save into multiple table

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C#

i have a form with one textbox and a gridview.
i am saving data from this form to two different tables (ServiceCategory, ServiceActivity)
ServiceCategory has two columns catid(pk) IDENTITY and category and
ServiceActivity has two columns catid(fk) and activity

when the save button is clicked the data is save to ServiceCategory before ServiceActivity
i used this sql statement but it is not working with ServiceActivity pls help me out

string sqlQuery = null;
            sqlQuery = "Insert into ServiceActivity values(Catid, Activity) SELECT Catid from ServiceCategory WHERE Category ='" + Category + "','" + Activity + "'";