mysql - 按频率顺序选择最常出现的值

Question

我有一列，我只想返回不匹配某些条件的不同值，按最常见值的降序排列。

该列包含以下记录:

this 
those
that
dog
these
here
there
cat
dog
hamster
hamster
there
blah
here
blah
blah
dog

所以我有:

SELECT DISTINCT(rcolumn)
FROM otable
WHERE reason != 'this'
AND reason != 'that'
AND reason != 'those'
AND reason != 'these'
AND reason != 'them'
AND reason != 'here'
AND reason != 'there'
AND reason != 'in between'
AND reason != 'all over'
AND reason != 'something'
AND reason != 'something else'
AND reason != 'anywhere'
AND reason != 'anywhere else'

将返回:

blah
cats
hamsters
dogs

但我希望它返回:

blah
dogs
hamsters
cats

按照这个顺序，表中出现最多的是“blah”，然后是狗，然后是仓鼠，然后是猫。

Answer 1

为了按频率顺序返回内容，您需要对值进行GROUP BY:

SELECT   rcolumn
FROM     otable
-- WHERE stuff
GROUP BY rcolumn;

这应该给出与您得到的答案基本相同的答案。然后您可以轻松计算每行的发生率并排序:

SELECT   rcolumn, COUNT(*) AS frequency
FROM     otable
GROUP BY rcolumn
ORDER BY frequency DESC;

如果您不想在结果中看到计算出的频率列，那么您可以直接按此订购:

SELECT   rcolumn
FROM     otable
GROUP BY rcolumn
ORDER BY COUNT(*) DESC;

一般来说，在使用 GROUP BY 子句时，请小心仅选择:

• 分组依据的列
• 聚合函数的结果

大多数 RDBMS 都会将此视为错误，但 MySQL 只会选择随机行并返回该数据。

最后，为了更加清晰，为什么不将长 WHERE 子句更改为更优雅的内容:

WHERE rcolumn NOT IN
    ('this'
    ,'that'
    ,'those'
    -- ...
)