Group By usage

Aggregate functions
SUM ()

COUNT function -- COUNT ()

MAX/MIN functions-MAX ()/MIN ()

Mean function-AVG ()

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Group by is a GROUP query. Generally, group by is used with Aggregate functions.

You use group by to group by the ITEM. ITEMNUM field. If the content of other fields is different, how can this problem be displayed?

A B
1 abc
1 bcd
1 asdfg

Select A, B from table group by
What are the results,

A B
Abc
1 bcd
Asdfg

How do the three entries on the right become one? So we need to use aggregate functions, such

Select A, count (B) quantity from table group by
The result is
A quantity
1 3

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Store_Information table

Store_name Sales Date

Los Angeles $1500 Jan-05-1999

San Diego $250 Jan-07-1999

Los Angeles $300 Jan-08-1999

Boston $700 Jan-08-1999

Let's get
SELECT store_name, SUM (Sales) FROM Store_Information group by store_name

Result:

Store_name SUM (Sales)

Los Angeles $1800

San Diego $250

Boston $700

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The principle of group by is that all columns after select do not use aggregate functions and must appear after group.