Executing Multiple SQL Statements

Executing Multiple SQL Statements
Typically, your C# program and the database will run on different computers and
communicate over a network. Each time you execute a command in your program, it has
to travel over the network to the database and be executed by the database, and any
results must be sent back across the network to your program. That's a lot of network
traffic! One way to potentially reduce network traffic is to execute multiple SQL
statements at a time.
In this section, you'll see how to execute multiple SELECT statements and retrieve
results, and you'll see how to execute multiple SELECT, INSERT, UPDATE, and
DELETE statements that are interleaved.
Executing Multiple SELECT Statements
Let's take a look at how you execute multiple SELECT statements and retrieve the
results. The following code first creates a SqlCommand object named mySqlCommand
and sets its CommandText property to three different SELECT statements:
SqlCommand mySqlCommand = mySqlConnection.CreateCommand();
mySqlCommand.CommandText =
"SELECT TOP 5 ProductID, ProductName " +
"FROM Products " +
"ORDER BY ProductID;" +
"SELECT TOP 3 CustomerID, CompanyName " +
"FROM Customers " +
"ORDER BY CustomerID;" +
"SELECT TOP 6 OrderID, CustomerID " +
"FROM Orders " +
"ORDER BY OrderID;";

Notice that all of the SELECT statements are separated by semi-colons.

Using Table Joins
Be careful to retrieve only the rows and columns you actually need. Also, make sure you

use SELECT statements that retrieve rows from multiple tables. For example, if you want
to see all the orders placed by the customer with the CustomerID of ALFKI, don't
perform two separate SELECT statements against the Customers and Orders tables.
Instead, use a table join, as shown in the following SELECT statement:
SELECT Customers.CustomerID, CompanyName, OrderID
FROM Customers, Orders
WHERE Customers.CustomerID = Orders.CustomerID
AND Customers.CustomerID = 'ALFKI';

To run earlier SQL statements, you call the ExecuteReader() method, which returns a
SqlDataReader object:
SqlDataReader mySqlDataReader = mySqlCommand.ExecuteReader();
The command will return three result sets, one for each SELECT statement. To read the
first result set, you call the Read() method of mySqlDataReader. The Read() method
returns false when there are no more rows to read. Once you're at the end of a result set,
you call the NextResult() method of mySqlDataReader before reading the next result set.
The NextResult() method advances mySqlDataReader onto the next result set and returns
false when there are no more result sets.
The following code illustrates the use of the Read() and NextResult() methods to read the
three result sets from the SELECT statements:
do
{
while (mySqlDataReader.Read())
{
Console.WriteLine("mySqlDataReader[0] = " +
mySqlDataReader[0]);
Console.WriteLine("mySqlDataReader[1] = " +
mySqlDataReader[1]);
}
} while (mySqlDataReader.NextResult());

Notice the use of the outer do…while loop, which tests the return value from
mySqlDataReader.NextResult() at the end. Because a do…while loop checks the
condition at the end of the loop, this means that the statements in the do…while loop
execute at least once. You want to put the call to NextResult() at the end because it first
attempts to advance mySqlDataReader to the next result set and then returns the Boolean
result that indicates whether there is another result set to move to. If you put the call to
NextResult() in a regular while loop, then mySqlDataReader would skip over the first
result set, and you don't want to do that!
Listing 9.4
illustrates how to execute multiple SELECT statements and read the results.
Listing 9.4: EXECUTEMULTIPLESELECTS.CS

/*
ExecuteMultipleSelects.cs illustrates how to execute
multiple SELECT statements using a SqlCommand object
and read the results using a SqlDataReader object
*/

using System;
using System.Data;
using System.Data.SqlClient;

class ExecuteSelect
{
public static void Main()
{
SqlConnection mySqlConnection =
new SqlConnection(
"server=localhost;database=Northwind;uid=sa;pwd=sa"
);

SqlCommand mySqlCommand = mySqlConnection.CreateCommand();

