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I am a student of a web application development module.
We have had a database examination, and I have a small question that you may be able to clarify for me.
It is an exercise that I think could be done in two ways, but according to my teacher only one option is correct.
I have looked and believe in this case two queries could apply. If you could clarify my confusion a bit, I thank you =)
I leave the question of the statement, the tables and two solutions. Would both be okay?
Exercise: It returns a list with the identifiers of the clients that have requested a command. Note that it should not display identifiers that are repeating. They must be ordered.
If there are foreigns keys. I add the keys to the tables. Thank you very much, it has been very useful.
Table comanda:
Table comanda
Table cliente:
Table cliente
Queries:
SELECT DISTINCT id_cliente as 'Identificador cliente'
FROM comanda
ORDER BY id_cliente ASC;
SELECT DISTINCT cliente.id
FROM cliente INNER JOIN comanda
ON cliente.id = comanda.id_cliente
ORDER BY cliente.id;
Many thanks =)
There's one subtle difference: If there are values in comanda.id_cliente that do not exist in cliente.id, the first query will output them, the second doesn't because they'll find no match for the join.
It is not unreasonable to assume that (comanda.id_cliente) is constrained to be a foreign key referencing (cliente.id) though. Such a declaration would make such orphaned values in comanda.id_cliente impossible. And such a constraint should normally in place, if the database is well designed. But the picture doesn't seem to indicate such a constraint (unless I'm missing something). So unless there's further information indeed showing that such a constraint exists, the assumption might be proven wrong.
So generally spoken, the queries are not equivalent, I'm afraid. Which one is correct in terms of the task leaves room for interpretation of the text of the task though.
Both queries work and would generate the result that is expected.
The first query is more efficient than the second one: there is no need to bring the client table to generate the result you want.
It should also be noted that using single quotes for identifiers (here, column alias Identificador cliente) should generally be avoided. In standard SQL, single quotes stand for literal strings. The standard quoting character for identifiers is double quotes.
Both answers could be accepted. But it depends a lot on the teacher and how much are the exercises clarified.
Query
Pro: Efficient and simple
Cons: There could be records with non-existing client id. In this case, the exercise should mention this.
Query
Pro: More precise
Cons: That join could be unnecessary
It would interesting to ask the teacher about the reason. A teacher could expect any of the answers.
I am an Oracle end user, not a database developer or admin.
Our database has some typos in column names, similar to a column being named ADRESS2 instead of ADDRESS2
I have to use these columns in my queries, and it's hard to remember which typo applies to which column. Also, I'm kind of anal persnickety about the way this makes my code look (not so professional).
I suspect there is no chance of getting these columns actually renamed in production -- it would be a low priority with lots of dependencies.
What is the best practice in this situation?
I can think of a couple options:
add comments that the typo is intentional
wrap the table with a select(bad_name good_name, * from table)
create a view based on the above
But is there any way to do something like:
DECLARE address2 CHAR(8) := 'adress2';
And use in a query, but returning the values of ADRESS2 instead of the char "adress2".
The desired solution would be a practice that:
wouldn't increase the complexity of the query
would be recognized by developers as a response to a situation where column names are unwieldy or misleading
I think about the only legitimate response is a view. If I were in your situation, I'd fix the column typos. Failing that, I would create a view.
But, something in your question makes me think that you're willing to consider something more "out there", so here it is:
alter table sample_table add address2
invisible generated always as ( addres2 || '') virtual;
This is very likely to not impact any of your applications, as nothing will see the ADDRESS2 column unless they explicitly ask for it.
But, it makes it available for you persnickity types.
Keep in mind that the function ADDRES2 || '' means your queries won't use indexes on the columns you access this way.
Repeat: I would change the column names or make a view.
I have building MYSQL queries with backticks. For example,
SELECT `title` FROM `table` WHERE (`id` = 3)
as opposed to:
SELECT title FROM table WHERE (id = 3)
I think I got this practice from the Phpmyadmin exports, and from what I understood, even Rails generates its queries like this.
But nowadays I see less and less queries built like this, and also, the code looks messier and more complicated with backticks in queries. Even with SQL helper functions, things would be simpler without them. Hence, I'm considering to leave them behind.
