
{c.Name|c \in Customers \land c.Balance < 0.0}

\pi[Name](\sigma[Balance<0.0](Customers))


Find the names of customers which have an unpaid order that was issued
before October:

{c.Name| c \in Customers \land \exists_o o \in Orders \land c.CID = o.CID \land o.status != 'paid' \land o.month < 10}

\pi[Name](\sigma[Status != 'paid' \land Month<10](Orders)\bowtie(Customers)

Find the names of all Basel customers to which a product that was stored in
Basel has been delivered since the beginning of September:

{c.Name, p.Label | c \in Customers \land p \in Products \land \exists_o o \in Orders \land o.CID = c.CID \land
o.PID = p.PID \amd c.City = 'Basel' \land p.Location = 'Basel' \land o.Status = 'delivered' \land o.Month >= 9}

 Find the names of all Basel customers to which a product that was stored in
Basel has been delivered since the beginning of September, together with the
label of the corresponding product:

\pi[Name](\sigma[Location = 'Basel](Products)\bowties(\sigma[Status = 'delivered' \land Month >= 9](Orders))\bowtie(\sigma[City = 'Basel'](Customers)))




