Question

表就像这样，假设它代表日志文件中的各种类型的事件

<type> <date>

我想选择前5种最常见的类型

select type,count(type) as c from log order by c desc limit 5

这工作正常，但我只想要类型列，所以我可以使用它是一个where in子查询。我怎么做？我无法弄清楚如何压制伯爵colum

Answer 1

您没有指定RDBMS，这在很大程度上取决于您使用的是哪一个。这里有一些选择。

-- works in postgres and mysql
select type from log group by type order by count(*) desc limit 5;

-- this variant works in mssql but probably no where else (maybe sybase)
select top(5) type from log group by type order by count(*) desc;

-- works in posgres and mssqlserver but not mysql or oracle
select
      type
  from (select
        type,
        row_number() over (order by count(*) desc) as r
    from
        log
    group by
        type
  ) as t
  where
    r <= 5
;

-- portable to all standards compliant RDMS
select
      type
  from (select
        type,
        row_number() over (order by c) as r
    from
        (select
                type,
                count(*) as c
            from
                log
            group by
                type
        ) as t
  ) as t
  where
    r <= 5
;

-- works if you don't have windowing functions, but returns more than 5 rows
select
        type
    from
        (select
                type,
                count(*) as c
            from
                log
            group by
                type
        ) as t
    order by
        c desc
;

Answer 2

非常简单：

SELECT type FROM log GROUP BY type ORDER BY COUNT(type) DESC LIMIT 5

Answer 3

也可以将其作为子查询：

SELECT TYPE
FROM LOG
WHERE TYPE IN
    (select type,count(type) as c 
     from log 
     order by c desc limit 5)

Answer 4

没用过sqlite，但是我会在SQL server中写这样的东西;也许有一些想法你可以偷？

select top (5) type from log group by type order by count(type) desc

选择按计数排序但没有计数的列

4 个答案: