Suppose you are no longer interested in the weather of Hayward. Then you can do the following to delete those rows from the table. Deletions are performed using the DELETE command:
DELETE FROM weather WHERE city = 'Hayward';
All weather records belonging to Hayward are removed.
SELECT * FROM weather;
city | temp_lo | temp_hi | prcp | date
---------------+---------+---------+------+------------
San Francisco | 46 | 50 | 0.25 | 1994-11-27
San Francisco | 41 | 55 | 0 | 1994-11-29
(2 rows)
One should be wary of statements of the form
DELETE FROM tablename;
Without a qualification, DELETE will remove all rows from the given table, leaving it empty. The system will not request confirmation before doing this!