// set the CommandText property of the SqlCommand object to
// the mutliple SELECT statements
mySqlCommand.CommandText =
"SELECT TOP 5 ProductID, ProductName " +
"FROM Products " +
"ORDER BY ProductID;" +
"SELECT TOP 3 CustomerID, CompanyName " +
"FROM Customers " +
"ORDER BY CustomerID;" +
"SELECT TOP 6 OrderID, CustomerID " +
"FROM Orders " +
"ORDER BY OrderID;";

mySqlConnection.Open();

SqlDataReader mySqlDataReader = mySqlCommand.ExecuteReader();

// read the result sets from the SqlDataReader object using
// the Read() and NextResult() methods
do
{
while (mySqlDataReader.Read())
{
Console.WriteLine("mySqlDataReader[0] = " +
mySqlDataReader[0]);
Console.WriteLine("mySqlDataReader[1] = " +
mySqlDataReader[1]);

}
Console.WriteLine(""); // visually split the results
} while (mySqlDataReader.NextResult());

mySqlDataReader.Close();
mySqlConnection.Close();
}
}

The output from this program is as follows:
mySqlDataReader[0] = 1
mySqlDataReader[1] = Chai
mySqlDataReader[0] = 2
mySqlDataReader[1] = Chang
mySqlDataReader[0] = 3
mySqlDataReader[1] = Aniseed Syrup
mySqlDataReader[0] = 4
mySqlDataReader[1] = Chef Anton's Cajun Seasoning
mySqlDataReader[0] = 5
mySqlDataReader[1] = Chef Anton's Gumbo Mix

mySqlDataReader[0] = ALFKI
mySqlDataReader[1] = Alfreds Futterkiste
mySqlDataReader[0] = ANATR
mySqlDataReader[1] = Ana Trujillo3 Emparedados y helados
mySqlDataReader[0] = ANTON
mySqlDataReader[1] = Antonio Moreno Taquería

mySqlDataReader[0] = 10248
mySqlDataReader[1] = VINET

mySqlDataReader[0] = 10249
mySqlDataReader[1] = TOMSP
mySqlDataReader[0] = 10250
mySqlDataReader[1] = HANAR
mySqlDataReader[0] = 10251
mySqlDataReader[1] = VICTE
mySqlDataReader[0] = 10252
mySqlDataReader[1] = SUPRD
mySqlDataReader[0] = 10253
mySqlDataReader[1] = HANAR

Executing Multiple SELECT, INSERT, UPDATE, and DELETE Statements
You can interleave multiple SELECT, INSERT, UPDATE, and DELETE statements.
This can save network traffic because you're sending multiple SQL statements to the
database in one go. The following code first creates a SqlCommand object named
mySqlCommand and sets its CommandText property to multiple interleaved SQL
statements:
mySqlCommand.CommandText =
"INSERT INTO Customers (CustomerID, CompanyName) "+
"VALUES ('J5COM', 'Jason 5 Company');" +
"SELECT CustomerID, CompanyName " +
"FROM Customers " +
"WHERE CustomerID = 'J5COM';" +
"UPDATE Customers " +
"SET CompanyName = 'Another Jason Company' " +
"WHERE CustomerID = 'J5COM';" +
"SELECT CustomerID, CompanyName " +
"FROM Customers " +
"WHERE CustomerID = 'J5COM';" +
"DELETE FROM Customers " +

"WHERE CustomerID = 'J5COM';";
The SQL statements are as follows:
•
The INSERT statement adds a new row to the Customers table.
•
The first SELECT statement retrieves the new row.
•
The UPDATE statement modifies the CompanyName column of the row.
•
The second SELECT statement retrieves the row again.
•
The DELETE statement removes the row.
You can use the same do…while loop as shown in the previous section
to retrieve the two
result sets returned by the SELECT statements. The same loop works even though the
example executes inter-leaved non-SELECT statements. It works because only the
SELECT statements return result sets and the NextResult() method returns true only for
the SELECT statements; it returns false for the other SQL statements. Therefore,