Insert only if Item is selected in ASP.Net DropDownList

HI

i have dropdown list in my page that bind it from database

  BindDropDownList(DDL3, "Guidcenter", "Centername", "ID");
DDL3.Items.Insert(0, new ListItem(" select all", "0"));

here when users don't select any thing from dropdown list in data base insert 'select all'  text but i want if users don't select any thing from DDL in database insert NULL 

how i can do it?

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