Question

I have two datasets named dataset1,dataset2. I want put the left outer join relation between them.

eg:dataset1 left outer join dataset2

Answer 1

Your question is a bit unclear so there are at least two possible answers.

If you do indeed have two different DataSets and they need to stay as separate DataSets then you could use LINQ (with Linq to DataSet) to create a left outer join. Here are a couple of blog post explaining how to create left outer joins using LINQ (one and two). Your code will probably look something along the lines of

var joinResult = from ds1Row in dataset1.Tables["some table"]
                 join ds2Row in dataset2.Tables["some other table"]
                 on ds1Row.Field<T>("some column") equals ds2Row.Field<T>("some other column") into tmpResult
                 from resultRow in tmpResult.DefaultIfEmpty()
                 select new 
                 {
                   //select whatever information you want
                   column1 = ds1Row.Field<T>("name column"),
                   column2 = ds1Row.Field<T>("name column"),
                   column3 = ds2Row.Field<T>("name column")
                 };

If you really meant doing a join between two DataTables or you can combine two DataSets into a single DataSet using the Merge method and then create a relationship between any two tables. Your code will look something along the lines of:

mergedDataset.Relations.Add("relation name",
mergedDataset.Tables["some table"].Columns["some column"],
mergedDataset.Tables["some other table"].Columns["some other column"]);