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How to present features of aggregate functions (NULL)?


How to present features of aggregate functions (NULL)?

By : Justin Ponczek
Date : October 18 2020, 11:12 AM
I wish this helpful for you I'm looking for 'textbook' example of database to illustrate salient features of the aggregate functions (Max, Min, Sum, Avg and Count) when NULL values are involved. , Use:
code :


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GROUP BY behavior when no aggregate functions are present in the SELECT clause

GROUP BY behavior when no aggregate functions are present in the SELECT clause


By : user1515189
Date : March 29 2020, 07:55 AM
it helps some times Read MySQL documentation on this particular point.
In a nutshell, MySQL allows omitting some columns from the GROUP BY, for performance purposes, however this works only if the omitted columns all have the same value (within a grouping), otherwise, the value returned by the query are indeed indeterminate, as properly guessed by others in this post. To be sure adding an ORDER BY clause would not re-introduce any form of deterministic behavior.
code :
When using this feature, all rows in each group should have the same values
for the columns that are omitted from the GROUP BY part. The server is free
to return any value from the group, so the results are indeterminate unless
all values are the same.
aggregate functions return null

aggregate functions return null


By : Iskra Georgieva
Date : March 29 2020, 07:55 AM
will help you You could utilize the fact that the COUNT of NULL and NOT NULL columns will be different. So the script can go like this:
code :
SELECT  CODE,
        MIN(START_DATE) AS START_DATE,
        CASE 
            WHEN    COUNT(*) > COUNT(END_DATE)
            THEN    NULL
        ELSE
            MAX(END_DATE)
        END AS END_DATE
FROM    SAMPLE
GROUP BY    CODE
Change aggregate functions to output NULL when a element is NULL

Change aggregate functions to output NULL when a element is NULL


By : GeneralChaos
Date : March 29 2020, 07:55 AM
should help you out The sum() function automatically ignores NULL. To do what you want, you need an explicit checK:
code :
select (case when count(foo) = count(*) then sum(foo) end)
from #tmp;
select (case when sum(case when foo is null then 1 else 0 end) > 0
             then sum(foo)
        end)
Aggregate function on categorical features with corresponding binary/ordinal features

Aggregate function on categorical features with corresponding binary/ordinal features


By : user2758402
Date : March 29 2020, 07:55 AM
Does that help I have a dataframe with startup data. , use:
code :
df_new=(df.groupby('company').agg({'exited':'first','funding_rounds':'first',
                      'funding_total':'first','founders_have_degree':'sum'}))
         exited  funding_rounds  funding_total  founders_have_degree
company                                                             
C1            0               6          120.0                     1
C2            1               2          250.0                     2
C3            0               5           50.0                     0
Null in Aggregate Functions

Null in Aggregate Functions


By : user3461893
Date : March 29 2020, 07:55 AM
I wish this help you https://modern-sql.com/concept/null#aggregates says: , Consider this data:
code :
a     b
1     NULL
NULL  2
3     3
1 + NULL  --> NULL
NULL + 2  --> NULL
3 + 3     --> 6
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