Question

A management organization of shows, concert halls, and ticket sales for shows manages a database with the following relational schema:
Show (show_id, title, start_date, duration, singer, hall_id)
Concert (concert_id, date, hour, show_id)
Hall (hall_id, name, address, capacity)

Express the following SQL queries in relational algebra:
What are the singers who performed at least one concert in all halls?

SELECT singer
FROM Show S1
WHERE NOT EXISTS
(SELECT *
FROM Hall H
WHERE NOT EXISTS
(SELECT *
FROM Show S2
WHERE S2.singer = S1.singer
AND H.hall_id = S2.hall_id))

Answer 1

'-' is for intersection. 'Not Exist' can be represented as '-' in relation algebra.

'x' for cartesian product.

select all singers then exclude them as per the mentioned query.