Summary: in this tutorial, you will learn how to delete data from a table in a PostgreSQL database from a C# program.

This tutorial begins where updating data in PostgreSQL using C# is left off.

How to delete data from PostgreSQL using C#

To delete a row from a table in a PostgreSQL database from a C# program, you use the following steps:

First, construct a DELETE statement that deletes one or more rows from a specified table:

var sql = "DELETE.. ";

In the DELETE statement, you can use parameters in the format @parameter. When executing the query, you need to bind values to these parameters.

Parameterized queries can prevent SQL injection attacks, especially when the data comes from untrusted sources.

Second, create a data source that represents the PostgreSQL database:

await using var dataSource = NpgsqlDataSource.Create(connectionString);

Third, create a NpgsqlCommand object and bind one or more values to the parameters:

await using var cmd = dataSource.CreateCommand(sql);

cmd.Parameters.AddWithValue("@parameter1", value1);
cmd.Parameters.AddWithValue("@parameter2", value2);
...

Finally, execute the DELETE statement by calling the ExecuteNonQueryAsync() method of the command object:

await cmd.ExecuteNonQueryAsync();

Note that the using statement will automatically close the database connection.

Deleting data from a table

The following C# program shows how to delete the row with id 1 from the students table:

using Npgsql;

// Construct a DELETE statement
var sql = @"DELETE FROM students WHERE id = @id";

// Get the connection string
string connectionString = ConfigurationHelper.GetConnectionString("DefaultConnection");
try
{

    // Create a data source
    await using var dataSource = NpgsqlDataSource.Create(connectionString);

    // Create a Command object
    await using var cmd = dataSource.CreateCommand(sql);

    // Bind parameters
    cmd.Parameters.AddWithValue("@id", 1);

    // Execute the DELETE statement
    await cmd.ExecuteNonQueryAsync();

    Console.WriteLine("The row has been deleted successfully.");

}
catch (NpgsqlException ex)
{
    Console.WriteLine($"Error: {ex.Message}");
}

How it works.

First, construct an DELETE statement that deletes a row specified by an id from the students table:

var sql = @"DELETE FROM students WHERE id = @id";

The @id is a placeholder for the parameter in the DELETE statement.

Second, get the connection string from the configuration:

string connectionString = ConfigurationHelper.GetConnectionString("DefaultConnection");

Third, create a data source that represents the PostgreSQL database:

await using var dataSource = NpgsqlDataSource.Create(connectionString);

Fourth, create a new NpgsqlCommand object and bind the values to its parameters:

await using var cmd = dataSource.CreateCommand(sql);
cmd.Parameters.AddWithValue("@id", 1);

Fifth, execute the DELETE statement by calling the ExecuteNonQueryAsync() method and display a success message:

await cmd.ExecuteNonQueryAsync();
Console.WriteLine("The row has been deleted successfully.");

Finally, display an error message if any exceptions occur when deleting the data:

// ...
catch (NpgsqlException ex)
{
    Console.WriteLine($"Error: {ex.Message}");
}

Verify the deletion

First, open a terminal and connect to the elearning database using the ed user:

psql -U ed -d elearning

It’ll prompt you to enter a password for the ed user. Input the valid password and press Enter to connect to the PostgreSQL.

Second, query the row with id 1 from the students table:

SELECT * FROM students
WHERE id = 1;

Output:

id | first_name | last_name | email | registration_date
----+------------+-----------+-------+-------------------
(0 rows)

The output indicates that the program has successfully deleted the row id 1.

Summary

  • Use the NpgsqlCommand object to execute an DELETE statement that deletes a row from a table.