user: Question: Which Diameter (km) has a Name of alma-merghen planitia, and a Year named smaller than 1997?
Columns: ['Name', 'Latitude', 'Longitude', 'Diameter (km)', 'Year named']
Types: ['text', 'text', 'text', 'real', 'real']
Sample row: ['Aibarchin Planitia', '73.0S', '25.0E', 1200.0, 1997.0]
SQL:

assistant: SELECT "Diameter (km)" FROM "Table" WHERE "Name" = 'alma-merghen planitia' AND "Year named" < 1997.0;