I've come across statements such as "the default clause" in the context of a switch statement or "the JOIN clause" in the context of SQL and I know what these statements mean but still I'd like to see a clear definition of the term.
It's pretty much the same as in English (or another language). A clause is an incomplete fragment of a sentence, or in this case a statement, that encapsulates an actor and an action. In your example of a join clause, the action is the join and the actor is the table being joined.
I think that there is a parallel between a sentence and a dependent clause in natural languages and between a statement and a clause in computer languages.
A clause does not stand by itself, but only makes sense within the context of a statement.
For example, the clauses
"at 2:30" (adverbial clause)
WHERE A.ID = B.ID (JOIN clause)
don't stand meaningfully on their own. However, those clauses make sense in the context of a sentence or statement.
"Meet me at 2:30."
SELECT A.NAME, B.ADDRESS FROM A, B WHERE A.ID = B.ID
A default clause in a switch statement of Java refers to the branch that is taken if none of the checked values match. The word "default" is somewhat problematic, as it usually means a failure of some sort (as in, "Your mortgage is in default"). With computer languages, it tends to mean "the unchosen choice."
It is a "clause" because it makes sense only in the context of other choices.
Like many terms in programming it simply means what it means in the English language.
When I first saw COALESCE in SQL for example, I thought "What the hell?". But you know what? It does what it says on the tin:
In phonetics and historical
linguistics, fusion, or coalescence,
is the merger of the features of two
segment into one.
Related
For example when i try this i get no sorting:
SELECT A,B
FROM Table
ORDER BY C
But i only want to display column A,B in the output
I am using MySQL
it can be achieved by adding Sub Query in 'from' clause.
You can do something like below
select
t.A,
t.B
from
(SELECT A,B,C FROM Table ORDER BY C) t;
Don't go with this solution it wrong one , i was not aware that is ORDER BY limited to the outermost "SELECT".
thanks #jariha and #Erwin for clarification in comments.
This should be possible, sanctioned by what the current version of the SQL standard calls "extended sort keys".
However :
It constitutes a departure from SQL's original intent, whereby there was a "conceptual order of execution" in which the ORDER BY was executed after the SELECT, meaning anything mentioned in the ORDER BY had to be contained in the SELECT.
It constitutes yet one more departure from Codd's foundational "information principle", because the result now "carries meaning" that is only expressed by the ordering of the rows as returned by the DBMS. For this reason, it might arguably be the case that SQL cannot even legitimately be seen as implementing a bag algebra, when what it should really have been was set algebra. Don't think it won't ever bite you ...
Although, being part of the core spec of the standard and not one of the (many) "optional features", it should be the case that all engines support this feature correctly, you might run into one that doesn't because it still goes by the elder rule. Or that supports this feature only if you install some optional module or change some configuration option or so.
Although the case you mention should work properly, there are quite a bit of cases where it isn't (and cannot be) allowed. Decrypting the rules in this respect is, as usual, not a very trivial exercise. For example, if you later want to add a LIMIT or TOP n clause or some such, it seems highly likely that this change will move the query into "ORDER BY C not allowed" territory.
For these reasons, it is not terribly unwise to follow "principle of cautious design" and just stick with the old rules.
So just do SELECT A,B,C FROM ... and ignore the C column in whatever processing you do on the client side.
We seem to have a need for a multi-table JOIN operation and I am referring to some notes from an RDBMS class that I took several years ago. In this class the instructor graphically depicted the structure of a generic N-table JOIN query.
The figure seems to conform to examples of multi-table JOINs that I have seen but I have a question. Does the WHERE clause, for providing filtering, necessarily have to be the last clause in the query? Intuitively it appears that we can impose filtering conditions before a following JOIN clause, in order to properly scope the data, before we input it to the next JOIN operation.
Syntactically, the where clause has to be at the end. But the query plan will take it into account and use it to filter wherever possible. Note that just because you specify the from and joins in a given order doesn't mean the query will actually execute that way; it may rearrange them to whatever order it thinks will work best (unless you specify straight_join).
That said, having the where at the end does make some queries actually harder to read.
SQL queries consist of a sequence of clauses. The diagram you have is rather misleading. Common clauses -- and the order they must appear for a valid query -- are:
SELECT
FROM
WHERE
GROUP BY
HAVING
ORDER BY
Note that JOIN is not a clause. It is an operator, and an operator that specifically appears only in the FROM clause.
So, the answer to your question is that WHERE clauses immediately follow the FROM clause. The only "sort-of" exception is when a "window" clause is included and that is syntactically between the FROM and the WHERE.
Next, multiple table joins are often quite efficient and there is no reason whatsoever to discourage their use. Support for joins, in fact, is one of the key design features that databases are designed around.
And finally. What actually gets executed is not the string that you create. A query, in fact, describes the result set you want. It does not describe the processing. SQL is a descriptive language, not a procedural language.
The SQL engine has two steps to convert your query string to an executable form (typically a directed acyclic graph). One is to compile the query, and the second is to optimize the query. So, where filtering actually occurs . . . that depends on what the optimizer decides. And where it occurs has little relationship to what you think of when you think of SQL queries (DAGs don't generally have nodes called "select" or "join").
