We Keep Coding

C# updating entries and passing columns in an SQL database

I need to make my code for where the data is inserted into 'Class_1' there are actually 3 score columns (Score_1, Score_2, Score_3) as the application I am building requires the database to keep a record of the last three entered scores. The problem with the code is that every time the data is added it will create a new entry and so what I need the program to do is check to see if the First_Name and Last_Name are present in another entry and if so, update that entry, then what I would like to do is check to see if score_1 has a value present and of course if this is true, skip score_1 and input the data into score_2 but I am completely new to C# so any help here would be much appreciated!, Thankyou!
if(inpClassNumber.Text == "Class 1")
{
con = new SqlConnection(#"Data Source = (LocalDB)\MSSQLLocalDB; AttachDbFilename = C:\Program Files (x86)\Flux Infusion\Projects\Primary School Quiz System\Primary School Quiz System\Scores.mdf; Integrated Security = True");
con.Open();
cmd = new SqlCommand("INSERT INTO Class_1 (First_Name,Last_Name,Score_1) VALUES (#First_Name,#Last_Name,#Score)", con);
cmd.Parameters.AddWithValue("#First_Name", txtFirstName.Text);
cmd.Parameters.AddWithValue("#Last_Name", txtLastName.Text);
cmd.Parameters.AddWithValue("#Score", lblScore.Text);
cmd.ExecuteNonQuery();
}

If you are running SQL Server 2008 or newer you can use a MERGE TSQL:
MERGE Class_1 AS target
USING (SELECT #First_Name, #Last_Name, #Score) AS source (fName, lName, score)
ON (target.First_Name = source.fName and target.Last_Name = source.lName)
WHEN MATCHED THEN
UPDATE SET
Score_1 = #Score,
Score_2 = Score_1,
Score_3 = Score_2
WHEN NOT MATCHED THEN
INSERT (First_Name, Last_Name, Score_1)
VALUES (#First_Name, #Last_Name, #Score);
.
The above code keeps the most recent score in column Score_1, the second most recent in Score_2, and so on.
Here is the link to the TSQL documentation.

Determining if user login already exists in database?

I am building a web application in asp.net using C#. I have the Form where the user should register and then can login. I am having a problem in making the web app know that the name which the user picks is either "already exists" or not. If it already exists it should not insert the same name and display a message saying "user name already exists". I have tried the SqlDataReader but no luck.
protected void Register_Button_Click(object sender, EventArgs e)
{
SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["BJ_Player_String"].ToString());
SqlCommand cmd = new SqlCommand();
SqlCommand cmd2 = new SqlCommand();
SqlDataReader data_reader;
String name = TextBox2.Text;
String date = TextBox3.Text;
try
{
conn.Open();
cmd = new SqlCommand("Insert into BJ_Player (Player_Name, D_O_B) Values (#Player_name, #D_O_B)", conn);
cmd = new SqlCommand("Select Player_Name from BJ_Player WHERE Player_Name = #Player_name", conn);
cmd.Parameters.Add("#Player_name", SqlDbType.NVarChar).Value = name;
cmd.Parameters.Add("#D_O_B", SqlDbType.Date).Value = date;
cmd.Connection = conn;
data_reader = cmd.ExecuteReader();
cmd.ExecuteNonQuery();
if (data_reader.HasRows)
{
lblPlayerNameExists.Visible = true;
}
else
{
// do nothing
}
}

Make Player_Name unique in database then it will give you exception when you try to insert. You have to use unique constraint.
You have to give command type also and check you assigned both queries to same cmd object

in your code you're inserting data in your DB and then you are examining that the name is the same or not.
first you should search the name in your DB and then if there isn't any record with that name ,you should add your record.

I usually do it in one of two ways:
Create stored procedure that will check for name uniqueness and insert new record if everything is ok. It should return status as numeric code that you will check.
Check for name uniqueness before saving it using as a part of validation process.

