What is semi-join in database?

Simple example. Let’s select students with grades using left outer join:

SELECT DISTINCT s.id
FROM  students s
      LEFT JOIN grades g ON g.student_id = s.id
WHERE g.student_id IS NOT NULL

Now the same with left semi-join:

SELECT s.id
FROM  students s
WHERE EXISTS (SELECT 1 FROM grades g
              WHERE g.student_id = s.id)

The latter is much more efficient.