user: Question: When did the ship that was commissioned July 1948 with a bow number of PS-31 launch?
Columns: ['Bow number', 'Ship name', 'Launched', 'Commissioned', 'Service']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['PS-18', 'BRP Datu Tupas', '14 November 1943', 'November 1975', 'Philippine Navy Patrol Force']
SQL:

assistant: SELECT "Launched" FROM "Table" WHERE "Commissioned" = 'July 1948' AND "Bow number" = 'PS-31';