usando exists()
para obtener todos los clientes y etiquetas cuando ese cliente tiene una etiqueta de índice 1:
select ct.customernumber, ct.tagindex, c.date
from customers c
inner join customers_tags ct
on c.customernumber = ct.customernumber
where exists (
select 1
from customers_tags i
where i.customernumber = ct.customernumber
and i.tagindex = 1
)
o usando in()
:
select ct.customernumber, ct.tagindex, c.date
from customers c
inner join customers_tags ct
on c.customernumber = ct.customernumber
where c.customernumber in (
select i.customernumber
from customers_tags i
where i.tagindex = 1
)