Develop Reference

check if values are in datatable

I have an array or string:
private static string[] dataNames = new string[] {"value1", "value2".... };
I have table in my SQL database with a column of varchar type. I want to check which values from the array of string exists in that column.
I tried this:
public static void testProducts() {
string query = "select * from my table"
var dataTable = from row in dt.AsEnumerable()
where String.Equals(row.Field<string>("columnName"), dataNames[0], StringComparison.OrdinalIgnoreCase)
select new {
Name = row.Field<string> ("columnName")
};
foreach(var oneName in dataTable){
Console.WriteLine(oneName.Name);
}
}
that code is not the actual code, I am just trying to show you the important part
That code as you see check according to dataNames[index]
It works fine, but I have to run that code 56 times because the array has 56 elements and in each time I change the index
is there a faster way please?
the Comparison is case insensitive

First, you should not filter records in memory but in the datatabase.
But if you already have a DataTable and you need to find rows where one of it's fields is in your string[], you can use Linq-To-DataTable.
For example Enumerable.Contains:
var matchingRows = dt.AsEnumerable()
.Where(row => dataNames.Contains(row.Field<string>("columnName"), StringComparer.OrdinalIgnoreCase));
foreach(DataRow row in matchingRows)
Console.WriteLine(row.Field<string>("columnName"));
Here is a more efficient (but less readable) approach using Enumerable.Join:
var matchingRows = dt.AsEnumerable().Join(dataNames,
row => row.Field<string>("columnName"),
name => name,
(row, name) => row,
StringComparer.OrdinalIgnoreCase);

try to use contains should return all value that you need
var data = from row in dt.AsEnumerable()
where dataNames.Contains(row.Field<string>("columnName"))
select new
{
Name = row.Field<string>("columnName")
};

Passing a list of values is surprisingly difficult. Passing a table-valued parameter requires creating a T-SQL data type on the server. You can pass an XML document containing the parameters and decode that using SQL Server's convoluted XML syntax.
Below is a relatively simple alternative that works for up to a thousand values. The goal is to to build an in query:
select col1 from YourTable where col1 in ('val1', 'val2', ...)
In C#, you should probably use parameters:
select col1 from YourTable where col1 in (#par1, #par2, ...)
Which you can pass like:
var com = yourConnection.CreateCommand();
com.CommandText = #"select col1 from YourTable where col1 in (";
for (var i=0; i< dataNames.Length; i++)
{
var parName = string.Format("par{0}", i+1);
com.Parameters.AddWithValue(parName, dataNames[i]);
com.CommandText += parName;
if (i+1 != dataNames.Length)
com.CommandText += ", ";
}
com.CommandText += ");";
var existingValues = new List<string>();
using (var reader = com.ExecuteReader())
{
while (read.Read())
existingValues.Add(read["col1"]);
}
Given the complexity of this solution I'd go for Max' or Tim's answer. You could consider this answer if the table is very large and you can't copy it into memory.

Sorry I don't have a lot of relevant code here, but I did a similar thing quite some time ago, so I will try to explain.
Essentially I had a long list of item IDs that I needed to return to the client, which then told the server which ones it wanted loaded at any particular time. The original query passed the values as a comma separated set of strings (they were actually GUIDs). Problem was that once the number of entries hit 100, there was a noticeable lag to the user, once it got to 1000 possible entries, the query took a minute and a half, and when we went to 10,000, lets just say you could boil the kettle and drink your tea/coffee before it came back.
The answer was to stick the values to check directly into a temporary table, where one row of the table represented one value to check against. The temporary table was keyed against the user who performed the search, so this meant other users searches wouldn't become corrupted with each other, and when the user logged out, then we knew which values in the search table could be removed.
Depending on where this data comes from will depend on the best way for you to load the reference table. But once it is there, then your new query will look something like:-
SELECT Count(t.*), rt.dataName
FROM table t
RIGHT JOIN referenceTable rt ON tr.dataName = t.columnName
WHERE rt.userRef = #UserIdValue
GROUP BY tr.dataName
The RIGHT JOIN here should give you a value for each of your reference table values, including 0 if the value did not appear in your table. If you don't care which one don't appear, then changing it to an INNER JOIN will eliminate the zeros.
The WHERE clause is to ensure that your search only returns the unique items that you are looking for at the moment - the design should consider that concurrent access will someday occur here (even if it doesn't at the moment), so writing something in to protect it is advisable.

Querying data from SQL

I need to query a table from database which has 400 rows and 24 columns. I need to query this table so that on each row and then on each column of row I can perform some C# code ( I can use column information to execute some code).
Now at the moment I am querying each row again and again from table using select statement and storing to a custom list and performing custom operations on it.
Is it best and fastest way of doing it ? or should I just query the whole table one time and store somewhere ? not sure where in a dataset and then run throw custom code to do some operation using information in each row ?

