An “anti-join” between two tables returns rows from the first table
where no matches are found in the second table. An anti-join is
essentially the opposite of a semi-join: While a semi-join returns one
copy of each row in the first table for which at least one match is
found, an anti-join returns one copy of each row in the first table
for which no match is found.
Anti-semijoin is anti join.
The antijoin can also be defined as the complement of the semijoin, as follows:
R ▷ S = R − R ⋉ S
Given this, the antijoin is sometimes called the anti-semijoin, and the antijoin operator is sometimes written as semijoin symbol with a bar above it, instead of ▷.