I wanted to find out if there is other implication in this practice such as SQL (MySQL in my case) interpretation speed, etc. What do you think?
Backticks also allow spaces and other special characters (except for backticks, obviously) in table/column names. They're not strictly necessary but a good idea for safety.
If you follow sensible rules for naming tables and columns backticks should be unnecessary.
Every time I see this discussed, I try to lobby for their inclusion, because, well, the answer is hidden in here already, although wryly winked away without further thought. When we mistakenly use a keyword as a field or table name, we can escape confusion by various methods, but only the keenly aware back-tick ` allows an even greater benefit!!!
Every word in a sql statement is run through the entire keyword hash table to see if conflicts, therefore, you've done you query a great favor by telling the compiler that, hey, I know what I'm doing, you don't need to check these words because they represent table and field names. Speed and elegance.
Cheers,
Brad
backticks are used to escape reserved keywords in your mysql query, e.g. you want to have a count column—not that uncommon.
you can use other special characters or spaces in your column/table/db names
they do not keep you safe from injection attacks (if you allow users to enter column names in some way—bad practice anyway)
they are not standardized sql and will only work in mysql; other dbms will use " instead
Well, if you ensure that you never accidentally use a keyword as an identifier, you don't need the backticks. :-)
You read the documentation on identifiers at http://dev.mysql.com/doc/refman/5.6/en/identifiers.html
SQL generators will often include backticks, as it is simpler than including a list of all MySQL reserved words. To use any1 sequence of BMP Unicode characters except U+0000 as an identifier, they can simply
Replace all backticks with double backticks
Surround that with single backticks
When writing handmade queries, I know (most of) MySQL's reserved words, and I prefer to not use backticks where possible as it is shorter and IMO easier to read.
Most of the time, it's just a style preference -- unless of course, you have a field like date or My Field, and then you must use backticks.
1. Though see https://bugs.mysql.com/bug.php?id=68676
My belief was that the backticks were primarily used to prevent erroneous queries which utilized common SQL identifiers, i.e. LIMIT and COUNT.
I'm curious to know how people are using table aliases. The other developers where I work always use table aliases, and always use the alias of a, b, c, etc.
Here's an example:
SELECT a.TripNum, b.SegmentNum, b.StopNum, b.ArrivalTime
FROM Trip a, Segment b
WHERE a.TripNum = b.TripNum
I disagree with them, and think table aliases should be use more sparingly.
I think they should be used when including the same table twice in a query, or when the table name is very long and using a shorter name in the query will make the query easier to read.
I also think the alias should be a descriptive name rather than just a letter. In the above example, if I felt I needed to use 1 letter table alias I would use t for the Trip table and s for the segment table.
There are two reasons for using table aliases.
The first is cosmetic. The statements are easier to write, and perhaps also easier to read when table aliases are used.
The second is more substantive. If a table appears more than once in the FROM clause, you need table aliases in order to keep them distinct. Self joins are common in cases where a table contains a foreign key that references the primary key of the same table.
Two examples: an employees table that contains a supervisorID column that references the employeeID of the supervisor.
The second is a parts explosion. Often, this is implemented in a separate table with three columns: ComponentPartID, AssemblyPartID, and Quantity. In this case, there won't be any self joins, but there will often be a three way join between this table and two different references to the table of Parts.
It's a good habit to get into.
I use them to save typing. However, I always use letters similar to the function. So, in your example, I would type:
SELECT t.TripNum, s.SegmentNum, s.StopNum, s.ArrivalTime
FROM Trip t, Segment s
WHERE t.TripNum = s.TripNum
That just makes it easier to read, for me.
As a general rule I always use them, as there are usually multiple joins going on in my stored procedures. It also makes it easier when using code generation tools like CodeSmith to have it generate the alias name automatically for you.
I try to stay away from single letters like a & b, as I may have multiple tables that start with the letter a or b. I go with a longer approach, the concatenation of the referenced foreign key with the alias table, for example CustomerContact ... this would be the alias for the Customer table when joining to a Contact table.
