Conversation so far:
What was Judy Garland's role in The Wizard of Oz?
Dorothy Gale.
Did she sing any songs?
"Over the Rainbow".
What was The  Wizard of Oz based on ?
a film based on the 1900 children's book by L.
Frank Baum.
How long did it take to complete the movie?
Shooting commenced on October 13, 1938, and was completed on March 16, 1939.
Was the Wizard of Oz successful?
The Wizard of Oz was a tremendous critical success.
Did she win any awards?
At the 1939 Academy Awards ceremony, Garland received her only Academy Award.

Question: What was the award for?
Self-contained rewrite: What did Judy Garland win an award for?