Core Concept Mastery • DBMS Module
Think of Venn Diagrams or Puzzle Pieces.
Inner Join: The intersection. Only where the two circles overlap. (Only Students who have a Library Card).
Left Join: The entire Left circle, plus any overlapping part of the Right circle. If there is no overlap, the Right side is blank (NULL). (All Students, whether they have a card or not).
Right Join: The entire Right circle.
Full Outer Join: Everything in both circles. If they match, great. If not, show them anyway with NULLs on the missing side.
Data is split into pieces to save space. Joins let us recombine them dynamically only when we need to see the full picture.
Inner Join: Show me students WITH a library card. Left Join: Show me ALL students, even those who haven't borrowed a book yet.
"Which join returns all rows from both tables, filling NULLs where there is no match?"
Full Outer Join.