user: Question: When has a Source of SurveyUSA and a Obama of 54%?
Columns: ['Source', 'Date', 'Clinton', 'Obama', 'Undecided']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['SurveyUSA', 'May 16–May 18, 2008', '42%', '55%', '1%']
SQL:

assistant: SELECT "Date" FROM "Table" WHERE "Source" = 'SurveyUSA' AND "Obama" = '54%';