Using the merge statement may help with this. Merge performs insert, update, or delete operations on a target table based on the results of a join with a source table.
Basically it inserts when needed, and updates when needed. Often times referred to as an upsert. but it gets the job done.
Here is a link to a site explaining how to use merge. Looks like a good article.
http://www.kodyaz.com/articles/sql-server-2008-t-sql-merge-statement-example.aspx

If you would like to write a model function to do that then
Leave it for ajax check which is pretty similar to the second
method
Issue "Select username from DB-table" to retrieve
usernames then check the username input against them before
displaying a view to report a problem if any or showing a message to
tell the user that "this name is valid", for example.

How to get UUID generated by MySQL into a C# variable

Here is the query:
string query = #"INSERT INTO session (PK_Id, user_id, login_time, machine_ip, machine_fingerprint)
VALUES (UUID(), #UId, #LogInTime, #MIp, #MFingerPrint);
";
Now I need this last inserted id back, which is a UUID generated by MySQL. As far as I read there is no select_last_insert_id() function for UUIDs!! And I read for php you could assign UUID() function first to a variable and then return that value. But how to go about that in C#?
Something like this, but not exactly:
string query = #"INSERT INTO session (PK_Id, user_id, login_time, machine_ip, machine_fingerprint)
VALUES (#UUID = SELECT UUID(), #UId, #LogInTime, #MIp, #MFingerPrint);
"; //how to do this here?
Here is more of my code:
string query = #"INSERT INTO session (PK_Id, user_id, login_time, machine_ip, machine_fingerprint)
VALUES (#UUID = SELECT UUID(), #UId, #LogInTime, #MIp, #MFingerPrint);
";
try
{
if (_conn.State != ConnectionState.Open)
_conn.Open();
MySqlCommand cmd = new MySqlCommand(query, _conn);
cmd.Parameters.AddWithValue("#UId", Utility.usr.Id);
cmd.Parameters.AddWithValue("#LogInTime", DateTime.Now);
cmd.Parameters.AddWithValue("#MIp", GetMachineIP());
cmd.Parameters.AddWithValue("#MFingerPrint", GetHardwareFingerPrint());
var s= Convert.ToString(cmd.ExecuteScalar()); //this returns an empty string :(
//I need to get it to any .NET data type, string, or Guid or byte[] or anything.
But I need this datatype of s to be used in another WHERE clause in a query like this:
string query = #"UPDATE session SET logout_time = #LogOutTime
WHERE user_id = #UId AND PK_Id = #SessionId";
try
{
if (_conn.State != ConnectionState.Open)
_conn.Open();
MySqlCommand cmd = new MySqlCommand(query, _conn);
cmd.Parameters.AddWithValue("#UId", Utility.usr.Id);
cmd.Parameters.AddWithValue("#SessionId", s);
cmd.Parameters.AddWithValue("#LogOutTime", DateTime.Now);
cmd.ExecuteScalar();
Here #"SessionId" is the UUID field in the same table. So basically, how can I get the MySQL varbinary field in C# so that I could use that type to update by specifying WHERE in another query?
In MySQL table the UUID field is varbinary (I hope to see some solution that is not another php link or that is not asking me to switch to char datatype in the database :) ).
Edit: The problem here is we have already added plenty of UUIDs generated by MySQL into the table, so I'm a bit apprehensive about changing MySQL UUID to .NET Guid. If that's the only workaround, I'll consider that. Just that this is the first time we needed the inserted UUID value back so that I can update in another query another point of time.
A sub question: Is .NET Guid exactly the same thing as MySQL UUID?

You can use the Guid type which is the MS implementation of UUID. You should be aware that when inserting data into the DB, you may need to convert the Guid to ByteArray if the MySQL driver isn't familiar with handling Guid's. See Store GUID in MySQL from C# for an example of this.

I think you can go ahead with your earlier implementation without having to rely on MS Guid, but I fear I am too late :)
string query = #"INSERT INTO session (PK_Id, user_id, login_time, machine_ip, machine_fingerprint)
VALUES (UUID(), #UId, #LogInTime, #MIp, #MFingerPrint);
SELECT PK_Id FROM session WHERE login_time=#LogInTime AND machine_fingerprint=#MFingerPrint; //or something similar which gives you the exact same id - UUID
";
try
{
if (_conn.State != ConnectionState.Open)
_conn.Open();
MySqlCommand cmd = new MySqlCommand(query, _conn);
cmd.Parameters.AddWithValue("#UId", Utility.usr.Id);
cmd.Parameters.AddWithValue("#LogInTime", DateTime.Now);
cmd.Parameters.AddWithValue("#MIp", GetMachineIP());
cmd.Parameters.AddWithValue("#MFingerPrint", GetHardwareFingerPrint());
MySqlDataReader r = cmd.ExecuteReader();
if (r.Read()) //ensure if it is read only once, else modify your `WHERE` clause accordingly
{
var s = (Guid)r[0];
}
//or even (Guid)cmd.ExecuteScalar() would work
Now you can query in update like this:
string query = #"UPDATE session SET logout_time = #LogOutTime
WHERE user_id = #UId AND PK_Id = #SessionId";
try
{
if (_conn.State != ConnectionState.Open)
_conn.Open();
MySqlCommand cmd = new MySqlCommand(query, _conn);
cmd.Parameters.AddWithValue("#UId", Utility.usr.Id);
cmd.Parameters.AddWithValue("#SessionId", s.ToByteArray());
cmd.Parameters.AddWithValue("#LogOutTime", DateTime.Now);
cmd.ExecuteNonQuery();
Note: Here I have converted the Guid variable s to byte array before querying. This is important, in WHERE clause, be it UPDATE or SELECT statements in query. I would ask you to move to binary field in MySQL table from varbinary.
Edit: If your table would grow dramatically large then inserting and selecting is a bad idea since SELECT query is an additional query being run. In that case #PinnyM's choice is better. I really do not think MySQL or any other database would have a default way to give back "custom" inserted ids which are not something database generated. So in short I advice you to not go for this..
Edit2: See this answer for getting binary value to .NET datatype. Sometimes casting do not work depending on MySQL .NET connector version..

How to save a SQL "Select" result in a variable in C#

I'm using Visual C# connected to MySQL for study purposes and I'm stuck in throwing an error to the user when he types a username that already exists.
Current code to put things into the database (it may be useless, once my question may be much more about SQL):
s = new sql(); // This calls a class that works as an adapter to connect form with the database
Conn = s.Connection;
Conn.Open();
coma = Conn.CreateCommand();
coma.CommandText = "INSERT INTO test.test (`user`,`password`) VALUES ('"+username.Text+"','"+password.Text+"');";
coma.ExecuteNonQuery();
What I want to do it compare "username.Text" ("username" is a TextBox) with the values on database's "test" table and, if some value match, evoke a MessageBox.Show("Hey guy, this username is already in use! Try something different)

Some points about your code sample
You want to be sure that you dispose of your connection and command objects. For my answer, I've wrapped them in using statements which will take care of that for me.
You do not want to go to the database with unsanitized inputs. I am going to use parameterized queries in the example.
It's not a good idea to store passwords in plain text. I am not going to demonstrate more secure techniques, just know to look for information about encrypting passwords, salt keys, etc.
And now for some code. In this, I'm using OleDb objects, retrofit to your particular database. And, of course, provide appropriate names to tables, columns, etc.
using (OleDbConnection connection = SomeMethodReturningConnection())
using (OleDbCommand command = SomeMethodReturningCommand())
{
command.Parameters.Add(new OleDbParameter("#username", username));
command.CommandText = "Select Count(*) From Users where Username = #username";
connection.Open();
int output = (int)command.ExecuteScalar();
if (output > 0)
{
// username already exists, provide appropriate action
}
else
{
// perform insert
// note: #username parameter already exists, do not need to add again
command.Parameters.Add(new OleDbParameter("#password", password));
command.CommandText = "Insert Into Users (Username, Password) Values (#username, #password)";
command.ExecuteNonQuery();
}
}

