l TIMESTAMPDIFF(unit , timestamp_expr1, timestamp_expr2)
timestampdiff函数是计算两个日期时间之间(timestamp_expr2-timestamp_expr1)的差值，并以unit形式返回结果。timestamp_expr1，timestamp_expr2必须是一个timestamp、timestamptz、date类型的值表达式。unit表示的是两个日期差的单位。
仅在兼容MY类型时（即创建数据库时指定dbcompatibility 'B'）有效，其他类型不支持该函数。
l year ：年份
gbase=# SELECT TIMESTAMPDIFF(YEAR, '2018-01-01', '2020-01-01');
timestamp_diff
----------------
2
(1 row)
l quarter ：季度
gbase=# SELECT TIMESTAMPDIFF(QUARTER, '2018-01-01', '2020-01-01');
timestamp_diff
----------------
8
(1 row)
l month：月份
gbase=# SELECT TIMESTAMPDIFF(MONTH, '2018-01-01', '2020-01-01');
timestamp_diff
----------------
24
(1 row)
l week：星期
gbase=# SELECT TIMESTAMPDIFF(WEEK, '2018-01-01', '2020-01-01');
timestamp_diff
----------------
104
(1 row)
l day ：天
gbase=# SELECT TIMESTAMPDIFF(DAY, '2018-01-01', '2020-01-01');
timestamp_diff
----------------
730
(1 row)
l hour ：小时
gbase=# SELECT TIMESTAMPDIFF(HOUR, '2020-01-01 10:10:10', '2020-01-01 11:11:11');
timestamp_diff
----------------
1
(1 row)
l minute ：分钟
gbase=# SELECT TIMESTAMPDIFF(MINUTE, '2020-01-01 10:10:10', '2020-01-01 11:11:11');
timestamp_diff
----------------
61
(1 row)
l second ：秒
gbase=# SELECT TIMESTAMPDIFF(SECOND, '2020-01-01 10:10:10', '2020-01-01 11:11:11');
timestamp_diff
----------------
3661
(1 row)
l microseconds ：秒域（百万级别），可包含小数。
gbase=# SELECT TIMESTAMPDIFF(MICROSECOND, '2020-01-01 10:10:10.000000', '2020-01-01 10:10:10.111111');
timestamp_diff
----------------
111111
(1 row)
l timestamp_expr含有时区
gbase=# SELECT TIMESTAMPDIFF(HOUR,'2020-05-01 10:10:10-01','2020-05-01 10:10:10-03');
timestamp_diff
----------------
2
(1 row)