I'm having problems with MySQL, I do not know how to pull media

To calculate the average in MySQL we use the function AVG .

In this way you filter the form of payment by the code, which in this case is 6 :

SELECT a.'Tipo', AVG(b.'ValorPagar') AS media FROM FormaPagamento a
INNER JOIN Pagamento b ON a.'CodigoPagForm' = b.'CodigoFormaPagamento'
WHERE a.'CodigoPagForm' = 6
GROUP BY a.'Tipo';

And this way you filter by type:

SELECT a.'Tipo', AVG(b.'ValorPagar') AS media FROM FormaPagamento a
INNER JOIN Pagamento b ON a.'CodigoPagForm' = b.'CodigoFormaPagamento'
WHERE a.'Tipo' = 'Boleto'
GROUP BY a.'Tipo';

See more about the AVG function here .     

Calculate the mean:

Select codigo, AVG(valor) FROM Tabela 

Calculate the mean with target values:

Select codigo, AVG(DISTINCT valor) FROM Tabela

Calculate the mean and sort the result by the identifier (id):

Select codigo, AVG(valor) from Tabela GROUP BY codigo

Calculates the mean of the target values and sorts the result by the identifier (id):

Select codigo, AVG(DISTINCT valor) from Tabela GROUP BY codigo

Example by W3sscoming .

Documentation MySql .