[英]What's the most efficient way to get the horizontal average in a MySQL query?
我有以下MySQL表
Id | One | Two | Three
----------------------------
1 | 10 | 30 | 20
2 | 50 | 60 | 20
3 | 60 | NULL | 40
编辑:当然,默认情况下表不需要为NULL,但我不希望它影响平均值(因此平均值计算为50而不是33,33)。
我希望看起来像这样,使用MySQL查询:
Id | Average
------------
1 | 20
2 | 43,33
3 | 50
实现这一目标的最有效方法是什么?
select id, (ifnull(one,0) + ifnull(two,0) + ifnull(three,0))/
((one is not null) + (two is not null) + (three is not null)) as average from table
如果您没有NULL值,下面的明显选项将起作用:
SELECT ((col_1 + col_2 + col_3) / 3) AS Average FROM table;
但是,正如jxac在另一个答案中建议的那样,你必须按如下方式进行:
SELECT id, (ifnull(col_1, 0) + ifnull(col_2, 0) + ifnull(col_3, 0)) /
((col_1 is not null) + (col_2 is not null) + (col_3 is not null)) as Average
FROM
table;
如果每列的所有值都为NULL,则除以零将为该行返回NULL。
采用:
SELECT a.id,
AVG(a.val) AS average
FROM (SELECT x.id,
x.one AS val
FROM TABLE x
UNION ALL
SELECT y.id,
y.two AS val
FROM TABLE y
UNION ALL
SELECT z.id,
z.three AS val
FROM TABLE z) a
GROUP BY a.id
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