Question

I need to select items with defined idBook from exemplar table which are currently not ordered(booked)- dateReturn field in "orders" table is not null.

Table orders can contain previously fulfilled orders with the same idExemplar!

If exemplar has at least one record in "orders" table with dateReturned=null then this exemplar should be excluded!

I’ve tried to use this query:

select * 
from exemplar 
join orders on orders.idExemplar=exemplar.idExemplar
where dateReturned is not null

but it gives wrong result if table orders contains previusly fulfilled orders with the same idExemplar

Answer 1

Similar to @vizier’s answer, but expressed differently:

1) Using NOT IN:

SELECT *
FROM exemplar
WHERE idExemplar NOT IN (SELECT idExemplar FROM orders WHERE dateReturned IS NULL)

2) Using LEFT JOIN + WHERE IS NULL:

SELECT e.*
FROM exemplar e
LEFT JOIN ON orders o ON e.idExemplar = o.idExemplar AND o.dateReturned IS NULL
WHERE o.idExemplar IS NULL