O que é GROUP BY?

Vitor,

A ideia do "Group By" é agrupar os resultados por um determinada coluna.

Por favor, leia os links abaixo e faça os exemplos:

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Cláusula GROUP BY no MySQL

Usamos a cláusula GROUP BY para agrupar registros em subgrupos baseados em colunas ou valores retornados por uma expressão.

http://www.bosontreinamentos.com.br/mysql/mysql-group-by-agrupamento-de-registros-26/

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MySQL GROUP BY Statement

The GROUP BY statement groups rows that have the same values into summary rows, like "find the number of customers in each country".

The GROUP BY statement is often used with aggregate functions (COUNT(), MAX(), MIN(), SUM(), AVG()) to group the result-set by one or more columns.

https://www.w3schools.com/mysql/mysql_groupby.asp

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12.20.3 MySQL Handling of GROUP BY

SELECT o.custid, c.name, MAX(o.payment)
  FROM orders AS o, customers AS c
  WHERE o.custid = c.custid
  GROUP BY o.custid;

https://dev.mysql.com/doc/refman/5.7/en/group-by-handling.html

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MySQL GROUP BY

https://www.mysqltutorial.org/mysql-group-by.aspx

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MySQL Tutorial » MySQL GROUP BY

Syntax:

SELECT
column1, column2,..., columnm, aggregate_function(columni)
FROM
target_table
WHERE
conditions_or_constraints
GROUP BY criteriacolumn1 , criteriacolumn2,...,criteriacolumnj;

https://www.educba.com/mysql-group-by/

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[]'s,

Fabio I.