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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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As a developer, a common mistake that I keep on repeating is assuming the data type of a column. I have read multiple articles regarding SQL column naming convention but have not seen any reference regarding data type as part of the column name - specifically for SQL Server.
E.g. Revenue_f for float, Organization_v for varchar, AccountNumber_i for integer and so on.
This must have been thought of already before but I want to know the reason why it is not being used, or an expert's input regarding the matter; pointing me to the right article/documentations will be greatly appreciated.
That is a horrible naming convention. Consider how awful it would be if you need to change AccountNumber to a character datatype. Do you then go back and rename the column and change every single query everywhere? Or do you leave the suffix in your column name even though it is no longer accurate? If you want to know the datatype of a column the ONLY way is to look at the definition of the table.
Also, a single character really is kind of useless. How do you handle nvarchar vs varchar? And what about the scale?
P.S. Even though I wrote an answer I am voting to close this question because it is primarily opinion based and as such is considered off topic for SO.
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I have noticed that some people in database columns instead of for example id, user, addressare using something like h_id, e_user, f_address etc...
Is that some kind of security aspect? or maybe these are some shortcuts of words?
Its because there might be many id fields like user_id,category_id,that's why they use so that code is understandable.
And talking about columns name like f_address, they are just shortcut for say first address. It doesn't have anything to do with security but to increase the query readability use proper name to fields so that people can understand just by seeing column name what data it saves.
If there are fields like category_id and sub_category_id , it is understandable from the field name, but if i denote it using c_id and s_id, its hard to depict.
Well, 'User' is a security object in SQL Server, so using that is kind of scary. 'ID' and 'address' are way too generic to provide any semantics when used as attribute names.
If a purpose of design is to be maintainable and readable, then there some words that simply don't work.
Definitely not security related.
Some use it for readability or speed (you don't have to remember which table you gave a certain name->see following example) when writing queries.
i.e.
select a.name, b.name from table1 a join table2 b on a.id=b.id
Like this you have to remember that table1 is named a and table2 is named b etc.
But if you use tablename_field (which you can shorten by using only the first letter of the tablename). That way you never have duplicate fieldnames when creating join queries.
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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.
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Hi guys so I have a question about nick name searches.
I have a very large database and in my Accounts Entity I have a firstname column. When a user searches for an account by first name, it is possible that they may be using a nickname. For example searching for Bob, should also return Robert.
The way I would think to do this would be to create a table called nickname, with two columns, the nickname, and name. That way we map bob->robert.
Then when doing the query make the where clause look like this "WHERE firstname IN (SELECT name FROM nickname WHERE nickname = 'bob')"
The two problems I have is, the query above seems very inefficient, and would be very slow over large data sets (I could be wrong here so please tell me if so, when I say large data set I mean 14 million rows).
The Second problem I have is where to get the Nickname data from. This is the only thing I have found so far: https://code.google.com/p/nickname-and-diminutive-names-lookup/downloads/list
Any help would be greatly appreciated.
One option would be to use full text search instead:
http://www.postgresql.org/docs/current/static/textsearch.html
This would allow you to add custom dictionaries, among other colorful features:
http://www.postgresql.org/docs/current/static/textsearch-dictionaries.html
I had to solve a similar problem. We had a table with name variations that were linked to an individual. This was for a database of authors.
We then created a mapping table with soundex and double metaphone entries for these names (pre-generated) then did queries against that table to find individuals.
If you're not familiar with soundex or double metaphone, they are phonetic algorithms to match when using misspellings and similiar names. Soundex was developed for the US Census.
In our case, we already had the variations of every name the person published as rather than a generic list of names. However, the soundex algorithm can help with similar spellings. You'll still need to get a nickname list from somewhere, but this should help with the performance.
The reason I suggested two algorithms is that we had a lot of collisions using just one, but with both of those together it was a rather good filter. Double Metaphone worked better for non Western European names.
I'd suggest adding a front end element to let your customer service people (or customers) add their nickname too. Customers can help you build up a nickname list and you can use known nicknames to help with fuzzy searches on others eventually.
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Just for reference I am using SQL Azure.
I noticed when I am trying to select data from a table based on a license plate and the state of that plate I get no results back if the state is "IN". I realize the word "IN" is reserved in SQL server; however, I am containing that within quotes in my query. I currently am in testing phase and have only one record in the table which has a lisence plate 287YGB and state IN.
If I write my query as follows I get nothing back.
SELECT MakeModel, CitizenID, VehicleID FROM tblVehicles WHERE tblVehicles.Lisence = '287YGB' AND tblVehicles.PlateState = 'IN'
If I write my query this way I get back my result. But this is not good enough.
SELECT MakeModel, CitizenID, VehicleID FROM tblVehicles WHERE tblVehicles.Lisence = '287YGB'
And finally, if I write my query this way I get the only row in the table.
SELECT MakeModel, CitizenID, VehicleID FROM tblVehicles
From these tests I can see that the last where parameter is causing the problem. I am assuming it is due to the fact that the word "IN" is reserved. Is there a way around this?
Reserved words usually only cause problems if you're using them as field names, and in that case you need to wrap them with brackets ("[]") to eliminate the problem. I will amost guarantee you that your PlateState has some garbage in it, so you need to either trim it first (LTRIM(RTRIM(PlateState)) = 'IN') or use Like '%IN%' instead, and this will return the results you expect.
try this
SELECT MakeModel, CitizenID, VehicleID FROM tblVehicles WHERE tblVehicles.Lisence = '287YGB' AND LTRIM(RTRIM(tblVehicles.PlateState)) = 'IN'