display an alert after both clicking a button and checking data from database

i want that after clicking a hyperlink "delete" located at each row of my gridview which lists trainings that i check if a trainee wasn't assign to the training concerned by the delete
For adding a delete prompt on a datatable (looks like its going to be a datagrid in UI), you would need to bind the javascript to prompt for this alert during the ItemDataBound/RowDataBound event of the datagrid.

In this event, while the data is binded row by row - add something like:

LinkButton l = (LinkButton)e.Row.FindControl("LinkButton1");
l.Attributes.Add("onclick", "javascript:return " +
    "confirm('Are you sure you want to delete this record')"); 

Alternatively,
if the link button in ItemTemplate of datagrid has OnclientClick event exposed (which is present in ASP.NET2.0 onwards), you can add something like:

OnClientClick = 'return confirm("Are you sure you want to delete this entry?");' 

Refer: MSDN: Adding Client-Side Confirmation when Deleting[^]


hen display a alert message if there is any trainee assigned to the training and if not to delete the training
For this, just show the thing in a label text. Showing it in an alert window would not be good idea for the purpose. Once DB update fails, show that as a red label text on the page.

you must be doing all the delete logic processing on server side. So you problem basically boils down to showing a messagebox(alert) to the user on some conditions that are met on serverside.

I think you can find this useful for your scenario

A Windows Form like MessageBox for ASP.NET Website[^]