So, I don't really understand the purpose of using an implicit join in SQL. In my opinion, it makes a join more difficult to spot in the code, and I'm wondering this:
Is there a greater purpose for actually wanting to do this besides the simplicity of it?
Fundamentally there is no difference between the implicit join and the explicit JOIN .. ON ... Execution plans are the same.
I prefer the explicit notation as it makes it easier to read and debug.
Moreover, in the explicit notation you define the relationship between the tables in the ON clause and the search condition in the WHERE clause.
Explicit vs implicit SQL joins
When you join several tables no matter how the join condition written, anyway optimizer will choose execution plan it consider the best. As for me:
1) Implicit join syntax is more concise.
2) It easier to generate it automatically, or produce using other SQL script.
So I use it sometimes.
Others have answered the question from the perspective of what most people understand by "implicit JOIN", an INNER JOIN that arises from table lists with join predicates in the WHERE clause. However, I think it's worth mentioning also the concept of an "implicit JOIN" as some ORM query languages understand it, such as Hibernate's HQL or jOOQ or Doctrine and probably others. In those cases, the join is expessed as a path expression anywhere in the query, such as e.g.
SELECT
b.author.first_name,
b.author.last_name,
b.title,
b.language.cd AS language
FROM book b;
Where the path b.author implicitly joins the AUTHOR table to the BOOK table using the foreign key between the two tables. Your question still holds for this type of "implicit join" as well, and the answer is the same, some users may find this syntax more convenient than the explicit one. There is no other advantage to it.
Disclaimer: I work for the company behind jOOQ.
For example:
SELECT *
FROM a
JOIN b ON a.b_id = b.id
AND b.col = 'something'
vs
SELECT *
FROM a
JOIN b ON a.b_id = b.id
WHERE b.col = 'something'
I would assume that MySQL's query optimizer would regard this the same query. Are they the same in all such cases, whether the WHERE column is on table a or table b?
These queries will be handled the same way by MySQL. You can verify this by placing EXPLAIN EXTENDED in front of either query and looking over the Query Execution Plan. If you need a good resource for understanding the output of the EXPLAIN query, check out http://www.sitepoint.com/using-explain-to-write-better-mysql-queries/ (if that link ever breaks, search the web for "Understanding MySQL Explain" and you'll have no trouble finding a resource.
In general, I would recommend the second form you used. It's not so much a matter of technical reasons related to query execution as it is related to ease of modifying code later on, when/if you need to. For example, suppose you added four or five more joins. It would be difficult to read this query if WHERE clauses were sprinkled all over the place.
Keeping your join clauses and your WHERE filters separate is definitely something I would consider a best practice, but it's to do with ease of reading/editing, not because you're gonna end up with a different query execution plan (unless you make a mistake, which I think most people would be more likely to do with the first query as opposed to the second)
Few months ago I was programming a simple application with som other guy in PHP. There we needed to preform a SELECT from multiple tables based on a userid and another value that you needed to get from the row that was selected by userid.
My first idea was to create multiple SELECTs and parse all the output in the PHP script (with all that mysql_num_rows() and similar functions for checking), but then the guy told me he'll do that. "Okay no problem!" I thought, just much more less for me to write. Well, what a surprise when i found out he did it with just one SQL statement:
SELECT
d.uid AS uid, p.pasmo_cas AS pasmo, d.pasmo AS id_pasmo ...
FROM
table_values AS d, sectors AS p
WHERE
d.userid='$userid' and p.pasmo_id=d.pasmo
ORDER BY
datum DESC, p.pasmo_id DESC
(shortened piece of the statement (...))
Mostly I need to know the differences between this method (is it the right way to do this?) and JOIN - when should I use which one?
Also any references to explanations and examples of these two would come in pretty handy (not from the MySQL ref though - I'm really a novice in this kind of stuff and it's written pretty roughly there.)
, notation was replaced in ANSI-92 standard, and so is in one sense now 20 years out of date.
Also, when doing OUTER JOINs and other more complex queries, the JOIN notation is much more explicit, readable, and (in my opinion) debuggable.
As a general principle, avoid , and use JOIN.
In terms of precedence, a JOIN's ON clause happens before the WHERE clause. This allows things like a LEFT JOIN b ON a.id = b.id WHERE b.id IS NULL to check for cases where there is NOT a matching row in b.
Using , notation is similar to processing the WHERE and ON conditions at the same time.
This definitely looks like the ideal scenario for a join so you can avoid returning more data then you actually need. This: http://www.w3schools.com/sql/sql_join.asp or this: http://en.wikipedia.org/wiki/Join_(SQL) should help you get started with joins. I'm also happy to help you write the statement if you can give me a brief outline of the columns / data in each table (primarily I need two matching columns to join on).
The use of the WHERE clause is a valid approach, but as #Dems noted, has been superseded by the use of the JOINS syntax.
However, I would argue that in some cases, use of the WHERE clauses to achieve joins can be more readable and understandable than using JOINs.
You should make yourself familiar with both methods of joining tables.