user: Question: What is the average opened year of mini estadi stadium in barcelona, spain?
Columns: ['Team', 'Stadium', 'Capacity', 'Opened', 'City']
Types: ['text', 'text', 'real', 'real', 'text']
Sample row: ['Amsterdam Admirals', 'Amsterdam ArenA', 51859.0, 1996.0, 'Amsterdam , The Netherlands']
SQL:

assistant: SELECT AVG("Opened") FROM "Table" WHERE "City" = 'barcelona, spain' AND "Stadium" = 'mini estadi';