SQL - select distinct

问题

I used the select disctinct statement with 2 fields from my sql database table. Here is my code.

myCommand = New SqlCommand("SELECT DISTINCT Author FROM tblBook UNION SELECT DISTINCT BookCode FROM tblBook",myConnection)
 myAdapter = New SqlDataAdapter(myCommand)
 myAdapter.Fill(myDataSet, "tblBook")
 cboAuthor.DataSource = myDataSet.Tables(0)
 cboAuthor.DisplayMember = "Author"
 cboAuthor.DisplayValue = "BookCode"

and it generates error : cannot bind to the new member. Parameter name:value. Please help

回答1:

you have selected only one column in your sql statement, Author. BookCode isn't present so it won't be in the dataset either.

Include the BookCode in the Sql Statement and it will be fixed

回答2:

Maybe you are looking for

SELECT DISTINCT Author, Bookcode FROM tblBook

Union is used to merge rows not columns..

So your query returns a single column where you have all the autors (distinct) followed in the same column by rows of all the bookcodes (distinct as well).

回答3:

A UNION operation does not work in this way.

You want a recordset containing two columns - Authors and BookCodes so you are going to need a query something like:

 SELECT DISTINCT Author, BookCode FROM tblBook

回答4:

You make a query of two distinct things.

You either want to...

1. Get the author of each book. In that case have to make a query joining the tables, so union is not an option

2. You indeed need the result as one column, in that case give the column an alias and bind the alias

   SELECT DISTINCT Author AS mycolumn FROM tblBook UNION SELECT DISTINCT BookCode AS mycolumn FROM tblBook

来源：https://stackoverflow.com/questions/6571485/sql-select-distinct