Table: violent_events
Columns: estimate
name
nat
ship_type
principal_victims
date

Question: What is the nationality of the ship when the principle victims are civilians?
SQL: SELECT "nat" FROM violent_events WHERE "principal_victims" = 'civilians'

Question: what is the name of the battleship with the battle listed on May 13, 1915?
SQL: SELECT "name" FROM violent_events WHERE "ship_type" = 'battleship' AND "date" = 'may 13, 1915'