Simple SQL - Union statement

SELECT orderno FROM saletable
UNION
SELECT orderno FROM saledetail

This is to show distinct value from the 2 tables by merging those duplicated values.

Eg, 
 OrderNo.SaleTable  
     SO1   
     SO2
OrderNo.SaleDetail
    SO1                  
    SO1
    SO2


output will be:  SO1, SO2  total record is 2


SELECT column_name(s) FROM table1
UNION ALL
SELECT column_name(s) FROM table2;

This is to include those duplicated values from above tables:

output will be :  5 records