“One of the things that confuse SQL users all the time is how DISTINCT and ORDER BY are related in a SQL query ... most people quickly understand:
SELECT DISTINCT length
FROM film
[that] returns results in an arbitrary order, because the database can (and might apply hashing rather than ordering to remove duplicates) ... Most people also understand:
SELECT length
FROM film
ORDER BY length
[that] will give us duplicates, but in order ... And, of course, we can combine the two:
SELECT DISTINCT length
FROM film
ORDER BY length
[But if] somewhat intuitively, we may want to order the lengths differently, e.g. by title:
SELECT DISTINCT length
FROM film
ORDER BY title
[m]ost databases [sic] fail this query with an exception like Oracle’s:
ORA-01791: not a SELECTed expression
At first sight ... this
SELECT length
FROM film
ORDER BY title
works after all ... So, how are these different? We have to rewind and check out the logical order of SQL operations (as opposed to the syntactic order). And always remember, this is the logical order, not the actual order executed by the optimiser.”
--How SQL DISTINCT and ORDER BY are Related, Jooq.org
8/18/18: I have re-written this post for a better explanation. If you read it prior to the revision, you should re-read it.
“I learned that there is no concept of order in terms of tuples (e.g. rows) in a table, but according to wikipedia "a tuple is an ordered list of elements". Does that mean that attributes do have an order? If yes why would they be treated differently, couldn't one add another column to a table (which is why the tuples don't have order)? [OTOH], "In this notation, attribute–value pairs may appear in any order." Does this mean attributes have no order?”
--Do the “columns” in a table in a RMDB have order?
“Is it possible to reorder rows in SQL database? For example, how can I swap the order of 2nd row and 3rd row's values? The order of the row is important to me since i need to display the value according to the order [and] 'Order by' won't work for me. For example, I put a list of bookmarks in database. I want to display based on the result I get from query. (not in alphabet order). Just when they are inserted. But user may re-arrange the position of the bookmark (in any way he/she wants). So I can't use 'order by'. An example is how the bookmark display in the bookmark in firefox. User can switch position easily. How can I mention that in DB?”
--How can I reorder rows in sql database
While some data professionals may know that rows and columns of "database tables" are "unordered", few of them know what that means, and understand why. This is due to two, not unrelated, of the many common misconceptions[1] rooted in the lack of foundation knowledge in the industry, namely that relational databases consist of tables[2], and logical-physical confusion (LPC)[3]. They obscure understanding of the RDM and its practical implications, which is reflected in the answers to the above questions. Instead of debunking them, this post fills the gap in knowledge such that you can debunk them yourself -- try it before and after you read it.
“The choice of good InnoDB primary keys is a critical performance tuning decision. This post will guide you through the steps of choosing the best primary key depending on your workload ... You would be surprised how many times I had to explain the importance of primary keys and how many debates I had around the topic as often people have preconceived ideas that translate into doing things a certain way without further thinking.”
--Yves Trudeau, Principal architect, Percona.com
I will be labeled "pedantic" and a "purist" for saying this, but there is no such thing as "PK tuning". "Choice of good PK for performance" reflects logical-physical confusion (LPC), which, in turn, is rooted in lack of foundation knowledge, and failure to grasp the RDM -- not just by database practitioners, but even by DBMS designers.