The other reason I don't mind longer name, is due to most of my stored procedures are being generated via code CodeSmith. I don't mind hand typing the few that I may have to build myself.
Using the current example, I would do something like:
SELECT TripNum, TripSegment.SegmentNum, TripSegment.StopNum, TripSegment.ArrivalTime
FROM Trip, Segment TripSegment
WHERE TripNum = TripSegment.TripNum
Can I add to a debate that is already several years old?
There is another reason that no one has mentioned. The SQL parser in certain databases works better with an alias. I cannot recall if Oracle changed this in later versions, but when it came to an alias, it looked up the columns in the database and remembered them. When it came to a table name, even if it was already encountered in the statement, it re-checked the database for the columns. So using an alias allowed for faster parsing, especially of long SQL statements. I am sure someone knows if this is still the case, if other databases do this at parse time, and if it changed, when it changed.
I use it always, reasons:
leaving full tables names in statements makes them hard to read, plus you cannot have a same table twice
not using anything is a very bad idea, because later you could add some field to one of the tables that is already present in some other table
Consider this example:
select col1, col2
from tab1
join tab2 on tab1.col3 = tab2.col3
Now, imagine a few months later, you decide to add column named 'col1' to tab2. Database will silently allow you to do that, but applications would break when executing the above query because of ambiguity between tab1.col1 and tab2.col1.
But, I agree with you on the naming: a, b, c is fine, but t and s would be much better in your example. And when I have the same table more than once, I would use t1, t2, ... or s1, s2, s3...
Using the full name makes it harder to read, especially for larger queries or the Order/Product/OrderProduct scenari0
I'd use t and s. Or o/p/op
If you use SCHEMABINDING then columns must be qualified anyway
If you add a column to a base table, then the qualification reduces the chance of a duplicate in the query (for example a "Comment" column)
Because of this qualification, it makes sense to always use aliases.
Using a and b is blind obedience to a bizarre standard.
In simple queries I do not use aliases. In queries whit multiple tables I always use them because:
they make queries more readable (my
aliases are 2 or more capital letters that is a shortcut for the table name and if possible
a relationship to other
tables)
they allow faster developing and
rewriting (my table names are long and have prefixes depending on role they pose)
so instead of for example:
SELECT SUM(a.VALUE)
FROM Domesticvalues a, Foreignvalues b
WHERE a.Value>b.Value
AND a.Something ...
I write:
select SUM(DVAL.Value)
from DomesticValues DVAL, ForeignValues FVAL
where DVAL.Value > FVAL.Value
and DVAL.Something ...
There are many good ideas in the posts above about when and why to alias table names. What no one else has mentioned is that it is also beneficial in helping a maintainer understand the scope of tables. At our company we are not allowed to create views. (Thank the DBA.) So, some of our queries become large, even exceeding the 50,000 character limit of a SQL command in Crystal Reports. When a query aliases its tables as a, b, c, and a subquery of that does the same, and multiple subqueries in that one each use the same aliases, it is easy for one to mistake what level of the query is being read. This can even confuse the original developer when enough time has passed. Using unique aliases within each level of a query makes it easier to read because the scope remains clear.
I feel that you should use them as often as possible but I do agree that t & s represent the entities better than a & b.
This boils down to, like everything else, preferences. I like that you can depend on your stored procedures following the same conventions when each developer uses the alias in the same manner.
Go convince your coworkers to get on the same page as you or this is all worthless. The alternative is you could have a table Zebra as first table and alias it as a. That would just be cute.
i only use them when they are necessary to distinguish which table a field is coming from
select PartNumber, I.InventoryTypeId, InventoryTypeDescription
from dbo.Inventory I
inner join dbo.InventoryType IT on IT.InventoryTypeId = I.InventoryTypeId
In the example above both tables have an InventoryTypeId field, but the other field names are unique.
Always use an abbreviation for the table as the name so that the code makes more sense - ask your other developers if they name their local variables A, B, C, etc!
The only exception is in the rare cases where the SQL syntax requires a table alias but it isn't referenced, e.g.
select *
from (
select field1, field2, sum(field3) as total
from someothertable
) X
In the above, SQL syntax requires the table alias for the subselect, but it isn't referenced anywhere so I get lazy and use X or something like that.
