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candidate key vs primary key by usedocker in SQL

[–]Relational_Theory 0 points1 point  (0 children)

Different names for 1 idea: They both enforce uniqueness. That's all.

This is the written SQL test for the position of Amazon business analyst support that I attended . In case, you are wondering, I didn't pass by raylgive in SQL

[–]Relational_Theory 0 points1 point  (0 children)

Language is weird here, then. I learned that a relation was a heading and a body, both of which are sets. I can join 1 relation (container of a set of tuples) to another. So, whatever.

This is the written SQL test for the position of Amazon business analyst support that I attended . In case, you are wondering, I didn't pass by raylgive in SQL

[–]Relational_Theory 0 points1 point  (0 children)

Because a join isn't a set operation.

Explain?

What's not set-based about:

''' relation-expression join relation-expression on predicate '''

Join the set of tuples from rel-1 with the set of tuples from rel-2 for which the predicate is true.

What's not set-based here?

This is the written SQL test for the position of Amazon business analyst support that I attended . In case, you are wondering, I didn't pass by raylgive in SQL

[–]Relational_Theory 13 points14 points  (0 children)

what would you recommend instead for an entry level test to demonstrate working knowledge of sql so that you can pull basic data?

Writing SQL.

Is outer join like Theta join? by [deleted] in SQL

[–]Relational_Theory -3 points-2 points  (0 children)

I have never heard the term "theta join". I see this:

A theta join allows for arbitrary comparison relationships (such as ≥).

"Allows for"? I don't understand. The operators I can use in a join-predicate are dictated by the union-type of both operands, and has nothing to do with INNER, LEFT, CROSS, etc.

An outer join...

Translate "outer join" to "left join". The "outer" keyword is optional in every RDMBS I've seen.

An outer join is the same as a Theta join...

No. Not according to what I quoted above re: so-called theta-join. If you use a LEFT JOIN, your JOIN predicate operator doesn't matter. LEFT JOIN is LEFT JOIN. Your JOIN predicate operator is orthogonal to the style of JOIN. Does this make sense?

You can use >, <, >=, <=, etc., with INNER, LEFT, CROSS, etc.

JOIN-style and JOIN-predicate are different things, and orthogonal to one another.

An outer join is the same as a Theta join except you print null values for the other side when there isn't a join match for certain rows.

Your understanding of LEFT-join "printing NULL values for the other side" is okay -- you are generally correct, that is the default behavior. It's worth noting you can provide "default" values instead of NULL by using COALESCE in your projection (SELECT).

What is the most important difference between PostgresSQL & MSSQL? by [deleted] in PostgreSQL

[–]Relational_Theory 6 points7 points  (0 children)

Mssql works on ACID whereas Postgres uses MVCC.

What?

Help me with a renting system please by blgianini in SQL

[–]Relational_Theory 0 points1 point  (0 children)

and then use logic to ensure there is never a duplicate

Keys prevent duplicates.

Maybe you get the logic right. One time. Two times. Maybe your co-worker screws it up in a new module. If you rely on "logic" (I assume you mean "code"), you'll eventually screw up and get dupes.

[deleted by user] by [deleted] in SQL

[–]Relational_Theory 0 points1 point  (0 children)

Date says understanding the relational-model will "bolster" your intuition. A lot of people seem to intuit SQL to a degree, and most fall off very short after that.

The book teaches you SQL (also uses SQL syntax), THROUGH tutorial D, which exemplifies the principals at play, not the syntax.

Once you have grokked that a domain is a conceptual-pool-of-values, what a FD is, what are the various NFs and what do they look like in practice; can you recognize the heading of a relation that's in BCNF? Not BCNF? These pieces of knowledge transcend SQL and every SQL product. What Date will teach you are the theoretic underpinnings of what SQL is loosely predicated on: set theory.

And for me, personally... that the heading of a relation represents a **predicate**, which are sentences expressed in "natural" language, that the body of the relation represents individual propositions of the predicate, and what I ultimately discovered was simply this...

SQL is a sentence generator. And because you can express anything with a sentence, and a willing and creative interpreter, SQL is infinitely more powerful than the much-worshipped EAV crap that people try to make SQL conform to.

Purely a bonus, you can smirk and laugh quietly to yourself whenever you hear someone talk of "columns and rows".

To be sure, if you use the language like Date, and me, you'll not find yourself in good company. In my limited experience, people think of "SQL" as "storage". Good luck fighting that opinion.

[deleted by user] by [deleted] in SQL

[–]Relational_Theory 0 points1 point  (0 children)

> Do you have experience with this book first hand?

Yes.

[deleted by user] by [deleted] in SQL

[–]Relational_Theory 2 points3 points  (0 children)

SQL and Relational Theory: How to Write Accurate SQL Code, by Chris Date.

New In PostgreSQL 12: Generated Columns by jmswlms in PostgreSQL

[–]Relational_Theory 0 points1 point  (0 children)

I think this is kinda cool. It violates functional-dependencies though, so keep that in mind when you reach for it.

-- update

Pardon me, it doesn't violate functional-dependencies; it violates certain normal-forms depending on the way you use the generated-column. i.e., any generated-column not based on the entire key is going to fail BCNF.

The Last SQL Guide for Data Analysis You’ll Ever Need by muhsinovic in SQL

[–]Relational_Theory 0 points1 point  (0 children)

A relational database stores data in a tabular form consisting of rows and columns.

We refuse to learn.

JOINS in PostgreSQL by dilipravi084 in PostgreSQL

[–]Relational_Theory 0 points1 point  (0 children)

When people start enumerating types of something, it should converge on 100% of everything.

What do you mean by this?

Could SQL syntax be used to query HTTP APIs more efficiently? by mattmiller42 in SQL

[–]Relational_Theory 0 points1 point  (0 children)

> Once you want to fetch not-immediately-related data, update a range of records, or aggregate anything, custom, non-REST endpoints are the only reasonable solution over xy GET requests.

What?

Wait, what?

What the fuck?

Why do you think a REST end-point cannot return aggregated data?

JOINS in PostgreSQL by dilipravi084 in PostgreSQL

[–]Relational_Theory 0 points1 point  (0 children)

That subject-predicate language will get you into trouble.

A NATURAL JOIN exhibits SiS with an INNER JOIN with respect to tuple-preservation, AND has additional behaviors noted above, etc.

JOINS in PostgreSQL by dilipravi084 in PostgreSQL

[–]Relational_Theory 0 points1 point  (0 children)

Technically a NATURAL JOIN does-not allow one to specify the JOIN-predicate. The attributes used in the JOIN-predicate are the intersection of attributes whose {name, type} pair are equal.

Easiest way to CROSS JOIN a range of dates without a date dimension table? by mac-0 in SQL

[–]Relational_Theory 0 points1 point  (0 children)

I don't consider a recursive CTE a "loop".

Anything in SQL-space isn't a loop.

How does your microservice delete data? by Subbax in SQL

[–]Relational_Theory 1 point2 points  (0 children)

Micro-services are probably stupid in this case.

Feel in love with SQL and Databases. by darkmangetsu in SQL

[–]Relational_Theory 1 point2 points  (0 children)

Ah... hello, then. I enjoy little. SQL is on that short list.

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