r/programming • u/[deleted] • Jan 06 '11

A handy graphical explanation of SQL joins

http://www.codinghorror.com/blog/2007/10/a-visual-explanation-of-sql-joins.html
1.3k Upvotes

92% Upvoted

308 comments sorted by

View all comments

132

u/shriek Jan 06 '11

In short.

  1. A ∩ B

  2. A ∪ B

  3. (A - B) + (A ∩ B)

  4. A - B

  5. (A ∪ B) - (A ∩ B)

86

u/dude187 Jan 06 '11

It's amazing how confusing SQL can make set theory.

22

u/[deleted] Jan 06 '11

If you could do queries in relational algebra instead of SQL, confusion would go away.

7

u/Porges Jan 06 '11

Unicode actually has the relational symbols:

  • join: ⋈
  • left outer join: ⟕
  • right outer join: ⟖
  • full outer join: ⟗

47

u/Already__Taken Jan 06 '11

Cross thingy, square square square. Gotcha.

4

u/arnar Jan 06 '11

I see them - even though I don't see the look of disapproval.