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Programatically erasing a row in SQL within a foreach loop?

I have a relational database connecting meal_ingredients and ingredient nutritional values (see here for further information), utilising PostgreSQL. Within a WinForms application, there is a button that takes values from a DataGridView and then places each row into an array.
Each row in the DataGridView is an ingredient with its nutritional values. Above the DataGridView, a textbox takes a string for the meal name. Upon clicking the button (code below), the array values do one of two things:
If the meal name (meal_name being the PK) already exists in the meal_ingredients table, all rows in the database containing this string are removed. The rows from the DataGridView are then inserted, effectively 'overwriting' the ingredients for that meal.
If the meal name does not exist in the meal_ingredients table, the rows plus the meal name, entered into the textbox, are simply appended to the table.
In my code, as you can see once the data is placed into the array, a connection is made with the database, and the results from the SELECT query loaded into a DataTable.
The loop which follows triggers a MessageBox if the meal_name field matches the string value in the textbox. This works fine.
My issue is as follows. For however many rows exist in the DataGridView, the MessageBox will fire off that many times; so with two rows, I will see two MessageBoxes, for example. This, per se, is not a problem, unless replacing this MessageBox with DELETE and INSERT statements would throw an error.
In place of MessageBox.Show("test");, I would instead place a SQL statement to remove any records where meal_name == txtMealName.Text and then a second SQL statement to insert new records based upon the DataGridView rows. Of course, if the MessageBox fires off according to the number of rows, I expect the SQL would also occur that many times. Again, this is fine in principle. But I am just wondering if this would cause a conflict of any kind (that is, for example, the SQL throwing an exception because there are no remaining rows to delete)?
private void btnMealAdd_Click(object sender, EventArgs e)
{
if (txtMealName.Text != "")
{
foreach (DataGridViewRow row in dgvMealIngredient.Rows)
{
List<string> macroList = new List<string>();
macroList.Add(row.Cells[0].Value.ToString());
macroList.Add(row.Cells[9].Value.ToString());
macroList.Add(txtMealName.Text);
String[] str = macroList.ToArray();
NpgsqlConnection conn = new NpgsqlConnection(Globals.connectionString());
conn.Open();
NpgsqlCommand comm = new NpgsqlCommand();
comm.Connection = conn;
comm.CommandType = CommandType.Text;
comm.CommandText = "SELECT * FROM meal_ingredients";
NpgsqlDataReader dr = comm.ExecuteReader();
DataTable dt = new DataTable();
dt.Load(dr);
foreach (DataRow dataRow in dt.Rows)
{
if (dataRow[0].ToString() == txtMealName.Text)
{
MessageBox.Show("test");
}
Debug.WriteLine(dataRow[0]);
}
}
}
else
{
MessageBox.Show("error: enter a meal name");
}
}
A simplified form of the database relation (note that qty is one of the fields in the DataGridView):

just whether replacing that with INSERT or DELETE will throw an error
An SQL DELETE can be typically run multiple times without error. There either will be some rows for it to delete or there will not but it will only normally result an error if there are dependent records in another table and no arrangement for them to be deleted or disconnected in cascade fashion. It is not an error for a DELETE statement to affect 0 rows
An SQL INSERT can typically only be run multiple times when it is not subsequently(after the first run) inhibited by the presence of a unique constraint on one or more of the columns. As most tables you design should really have a primary key, you can only insert a row with a unique value for the key column. If you aren't devolving generation of the value to the database then re-running an identical INSERT will fail on the second run. If the table depends on another table to have a related row and a foreign key constraint backs this up, then an insert that doesn't relate to a row in the parent table will fail on first run

Why is the unique constraint missing when filling the datatable?

