放进数据显示控件中,需要将其放入另一个DataTable中,如果直接放入原DataTable或将原DataTable的行全部清除再放入查询所得的数据都是不行的,程序会显示错误,错误提示是“表中已有此行”。
另外,在一个新表中插入行,不能直接用DataTable.Rows.Add(DataRow)的方式,因为这样是插入一个新表,也就是DataRow是空的。要用导入行的方式,DataTable.ImportRow(DataRow)。当然,前提是这个新表要有与原数据表一样的结构。
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Public Function SDEResearch(ByVal InputDT As DataTable, ByVal SearchStr As String) As DataTable
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'用来存储再查询后的数据表
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Dim ReSearchDT As DataTable = InputDT.Clone() '保证有与源数据表相同的表结构
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'用来存储查询后返回的datarow数组
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Dim ReSearchDR() As DataRow = Nothing
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Try
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ReSearchDR = InputDT.Select("NAME LIKE '%" + SearchStr + "%'") '只是从数据表中映射出来datarow(),所以不能删除原表中的行
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Catch ex As Exception
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Return Nothing
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End Try
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For i As Int16 = 0 To ReSearchDR.Length - 1
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ReSearchDT.ImportRow(ReSearchDR(i))
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Next
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Return ReSearchDT
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End Function
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