SQL aggregate function handling null [easy to understand]

Hello everyone , I meet you again , I'm your friend, Quan Jun .

Aggregate functions

Count type （count）

SELECT  COUNT(*)
FROM    (
            SELECT  1 AS num
            UNION ALL
            SELECT  1 AS num
            UNION ALL
            SELECT  2 AS num
            UNION ALL
            SELECT  NULL AS num
        ) 
;

SELECT  COUNT(num)
FROM    (
            SELECT  1 AS num
            UNION ALL
            SELECT  1 AS num
            UNION ALL
            SELECT  2 AS num
            UNION ALL
            SELECT  NULL AS num
        ) 
;

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The two results are 4 and 3

count(*) and count(column) The difference can be seen in the previous article

meanwhile sum(column) Will also be ignored directly null value

Mathematical functions （ variance :var_pop Standard deviation :stddev etc. ）

SELECT  var_pop(num)
FROM    (
            SELECT  3 AS num
            UNION ALL
            SELECT  6 AS num
            UNION ALL
            SELECT  9 AS num
            UNION ALL
            SELECT  NULL AS num
        ) 
;

SELECT  stddev(num)
FROM    (
            SELECT  3 AS num
            UNION ALL
            SELECT  6 AS num
            UNION ALL
            SELECT  9 AS num
            UNION ALL
            SELECT  NULL AS num
        ) 
;

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The two results are 6.0 and 2.449489742783178

[3 6 9] The variance of 6 , The standard deviation is 6 The square root of

The comparison of the two results shows that , Also directly ignore null It's worth it , Not as 0 Handle

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