You can fetch the table from database once and store it in datatable and then just use linq to select the column something like this
var data = dt.AsEnumerable().Select(s => s.Field<string>("myColumnName")).ToArray<string>();
and If you don't want to use other columns anywhere in your code then you should select only useful column from the database.
You can also select multiple columns of a database using linq. The values will be stored in anonymous type of object.
var mutipleData = from row
in dt.AsEnumerable()
select new
{ Value1 = row["Column1"].ToString(),
Value2 = row["Column2"].ToString()
};

Assuming that each field is 1000 bytes, the total memory to hold your 400 rows would be 9.6MB. Peanuts! Just read the whole table in a DataTable and process it as you wish.

Select all the records from DB table which are f your concern
Copy the select records into DataTable.
Pseudo Code:
--dt is datatable
foreach(datarow dr in dt.rows)
{
--perform operation
string str=dr["columnname"].tostring
}

400 rows isn't a massive amount, however it depends on the data in each column and how often you are likely to run the query. If all you are going to do is run the query and manipulate the output, use a DataReader instead.

If it's only 400 records, I'd fetch them all at once, store them in a class and iterate over each instance.
Something like:
Class
public class MyTableClass(){
string property1 {get; set;}
string property2 {get; set;}
int property3 {get; set;}
// etc.
}
Logic:
ICollection<MyTableClass> lstMyTableClass = (
from m in db.mytableclass
select m
).ToList();
return lstMyTableClass;
And then a loop:
foreach(var myTableInstance in lstMyTableClass){
myTableInstance.DoMyStuff();
}

If the number of records will always be below thoussand, just query all the records and keep it in a List.
Once the data is in the List you can query the List n number of times using LINQ without hitting the database for each request.

What am I doing wrong with this linq query (querying datatable)

I am connecting my Windows Form app to their Access DB (ugh, I know), and cannot get my linq query to return anything.
var matchDateField = from myRow in boilerDT.AsEnumerable()
where myRow.Field<DateTime>("EntryDate").ToShortDateString() == dateTimePicker1.Value.ToShortDateString()
select myRow;
Any suggestions?

Get all the rows:
IEnumerable<DataRow> dateFieldQuery =
from myRow in boilerDT.AsEnumerable()
select myRow;
Filter by date:
IEnumerable<DataRow> matchDateField =
dateFieldQuery.Where(p => p.Field<DateTime>("EntryDate").Date == dateTimePicker1.Value);
So here you're using deferred execution which enables multiple queries to be combined or a query to be extended. When a query is extended, it is modified to include the new operations, and the eventual execution will reflect the changes.
The first query returns all the rows and the second query extends the first by using Where to return all the rows with specific date.

Querying inside a Dataset C#

I have an ADO.NET dataset which is set by a certain query,
say
SELECT ID,USER,PRODUCT,COUNT FROM PRODUCTION
Without using a where clause I need to derive some results from the dataset. Say I want to get the User and Product count of the user who has the maximum product count. (And I want to do it by using the existing dataset. I can't derive this from dataset.)
Any idea of a way to query inside the dataset? Since there are Datatables my thought was there is some way to query it.

Traditional SQL queries cannot be applied to the DataSet. The following is possible, however:
Filter rows using DataTable.Select. See here for detailed information about expressions in DataTables.
Calculate totals etc. using DataTable.Compute.
If these two don't do the trick, there's always LINQ.
Quick-and-dirty LINQ example: (which doesn't return a DataTable, but a list containing an anonymous type):
var joinedResult = dataTable1
// filtering:
.Select("MyColumn = 'value'")
// joining tables:
.Join(
dataTable2.AsEnumerable(),
row => row.Field<long>("PrimaryKeyField"),
row => row.Field<long?>("ForeignKeyField"),
// selecting a custom result:
(row1, row2) => new { AnotherColumn = row1.Field<string>("AnotherColumn") });
AsEnumerable converts a DataTable into an IEnumerable on which LINQ queries can be performed. If you are new to LINQ, check out this introduction.

Yes, you can use DataTable.Select method.
DataTable table = DataSet1.Tables["Orders"];
// Presuming the DataTable has a column named Date.
string expression;
expression = "Date > #1/1/00#";
DataRow[] foundRows;
// Use the Select method to find all rows matching the filter.
foundRows = table.Select(expression);
// Print column 0 of each returned row.
for(int i = 0; i < foundRows.Length; i ++)
{
Console.WriteLine(foundRows[i][0]);
}
Also see this link.

You can do cross-table queries of a Dataset object using LINQ to DataSet:
msdn.microsoft.com/en-us/library/bb386969.aspx

C# DataTable LINQ & GROUP BY

I have a DataTable with 20 columns (I only need 3 of them.) I need to perform the following query on it and then save the results as an array. I've done some searching, but I can't figure out how to perform the mathematical operation. I know LINQ should be used, but I'm not getting anywhere. Any help is greatly appreciated!
SELECT DISTINCT columnZ, (columnX + columnY) / 2 FROM DataTable
*EDIT - corrected SQL statement

Answering your last comment (I suggest you update the question):
var result =
(from row in dataTable.AsEnumerable()
let average = ((double)row["columnX"] + (double)row["columnY"])/2
select new
{
ColumnZ = (string)row["columnZ"],
Average = average
}).Distinct();
Use your actual data types.