vb.net - select distinct not get the result wanted

I used the following code to select unique values from the database

myCommand = New SqlCommand("SELECT DISCTINCT Visitor, BookCode FROM tblBook", myConnection)
 myAdapte开发者_Python百科r = New SqlDataAdapter(myCommand)
 myAdapter.Fill(myDataSet, "tblBook")
 cboAuthor.DataSource = myDataSet.Tables(0)
 cboAuthor.DisplayMember = "Author"
 cboAuthor.ValueMember = "BookCode"

And it does not retrieve the unique values, it remains the same. But if I use only SELECT DISTINCT Author FROM tblBook it works fine.

Please help.

---

SELECT DISTINCT will ensure that no duplicate records are returned in the result set.

Therefore if you only put SELECT DISTINCT Author, you will get a list of unique authors.

Putting SELECT DISTINCT Author, Visitor, BookCode may return duplicate authors, with different visitor or bookcode.

---

SELECT DISTINCT would get all distinct values of ALL fields.

---

everything is right. select DISTINCT returns all unique combinations of all fields that are in SELECT clause.