Thank you Anthony! Your answer put me on the right track. Although there is something that the people who will read this post should change from your code in order to get it working with Odbc connectors: the way as parameters are parsed and the way as the textbox content is extracted:
using (OdbcConnection connection = SomeMethodReturningConnection())
using (OdbcCommand command = SomeMethodReturningCommand())
{
command.Parameters.Add(new OdbcParameter("#username", username.Text));
command.CommandText = "Select Count(*) From Users where Username = ?";
connection.Open();
int output = (int)command.ExecuteScalar();
if (output > 0)
{
// username already exists, provide appropriate action
}
else
{
// perform insert
// note: #username parameter already exists, do not need to add again
command.Parameters.Add(new OdbcParameter("#password", password.Text));
command.CommandText = "Insert Into Users (Username, Password) Values (?,?)**";
command.ExecuteNonQuery();
}
}
Thank you anyway!

How to update a large table using ADO.NET

Ok, so here's the problem I have to solve. I need to write a method in C# that will modify a table in SQL Server 2008. The table could potentially contain millions of records. The modifications include altering the table by adding a new column and then calculating and setting the value of the new field for every row in the table.
Adding the column is not a problem. It's setting the values efficiently that is the issue. I don't want to read in the whole table into a DataTable and then update and commit for obvious reasons. I'm thinking that I would like to use a cursor to iterate over the rows in the table and update them one by one. I haven't done a whole lot of ADO.NET development, but it is my understanding that only read-only server side (firehose) cursors are supported.
So what is the correct way to go about doing something like this (preferably with some sample code in C#)? Stored procedures or other such modifications to the DB are not allowed.

jpgoody,
Here is an example to chew on using the NerdDinner database and some SQLConnection, SQLCommand, and SQLDataReader objects. It adds one day to each of the Event Dates in the Dinners table.
using System;
using System.Data.SqlClient;
namespace NerdDinner
{
public class Class1
{
public void Execute()
{
SqlConnection readerConnection = new SqlConnection(Properties.Settings.Default.ConnectionString);
readerConnection.Open();
SqlCommand cmd = new SqlCommand("SELECT DinnerID, EventDate FROM Dinners", readerConnection);
SqlDataReader reader = cmd.ExecuteReader();
SqlConnection writerConnection = new SqlConnection(Properties.Settings.Default.ConnectionString);
writerConnection.Open();
SqlCommand writerCommand = new SqlCommand("", writerConnection);
while (reader.Read())
{
int DinnerID = reader.GetInt32(0);
DateTime EventDate = reader.GetDateTime(1);
writerCommand.CommandText = "UPDATE Dinners SET EventDate = '" + EventDate.AddDays(1).ToString() + "' WHERE DinnerID = " + DinnerID.ToString();
writerCommand.ExecuteNonQuery();
}
}
}
}

Your problem looks like something that you should be solving using T-SQL and not C#, unless there is some business rule that you are picking up dynamically and calculating the column values T-SQL should be the way to go. Just write a stored procedure or just open up Management studio and write the code to make your changes.
If this does not help then please elaborate on what exactly you want to do to the table, then we can help you figure out if this can be done via T-SQL or not.
[EDIT] you can do something like this
string sql = " USE " + paramDbName;
sql+= " ALTER TABLE XYZ ADD COLUMN " + param1 + " datatype etc, then put semicolon to separate the commands as well"
sql+= " UPDATE XYZ SET Columnx = " + some logic here
cmd.CommandText = sql;
cmd.ExecuteNonQuery();
get this executed on the required instance of Sql Server 2008.
If you have too many lines of text then use StringBuilder.

Here's a suggestion:
You can read data using a DataReader , create a update command for current row and add it to a list of commands.Then run update commands in a transaction.
something like this:
var commands=new List<SqlCommand>();
while(dr.Read())
{
var cmd=new SqlCommand();
cmd.CommandText="Add your command text here";
commands.Add(cmd);
}
using(var cnn=new SqlConnection("Connection String"))
{
IDbTransaction transaction;
try
{
cnn.Open();
transaction=cnn.BeginTransaction();
foreach(var cmd in commands)
{
cmd.Transaction=transaction;
cmd.ExecuteNonQuery();
cmd.Dispose();
}
transaction.Commit();
}
catch(SqlException)
{
if(transaction!=null)
transaction.Rollback();
throw;
}
}