Over a relation Borrow$($reader$,$book,date$),$ consider the following SQL expression:

select date from Borrow

except

select date from Borrow where reader$=$'Ali'

Which statements are correct? $($In relational algebra we use project to indicate projections and sigma to indicate selections.$)$

An equivalent SQL expression is the following:

select date from Borrow where reader $<>$ 'Ali'

The expression returns all dates when a book was borrowed by someone else than Ali.

The expression returns all dates when the only reader who borrowed something was Ali.

The following relational algebra expression is NOT equivalent:

project$[$date$]($Borrow minus sigma$[$author$=$'Ali'$]($Borrow$))$

We can write the same query in relational algebra as follows:

project$[$B$1.$date$]$

sigma$[$B$1.$reader $<>$ 'Ali'$]$

sigma$[$B$1.$date$=$B$2.$date$]$

$($rename$[$B$1]($Borrow$)$ cart$-$prod rename$[$B$2]($Borrow$)$

The expression returns all dates when Ali did not borrow a book.