我想选择sql SELECT "year-month" from table group by "year-month" AND order by date：，其中year-month-格式为日期“ 1978-01”，“ 1923-12”。 选择couse work的to_char ，而不是“正确”的顺序：
SELECT "year-month" from table group by "year-month" AND order by date
to_char(timestamp_column, 'YYYY-MM')
date_part(text, timestamp)
例如
date_part('month', timestamp '2001-02-16 20:38:40'), date_part('year', timestamp '2001-02-16 20:38:40')
http://www.postgresql.org/docs/8.0/interactive/functions- datetime.html
