Doubt in an SQL query

Let's start by looking at your table diagram.

Given your problem, only two all informed tables are important to get you what you need.

Notice that there is a Cliente table and a Conjuge table. Both are related through the column Cod_Cli , as you can see by the yellow "little key" that connects one to another.

That said, we can understand that only married (or dating) clients will have a spouse, that is, they will have their code referenced in the Conjuge table.

So we set up an SQL statement that will count how many clients do not have their code referenced in the Conjuge table, thus identifying single clients:

SELECT COUNT(cliente.Cod_Cli) 
FROM Cliente cliente 
WHERE cliente.Cod_Cli NOT INT (SELECT conj.Cod_Cli FROM Conjuge conj);