ps: over()里头的分组以及排序的执行晚于 where 、group by、 order by 的执行
1、 这样的结果是先按照stationcode分组,再按照时间进行排序。第一列为序列号
select row_number() over(partition by stationcode ORDER BY time),stationcode,time from dcsrealtime
2、按照stationcode进行分组后再按照time排序,然后筛选出rank为1的记录
select * from (select row_number() OVER(partition by stationcode ORDER BY time) rank,* from dcsrealtime rank) temp where temp.rank='1'
3、查询表占用大小
查询占用字节:select pg_relation_size('tablename');
查询占用MB:select pg_size_pretty(pg_relation_size('tablename'));
清空指定表:TRUNCATE TABLE tablename
4、将时间戳转为指定格式
select to_char(to_timestamp(updatetime/1000),'yyyy-MM-dd HH:MI:SS') from dangersource
5、空值替换函数(mysql使用ifnull,pgsql使用coalesce)
select level,coalesce(level,'蓝') from alarms
6、count中添加条件
select count(字段名 LIKE '%true%' or null) as 字段名 from tablename
7、查询表中:指定字段有重复的记录
select * from organization where code in (select code from organization GROUP BY code HAVING count(code)>1) ORDER BY name
8、字符连接函数
-- pgsql的三种字符链接函数,将多列的字段连接成一条数据
-- arrary_agg () string_agg( ) xmlagg() 分别来处理数组,字符串和xml文档。
-- array_agg字符链接后是json格式
SELECT array_agg(name ORDER BY name DESC) FROM monitoritem GROUP BY dangersourceid;
-- string_agg可以指定分隔符
SELECT string_agg(name, ',' ORDER BY name) FROM monitoritem GROUP BY dangersourceid;
-- ps:mysql的字符链接函数
select d.name,wm_concat(m."name") as roleName from dangersource d join monitoritem m on d.id = m.dangersourceid