I find it nothing more than a preference. As mentioned above, aliases save typing, especially with long table/view names.
One thing I've learned is that especially with complex queries; it is far simpler to troubleshoot six months later if you use the alias as a qualifier for every field reference. Then you aren't trying to remember which table that field came from.
We tend to have some ridiculously long table names, so I find it easier to read if the tables are aliased. And of course you must do it if you are using a derived table or a self join, so being in the habit is a good idea. I find most of our developers end up using the same alias for each table in all their sps,so most of the time anyone reading it will immediately know what pug is the alias for or mmh.
I always use them. I formerly only used them in queries involving just one table but then I realized a) queries involving just one table are rare, and b) queries involving just one table rarely stay that way for long. So I always put them in from the start so that I (or someone else) won't have to retro fit them later. Oh and BTW: I call them "correlation names", as per the SQL-92 Standard :)
Tables aliases should be four things:
Short
Meaningful
Always used
Used consistently
For example if you had tables named service_request, service_provider, user, and affiliate (among many others) a good practice would be to alias those tables as "sr", "sp", "u", and "a", and do so in every query possible. This is especially convenient if, as is often the case, these aliases coincide with acronyms used by your organization. So if "SR" and "SP" are the accepted terms for Service Request and Service Provider respectively, the aliases above carry a double payload of intuitively standing in for both the table and the business object it represents.
The obvious flaws with this system are first that it can be awkward for table names with lots of "words" e.g. a_long_multi_word_table_name which would alias to almwtn or something, and that it's likely you'll end up with tables named such that they abbreviate the same. The first flaw can be dealt with however you like, such as by taking the last 3 or 4 letters, or whichever subset you feel is most representative, most unique, or easiest to type. The second I've found in practice isn't as troublesome as it might seem, perhaps just by luck. You can also do things like take the second letter of a "word" in the table as well, such as aliasing account_transaction to "atr" instead of "at" to avoid conflicting with account_type.
Of course whether you use the above approach or not, aliases should be short because you'll be typing them very very frequently, and they should always be used because once you've written a query against a single table and omitted the alias, it's inevitable that you'll later need to edit in a second table with duplicate column names.
Because I always fully qualify my tables, I use aliases to provide a shorter name for the fields being SELECTed when JOINed tables are involved. I think it makes my code easier to follow.
If the query deals with only one source - no JOINs - then I don't use an alias.
Just a matter of personal preference.
Always. Make it a habit.
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What are the pros and cons of using table aliases in SQL? I personally try to avoid them, as I think they make the code less readable (especially when reading through large where/and statements), but I'd be interested in hearing any counter-points to this. When is it generally a good idea to use table aliases, and do you have any preferred formats?
Table aliases are a necessary evil when dealing with highly normalized schemas. For example, and I'm not the architect on this DB so bear with me, it can take 7 joins in order to get a clean and complete record back which includes a person's name, address, phone number and company affiliation.
Rather than the somewhat standard single character aliases, I tend to favor short word aliases so the above example's SQL ends up looking like:
select person.FirstName
,person.LastName
,addr.StreetAddress
,addr.City
,addr.State
,addr.Zip
,phone.PhoneNumber
,company.CompanyName
from tblPeople person
left outer join tblAffiliations affl on affl.personID = person.personID
left outer join tblCompany company on company.companyID = affl.companyID
... etc
Well, there are some cases you must use them, like when you need to join to the same table twice in one query.
It also depends on wether you have unique column names across tables. In our legacy database we have 3-letter prefixes for all columns, stemming from an abbreviated form from the table, simply because one ancient database system we were once compatible with didn't support table aliases all that well.
If you have column names that occur in more than one table, specifying the table name as part of the column reference is a must, and thus a table alias will allow for a shorter syntax.
Am I the only person here who really hates them?