I have the following table in SQL server
projects
id(PK, int, not null)
name (varchar(255), not null)
public_key_token (varchar(50), null)
I have added a unique constraint to the name column using
ALTER TABLE dbo.projects
ADD CONSTRAINT name_unique UNIQUE (name);
which results in a Unique, Non-Clustered index on the table (trusting SSMS).
In the code I'm retrieving the table data using
using (SqlDataAdapter adapter = new SqlDataAdapter("select * from " + DbTabName, con))
{
using (DataTable table = new DataTable(DbTabName))
{
DataTable dt = adapter.FillSchema(table, SchemaType.Source);
PkColumns = dt.PrimaryKey.Select(c => c.ColumnName).ToList();
AutoIncrementColumns = dt.PrimaryKey.Where(c => c.AutoIncrement).Select(c => c.ColumnName).ToList();
UniqueColumns = dt.Columns.Cast<DataColumn>().Where(c => c.Unique).Select(c => c.ColumnName).ToList();
...
}
}
The PKs and AutoIncrement columns are OK but in UniqueColumns I only get the PK column again.
The name column arrives in C# without the Unique constraint.
Changing the SchemaType to Mapped did not alter the result.
Why do I lose this constraint on the way to C#? Am I missing something on the SQL Server side or in C#?
[UPDATE]
#Tim-Schmelter's answer only solves the problem half way.
Just adding the index did not work. Also adding the index when the PK on the id column exists doesn't work.
The only way I got it to work was delete the table, recreate it without any key and indexes and then add the unique index as in Tim's answer. However, after adding the PK for the id column once more I'm back to the old behaviour that only the id is listed as unique column.
This is really weird.

Use this to create a unique index or use the the gui of SSMS.
CREATE UNIQUE NONCLUSTERED INDEX [name_unique] ON [dbo].[projects]
(
name ASC
)
You have added a unique constraint not a unique index.
Your code now successfully retrieves the UniqueColumns(the single column name).

Get Inserted Row after .Net Dataset Row Added

I have a .Net dataset and am adding a row to a table. This works and the record is saved to the database. How do I get the updated version of my row after the insert. Or, alternatively, how do I know the ID of the item that was added (so that I can then use it in a subsequent child table insert.
MyDataSet.ProjectRow r = dsMyDataSet.Projects.AddProjectRow(txtTitle.Text);
m_daProjects.Update(dsMyDataSet.Projects);
// What is the ID of the new item here?

If the column is an identity column you can find the new ID's in the inserted rows.
You: "thanks. which object maintains a list of inserted rows?"
You can use DataTable.GetChanges(DataRowState.Added) to get a DataTable with all DataRows which are going to be added. You need to use it before AcceptChanges was called. If i remember correctly TableAdapter.Update calls AcceptChanges at the end. Then you need to use it before m_daProjects.Update(dsMyDataSet.Projects):
DataTable addedRows = ds.modModel.GetChanges(DataRowState.Added);
MyDataSet.ProjectRow r = dsMyDataSet.Projects.AddProjectRow(txtTitle.Text);
m_daProjects.Update(dsMyDataSet.Projects);
now addedRows contains all DataRows with the new identity value in each row
foreach(DataRow addedRow in addedRows.Rows)
Console.WriteLine("New ID: {0}", addedRow.Field<int>("IdColumn"));
Update: However, in your case it's simpler. You have already the single row that you want to insert. So you dont need to call DataTable.GetChanges at all.
You can see the new identity value in the (typed DataRow) ProjectRow r after Update.

Thanks to Tim Schmelter. In the link he posted there's a reference to an article on Beth Massi's blog with a complete walkthrough of the solution. It worked for me.
http://blogs.msdn.com/bethmassi/archive/2009/05/14/using-tableadapters-to-insert-related-data-into-an-ms-access-database.aspx
The basic steps are:
1) Add RowUpdated event handler on the strongly typed table adapter. This event handler issues a new OleDBCommand to the database to retrieve ##Identity and then assigns the integer to the member column of the table.
public void _adapter_RowUpdated(dynamic sender, System.Data.OleDb.OleDbRowUpdatedEventArgs e)
{
HMUI.Classes.AccessIDHelper.SetPrimaryKey(this.Connection, e);
}
public static void SetPrimaryKey(OleDbConnection trans, OleDbRowUpdatedEventArgs e)
{
if (e.Status == System.Data.UpdateStatus.Continue && e.StatementType == System.Data.StatementType.Insert)
{
if (pk != null)
{
OleDbCommand cmdGetIdentity = new OleDbCommand("SELECT ##IDENTITY", trans);
// Execute the post-update query to fetch new ##Identity
e.Row.Table.Columns[pk(0)] = Convert.ToInt32(cmdGetIdentity.ExecuteScalar());
e.Row.AcceptChanges();
}
}
}
2) In the constructor of the form using the dataset and table adapter I attach the function in step 1 to the RowUpdated event on the table adapter's internal data adapter.
// Event to handle inserted records and retrieve the primary key ID
m_daDataSources.Adapter.RowUpdated += new System.Data.OleDb.OleDbRowUpdatedEventHandler(m_daDataSources._adapter_RowUpdated);

How to delete selected row on datagridview and that also reflects to the database?

