Table: vienna_u_bahn
Columns: line
colour
route
length
stations

Question: What is the total number of stations with a line that is u 6?
SQL: SELECT SUM("stations") FROM vienna_u_bahn WHERE "line" = 'u 6'

Question: Which route has 21 stations?
SQL: SELECT "route" FROM vienna_u_bahn WHERE "stations" = 21

Question: Which line was green?
SQL: SELECT "line" FROM vienna_u_bahn WHERE "colour" = 'green'