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I'm stuck with relational algebra.
I mean, how can I express functions like "SUM(), COUNT()," etc in RA?
Thanks, any help will be kindly appreciated
To start, you simply have to know that SUM and COUNT are called aggregate functions. Then, just google "relational algebra for aggregate functions".
I don't know how to subscript/superscript on Stack Overflow, but this wikipedia entry has an example at the end of the paragraph. (You'll see GMax(Balance)(Account).)
Also promising: go to this link, and search the page for "Aggregate"... you'll see Fsum(salary)(E) with subscripts.
something like this?
SQLFiddle example
what cannot you understand? be more specific please.
You can find more details in your DBMS documentation
Or visit the wiki
Basically aggregate functions will make some kind of "cumulative" operation over a set of rows and usidng one column to (SUM, AVERAGE, etc) based on one or more "key" columns.
Example: You have a dataset containing demographic data from all cities of the nation, it happens to have a population count and a region identity column. You can use it to create a report of total population by region.
Related
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CrimeData Table for 12 months
Crime Took place in Easternmost
I need to find the following:
Q.7 What type of Crime takes place in the …
(a) Easternmost ………………..
(b) Westernmost ………………..
(c) Northernmost………………..
(d) Southernmost………………..
I tried to find the crime took place in the Easternmost using the following SQL code
SELECT Max(CrimeData.Easting) AS MaxOfEasting, CrimeData.Type
FROM CrimeData
GROUP BY CrimeData.Type;
but I got more than one crime and also other Easting numbers. Can you please tell me if there are other good ways to find the solution.
Please see the attached pictures :)
Rather than using Max/Min, have a look at the TOP keyword in SQL. Some SQL might look like:
SELECT TOP 1 CD.*
FROM CrimeData CD
ORDER BY CD.Easting DESC;
Regards,
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So this isn't a technical question, but rather questioning why a language is designed the way it is.
I've been learning SQL and one thing that's been bothering me greatly is how SQL asks you to name the column you want and THEN name the table you want to get it from. To me, it would make more sense that you refer to the parent body (which is the table) and THEN the column it has. But SQL seems to forces users to do it the other way around. Why?
I'm just curious as to why the language is designed this way.
SELECT column
FROM table
why not
FROM table
SELECT column
SQL tries to mimic English language to some extent, so that it feels natural to formulate the query.
In spoken English you would say something like "I want the names of the employees". You would not say "I want of the employees their names" or something like that.
But you are right, it might have been a good idea to have the query represent the order of execution. And "From the employee table I want the names" would not be so far off the mark :-)
SQL is a descriptive language, not a procedural language. It describes the result set being produced. And, you can think of that result set as a report, with column headers.
As such, the basic querying construct returns those column headers. The rest of the query describes how they are produced.
You may find this post useful. Starting with FROM is the most logical way to think about a query (Why would anyone write SELECT before knowing what to SELECT from?). However, SQL guidelines were designed as if your query were a command. Thus, you are commanding the system to SELECT the data for you, and the FROM further specifies that command.
Of course, the actual execution is distinct from the lexical and logical orders above.
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I have a problem writing regular expression. I want to write a regular expression that replaces all double consonants with a single consonant.
Please help me to write such a rule in only one line.
Thanks in advance.
Here's a .NET regex that'll find any group of exactly two non-vowels:
[^aeiou]{2}
The following will work for groups longer than 2:
[^aeiou]{2,}
For example, this will match "llst" in "allstar."
Slightly uglier, but will match groups of 2 consonants, case-insensitive:
[QqWwRrTtYyPpSsDdFfGgHhJjKkLlZzXxCcVvBbNnMM]{2}
The following will match two identical non-vowels:
([^aeiou])\1
For example, this would match the "ll" in "all."
Once you have your regex, just use your chosen language's Regex.Replace function.
Since you did not specify the language, I'm going to go ahead and assume Javascript.
This should get you started:
console.log('babble bubble http htttp www'.replace(/([^aeiou\.,\/=?:\d&\s!##$%^*();\\|<>"'_+-])\1{1}/gi, "$1"));
See more here:
http://regexr.com/3ee47
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I have an SQL in Access 2010 that was written by someone else that gives a Wrong Number of Arguments error when I try and run it. It's supposed to filter a report from a search page.
SELECT Activity.[ProjNo], Activity.[Code], Activity.[Type], Activity.[ProjNoStatus],
Activity.[Preliminary], Activity.[Planner], Activity.[Designer],
Activity.[Officer], Activity.[Manager], Activity.[Staff], Activity.[Analyst],
Activity.[Manager], Activity.[DeptHead], Activity.[ContractNumber],
Activity.[InfoOfficer],Activity.[ProjNoDesigner]
FROM Activity
WHERE Activity.ProjNo=Index.ProjNo AND (((IIf([Forms]![SearchForm]![txtCode]="",
"*",[Activity].[Code]=[Forms]![SearchForm]![txtCode]))<>False)
AND ((IIf([Forms]![SearchForm]![txtType]="","*",[Activity].[ Type]="",
"*", [Activity].[Type]=[Forms]![SearchForm]![txtType]))<>False) AND
((IIf([Forms]![SearchForm]![txtProjNoStatus]="","*",
[Activity].[ProjNoStatus]<=[Forms]![SearchForm]![txtProjNoStatus]))<>False));
I'm not very experienced with SQL and, like I said, I didn't write this code (the person who did has long since retired) so any help would be great.
That query defines just one data source (table or query):
FROM Activity
But then the WHERE clause appears to reference another data source named Index:
WHERE Activity.ProjNo=Index.ProjNo
Since Index is not included in the FROM clause, Access will object when you try to use it in the WHERE clause.
However, I'm not sure that is the cause of the first error Access complains about. It may help to show us the full text of that error message.
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I am new to databases. And our teacher gave us pretty hard assignment. There are two tables. First table nickname is abilities(of superhero's:) ) and second table name superheros.
We have to select nick of Superhero and his average(medial) range for those who has two abilities?
Image of both tables:
Original here: http://postimg.org/image/85pqbc47n/
I will not give you solution - after all, it's homework and you have to learn something :) But I can give you an advice - try to do one task at a time
first, find those superheroes who has only 2 abilities (actually, you can do this by quering only table with abilities)
second - try to find average range of abilities for all superheroes (here you'll need join)
combine your queries
take a look at join, group by, count and having
Don't feel bad if you can't write it at first attempt, your query is not super easy, but 'm sure you can do this.
You can use HAVING and AVG() for this:
SELECT s.NickName, AVG(a.Range)
FROM abilities a
JOIN superhero s
ON a.ID_SuperHero = s.ID_SuperHero
GROUP BY s.NickName
HAVING COUNT(DISTINCT a.Abilities > 1)