PostgreSQL中有以下格式化函数：

函数	返回类型	描述	例子
`to_char(timestamp, text)`	`text`	把时间戳转成字符串	`to_char(current_timestamp, 'HH12:MI:SS')`
`to_char(interval, text)`	`text`	把间隔转成字符串	`to_char(interval '15h 2m 12s', 'HH24:MI:SS')`
`to_char(int, text)`	`text`	把整数转成字符串	`to_char(125, '999')`
`to_char(double precision, text)`	`text`	把实数或双精度转成字符串	`to_char(125.8::real, '999D9')`
`to_char(numeric, text)`	`text`	把数字转成字符串	`to_char(-125.8, '999D99S')`
`to_date(text, text)`	`date`	把字符串转成日期	`to_date('05 Dec 2000', 'DD Mon YYYY')`
`to_number(text, text)`	`numeric`	把字符串转成数字	`to_number('12,454.8-', '99G999D9S')`
`to_timestamp(text, text)`	`timestamp with time zone`	把字符串转成时间戳	`to_timestamp('05 Dec 2000', 'DD Mon YYYY')`

将字符串转换成double类型(to_number())

select count(*) as tt, sum(to_number( length, '999999.9999') ) as x, sum(to_number( area, '9999999.9999') ) as x from szt_road