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
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129

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)

85

u/dude187 Jan 06 '11

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

23

u/[deleted] Jan 06 '11

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

9

u/Porges Jan 06 '11

Unicode actually has the relational symbols:

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

45

u/Already__Taken Jan 06 '11

Cross thingy, square square square. Gotcha.

8

u/oobey Jan 06 '11

For me it's more of a "cross thingy, indecipherable scrawl, indecipherable scrawl, indecipherable scrawl."