按30分钟统计：
shool_time格式为varchar， 例如
201911050808、201911050820、201911050842
分组后结果：
20191105 0800
20191105 0830
20191105 0900
20191105 0930
注：小于30分钟的按00统计，大于30分钟的按30统计
SELECT name,sex,(SELECT (SELECT to_char( to_timestamp( school_time, 'yyyyMMddhh24mi' ), 'yyyyMMdd hh24' ) ) || ':' || to_char(to_timestamp(( SELECT FLOOR ( (SELECT to_char( to_timestamp( school_time, 'yyyyMMddhh24mi' ), 'mi' ) ) :: INTEGER / 30 ) * 30 ),'mi' ),'mi' ) ) school_time
FROMstudent
GROUP BYname,sex,school_time
按小时统计：
SELECTname,sex,( SELECT to_char( to_timestamp( school_time, 'yyyyMMdd hh24:mi' ), 'yyyyMMdd hh24' ) ) school_time
FROMstudent
GROUP BYname,sex,school_time
按天统计：
SELECTname,sex,( SELECT to_char( to_timestamp( school_time, 'yyyyMMdd hh24:mi' ), 'yyyyMMdd' ) ) school_time
FROMstudent
GROUP BYname,sex,school_time
按周统计：
注意:
DOW 周日(0) -> 周一 (6)
isodow 周一(1) -> 周日 (7)
GROUP BY extract(isodow from timestamp '2019-11-03 20:38:40')
GROUP BY extract(DOW from timestamp '2019-11-03 20:38:40')
postgresql-参考文章
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