string username = SPContext.Current.Web.CurrentUser.Name;
            if (username != null)
            {
                SPSecurity.RunWithElevatedPrivileges(delegate()
                {
                    using (site = new SPSite(SPContext.Current.Site.ID))
                    {
                        using (web = site.OpenWeb())
                        {



                            string where = @"<Where><And><Contains><FieldRef Name='AssignedTo' /><Value Type='User'>" + username + "</Value></Contains><And><Leq><FieldRef Name='DueDate' /><Value Type='DateTime'>" + SPUtility.CreateISO8601DateTimeFromSystemDateTime(DateTime.Now) + "</Value></Leq><Or><Eq><FieldRef Name='Status' /><Value Type='Choice'>Not Started</Value></Eq><Eq><FieldRef Name='Status' /><Value Type='Choice'>In Progress</Value></Eq></Or></And></And></Where>";

                            SPSiteDataQuery query = new SPSiteDataQuery();
                            query.Webs = "<Webs Scope='SiteCollection'>";
                            query.Lists = "<Lists ServerTemplate='171'/>";
                            query.ViewFields = "<FieldRef Name='Title' Type='Text'/>";
                            query.ViewFields += "<FieldRef Name='StartDate' />";
                            query.ViewFields += "<FieldRef Name='DueDate' />";
                            query.ViewFields += "<FieldRef Name='Status' />";
                            query.ViewFields += "<FieldRef Name='Assigned_To' Type='UserMulti' Nullable='TRUE' />";
                            query.ViewFields += "<FieldRef Name='TestUser' Type='User' Nullable='True'/>";
                            query.ViewFields += "<FieldRef Name='PercentComplete' Type='Number' Nullable='TRUE' />";
                            query.ViewFields += "<FieldRef Name='Remarks' />";
                            query.Query = where;
                            DataTable dt = web.GetSiteData(query);
                            //foreach (DataRow row in dt.Rows)
                            //{
                            //    string name = row["Assigned_To"].ToString();
                            //}
                            if (dt.Rows.Count > 0)
                            {
                                gvlist.DataSource = dt;
                                gvlist.DataBind();
                              
                            }
                            else
                            {
                                lblEmpty.Text = "No Records Found";
                            }
                        }
                    }
                });
            }

SPquery
resuable workflow
What are SPQuery and SPSiteDataQuery?
SPSiteDataQuery properties?
Difference between SPQuery and SPSiteDataQuery?
What are the limitations of both?
How the SPQuery and SPSiteDataQuery return?
How to set the row limit to the both?
Searching scope levels of SPQuery and SPSiteDataQuery?
What is the max limit of querying the lists using SPSiteDataQuery?
Is it possible to query the records across the site collection level using SPSiteDataQuery?
What is the default row limit value for the SPQuery?
SPQuery.RowLimit Property?
To Query all Web sites in this site collection?
How to query the specific lists only using SPSiteDataQuery?
Difference between Caml and Linq queries?
How to set the Max lists limit?
what all are Errors you faced while working on SPSiteDataQuery?
I have created a custom list called “Employees” in all the
sites in a site collection and have added the data in all
the lists. Now I want to query those specific lists only. What is the approach?
How can consider hidden lists also in SPSiteDataQuery?
Is it possible to do joins lists using SPQuery?
How to query with the type: Single line of text, multiline of text, persons or groups, DateTime etc..
How to get the folders and sub folders levels documents from the document library?
How to query specific list using SPSiteDataQuery?