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0 pointsnendroid5y ago0 comments

It's not about flipping. The two concepts are actually commutative. You could in theory create syntax that looks like this:

  FROMCLAUSE * SELECTCLAUSE * WHERECLAUSE = SQLEXPRESSION
  SELECTCLAUSE * WHERECLAUSE * FROMCLAUSE = SQLEXPRESSION
  ...

The issue is that not only does SQL syntax force an artificial order on these clauses, but that these clauses Cannot be decomposed to be used elsewhere. I cannot reuse a WHERECLAUSE or a SELECTCLAUSE in another expression.

0 comments

nendroidOP5y ago

Some detail here. What goes on in relational algebra is that the FROMCLAUSE is encoded into an axiomatic primitive called a RELATION and you get stuff like this:

   SELECTCLAUSE(WHERECLAUSE(relation)) = relation
   WHERECLAUSE(SELECTCLAUSE(relation)) = relation

Basically every operation in relational algebra produces a primitive of type RELATION which allows for all operations to be composed like unix pipes.

My example in the previous post has a flaw where it's not clear what:

   SELECTCLAUSE * WHERECLAUSE = ????

will output because there's no meaning to a SQLEXPRESSION without a FROMCLAUSE. But hopefully it illustrates the point. I'm putting this here for anyone who's nitpicky about the details. The relational algebra syntax is much more elegant.

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