Return a sum in different rows in mysql

Replace column and table names:

select sum(case when operacao='01' then valor else 0 end) deposito,
       sum(case when operacao='02' then valor else 0 end) retirada,
       sum(case when operacao='01' then valor else -1*valor end) saldo
  from tabela
 where id_cliente = 51298

MySQL already has its own tool for conditional situations. It is the IF function:

IF( expressão de teste, valor se verdadeiro, valor se falso )

Applying to the question case:

Just a single SUM with IF to get the balance calculated in a single pass through the DB:

SELECT SUM( IF( operacao='01', valor, -valor ) ) AS saldo FROM tabela

Result:

saldo
66.66

See working on SQL Fiddle .

Variants:

If it is to see the balance of a specific customer:

SELECT SUM(IF(operacao='01',valor,-valor )) AS saldo FROM tabela WHERE id_cliente=51298

If it is for multiple clients in the same search:

SELECT SUM( IF( operacao='01', valor, -valor ) ) AS saldo FROM tabela GROUP BY id_cliente

You can try this way

select sum(case when operacao=2 then valor*-1 else valor end) 
from tabela 
GROUP by id_cliente

Ready:

select 
   (sum(valor) from tabela where operacao = '01') - 
   (sum(valor) from tabela where operacao = '02') saldo

I'm considering that the operation field is a varchar / string. If it is numeric, remove the quotation marks.