user: Question: What is the maximum diameter of the Mid West Gas Pipeline (1999)?
Columns: ['Name (year commissioned)', 'Owner/operator', 'Length', 'Maximum diameter', 'From/to', 'Licence number']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Dampier to Bunbury Natural Gas Pipeline (1984)', 'Dampier Bunbury Pipeline', '1,530km', '660 mm', 'Dampier to Bunbury', 'PL 40']
SQL:

assistant: SELECT "Maximum diameter" FROM "Table" WHERE "Name (year commissioned)" = 'Mid West Gas Pipeline (1999)';