I am working with C# Window Form, I have created a form connect to mySQL database and it display list of databases, list of tables on each database and also tables contents of each table.
Questions that I have here:
After I selected a random cell in the table (datagridview) and click Delete button, I want that row (corresponding to the selected cell) to be deleted on the database.
I also need that the datagridview table content also will be refreshed and updated (with that row has been removed). (This part I think I can do if I know how to do the part 1)
So I need help with the question 1, one of those things I can't not figure out is that how can I write the SQL statement to put in the SQLadapter or SQLcommandbuilder or whatever it is. I know the SQL statement should be like:
Delete from (selected Table) Where (THIS IS THE PART WHERE I STUCK AT) => I dont know what to put in this condition, how to get it?
Any helps and advises is really appreciated!

The delete statement should consider all the selected table primary key columns and the selected row from the datagridview.
How to get the primary key columns:
SELECT `COLUMN_NAME`
FROM `information_schema`.`COLUMNS`
WHERE (`TABLE_SCHEMA` = 'dbName')
AND (`TABLE_NAME` = 'tableName')
AND (`COLUMN_KEY` = 'PRI');
Source: A better way to get Primary Key columns
How your delete statement should look like:
DELETE FROM <TABLE>
WHERE <PRIMARY_KEY_COLUMN_1> = <ROW_VALUE_1>
AND <PRIMARY_KEY_COLUMN_2> = <ROW_VALUE_2>
You see, the table could have multiple columns uniquely identifying a row. There is also the possibility of existing a reference for that very row on another table, which would prevent you from deleting it.
It would look like this:
List<string> primaryKeyColumns = GetPrimaryKeyColumns(SelectedDB, SelectedTable);
string deleteWhereClause = string.Empty;
foreach (string column in primaryKeyColumns)
{
DataGridViewRow row = datagridview.CurrentCell.OwningRow;
string value = row.Cells[column].Value.ToString();
if (string.IsNullOrEmpty(deleteWhereClause))
{
deleteWhereClause = string.Concat(column, "=", value);
}
else
{
deleteWhereClause += string.Concat(" AND ", column, "=", value);
}
}
string deleteStatement = string.Format("DELETE FROM {0} WHERE {1}", SelectedTable, deleteWhereClause);
The method GetPrimaryKeyColumns returns the names of all the primary key columns of the selected table using the select statement i posted.
You would also have to deal with other types of columns such as dates and strings, but that's basically what you will have.

Database delete query between two tables with a foreign key

I have a Profile(as shown in below image) Table Which whenever I try to remove a row from it, I face this error which indicates there is still data in foreign key in Tempprice table which is related to the Id in Lots table .
the problem is when its Lots table turn to Delete , there is still rows in Tempprice which have different Userid but same lotsid that have Ownerid or Winnerid which I want to Delete.
And I don't know how to delete this rows!? or which query is needed?
Database Relationships and Tables:
Code:
while (checkbox.Checked)
{
// Retreive the ID
int ida = Convert.ToInt32(GridView1.DataKeys[row.RowIndex].Value);
// Pass the value of the selected Id(s) to the Delete //command.
//These numbers indicate in which order tables shoulde be deleted
/*1*/
new BLL.LoginBLL().Delete(ida);
/*2*/
new BLL.MessageBLL().Delete(ida);
/*3*/
new BLL.JointBLL().Delete(ida);
/*4*/
new BLL.TemppriceBLL().Delete(ida);
/*5*/
new BLL.LotsBLL().Delete(ida);
/*6*/
new BLL.AuctionBLL().Delete(ida);
/*7*/
new BLL.ProfileBLL().DeleteProfile(ida);
checkbox.Checked = false;
}
ShowUsers();

To delete references, you need to set the relation (by selecting the line connecting the two objects) to Cascade for the End 1 On Delete property.
Then you need to Load the referenced object or collection. Calling Delete in this case, will delete your object and all objects it references.