SQL Nested Queries in C# and Connection Open Problem

Question

I have three tables and my requirement is to fetch their records by using the parent record of each table.

So I have three queries. Here is a code sample:

First Query

SQLConnection con = new SQLConnection("connectionstring");

SQLCommand cmd1 = new SQLCommand (1stQuery,con);
SQLDataReader rs1 = cmd1.DataReader();

while (rs1.Read()) {

// Making 2nd Query, 2nd Query is using 1st Query result

SQLCommand cmd2 = new SQLCommand(2ndQuery,con);
SQLDataReader rs2 = cmd2.DataReader();

while (rs2.Read()){

//Making 3rd Query using 2nd Query result
}
}

It gives the error that there is already an open DataReader. I dont want to close the SQL connection and open it for each query, as this is bad for performance.

I want to use only one SQL Connection for all the queries.

Answer 1

You don't need to close the connection. The problem (as the error suggests) is that you're not closing one DataReader before starting the next.

As Oded says, you have a "select N+1" problem, but let's disregard that for the sake of this answer.

The easiest thing to do is read the entire first dataset into memory before you start reading the next one:

var list1 = new List<Foo>();
using (var rdr1 = cmd1.ExecuteReader())
{
    list1.Add(...);
}

var list2 = new List<Bar>();
var cmd2 = ...;
foreach (var item in list1)
{
    // assign parameters to cmd2 from item

    using (var rdr2 = cmd2.ExecuteReader())
    {
        list2.Add(...);
    }
}

... and so on.

The other option is to enable MARS on your connection, which lets you have multiple DataReaders active simultaneously. This is almost always a bad idea so let's pretend I never suggested it.

Answer 2

If you are nesting calls to the database, you will need to use different connections.

You have a worse problem here with a call to the DB in a loop - also known as the N+1 problem .

I suggest rewriting your database and application logic - use a join in the database to consolidate the separate queries into one and change the logic to use the result.

Answer 3

I suggest to use ExecuteDataSet . It will contain the result of all three queries. Moveover, These queries should be executed in stored procedure.

Otherwise you are inviting Sql Injections

Sql Injections

Answer 4

Set MultipleActiveResultsets property of your connection string to true, but the performance of your code is poor. Try wrapping your queries in a stored procedure to avoid loops.