StackOverflow2013

Note that there are some explanatory texts on larger screens.

plurals

PO
text
Body
copied!<p>Answering to your three distinct "second questions" :</p> <p>"Sometimes some instructors or database managers want the relational algebra of the queries?"</p> <p>I wouldn't know about instructors, but I'm fairly certain database managers never will (alas). They just want "the job done". Oh wait, and they usually don't want to see any "exotic" queries emerge on their precious databases either. If they do, they'll come and tell you something that basically means that you should <em>NOT</em> have used the full expressive power of the algebra in that query of yours they find so suspicious. Sometimes with good reason, sometimes not.</p> <p>Knowing the relational algebra of the queries, why important?</p> <p>Because if you don't understand the algebra, you basically just don't have a clue about what your SQLs mean/do. But that needn't worry you - the vast majority of so-called and more often than not self-proclaimed "database professionals" don't have a clue either and they can still make a living out of their ignorance. Ask a dozen people in your environment whether anyone can tell you something sensible about generalized transitive closure and you'll see what I mean.</p> <p>And can we all implement all these complex queries by using relational algebra?</p> <p>Yes. Relational algebra/Relational calculus (distinct but equivalent) are the foundation. And they are commonly regarded as "expressively complete" (meaning : they are sufficient to express just any query one could possibly imagine).</p>

Querying!

Guidance

An individual column

Larger individual text columns get their own page to allow for proper reading.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload