The use of group by in SQL

Source: Internet
Author: User

Group BY is a grouped query, and the general group by IS used with aggregate functions, you can think about

You have used group by to press ITEM. Itemnum This field group, the other field content is different, become a pair of more and change how to display it, such as the following

A B
1 ABC
1 BCD
1 ASDFG

Select A b from table group by A
You said it was a result of finding out.

A B
Abc
1 BCD
Asdfg

The 3 on the right becomes one, so you need to use aggregate functions, such as

Select A,count (B) Quantity from table group by A
So the result is
A Quantity
1 3

Group by has a principle that all columns following the Select Do not have columns that use aggregate functions and must appear behind GROUP by

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