Generally, I don't use them unless I have to. I just really hate having to read something like
select a.id, a.region, a.firstname, a.blah, b.yadda, b.huminahumina, c.crap
from table toys as a
inner join prices as b on a.blah = b.yadda
inner join customers as c on c.crap = something else
etc
When I read SQL, I like to know exactly what I'm selecting when I read it; aliases actually confuse me more because I've got to slog through lines of columns before I actually get to the table name, which generally represents information about the data that the alias doesn't. Perhaps it's okay if you made the aliases, but I commonly read questions on StackOverflow with code that seems to use aliases for no good reason. (Additionally, sometimes, someone will create an alias in a statement and just not use it. Why?)
I think that table aliases are used so much because a lot of people are averse to typing. I don't think that's a good excuse, though. That excuse is the reason we end up with terrible variable naming, terrible function acronyms, bad code...I would take the time to type out the full name. I'm a quick typer, though, so maybe that has something to do with it. (Maybe in the future, when I've got carpal tunnel, I'll reconsider my opinion on aliases. :P ) I especially hate running across table aliases in PHP code, where I believe there's absolutely no reason to have to do that - you've only got to type it once!
I always use column qualifiers in my statements, but I'm not averse to typing a lot, so I will gladly type the full name multiple times. (Granted, I do abuse MySQL's tab completion.) Unless it's a situation where I have to use an alias (like some described in other answers), I find the extra layer of abstraction cumbersome and unnecessary.
Edit: (Over a year later) I'm dealing with some stored procedures that use aliases (I did not write them and I'm new to this project), and they're kind of painful. I realize that the reason I don't like aliases is because of how they're defined. You know how it's generally good practice to declare variables at the top of your scope? (And usually at the beginning of a line?) Aliases in SQL don't follow this convention, which makes me grind my teeth. Thus, I have to search the entire code for a single alias to find out where it is (and what's frustrating is, I have to read through the logic before I find the alias declaration). If it weren't for that, I honestly might like the system better.
If I ever write a stored procedure that someone else will have to deal with, I'm putting my alias definitions in a comment block at the beginning of the file, as a reference. I honestly can't understand how you guys don't go crazy without it.
Good
As it has been mentioned multiple times before, it is a good practice to prefix all column names to easily see which column belongs to which table - and aliases are shorter than full table names so the query is easier to read and thus understand. If you use a good aliasing scheme of course.
And if you create or read the code of an application, which uses externally stored or dynamically generated table names, then without aliases it is really hard to tell at the first glance what all those "%s"es or other placeholders stand for. It is not an extreme case, for example many web apps allow to customize the table name prefix at installation time.
Microsoft SQL's query optimiser benefits from using either fully qualified names or aliases.
Personally I prefer aliases, and unless I have a lot of tables they tend to be single letter ones.
--seems pretty readable to me ;-)
select a.Text
from Question q
inner join Answer a
on a.QuestionId = q.QuestionId
There's also a practical limit on how long a Sql string can be executed - aliases make this limit easier to avoid.
If I write a query myself (by typing into the editor and not using a designer) I always use aliases for the table name just so I only have to type the full table name once.I really hate reading queries generated by a designer with the full table name as a prefix to every column name.
I suppose the only thing that really speaks against them is excessive abstraction. If you will have a good idea what the alias refers to (good naming helps; 'a', 'b', 'c' can be quite problematic especially when you're reading the statement months or years later), I see nothing wrong with aliasing.
As others have said, joins require them if you're using the same table (or view) multiple times, but even outside that situation, an alias can serve to clarify a data source's purpose in a particular context. In the alias's name, try to answer why you are accessing particular data, not what the data is.
I LOVE aliases!!!! I have done some tests using them vs. not and have seen some processing gains. My guess is the processing gains would be higher when you're dealing with larger datasets and complex nested queries than without. If I'm able to test this, I'll let you know.
You need them if you're going to join a table to itself, or if you use the column again in a subquery...
Aliases are great if you consider that my organization has table names like:
SchemaName.DataPointName_SubPoint_Sub-SubPoint_Sub-Sub-SubPoint...
My team uses a pretty standard set of abbreviations, so the guesswork is minimized. We'll have say ProgramInformationDataPoint shortened to pidp, and submissions to just sub.
The good thing is that once you get going in this manner and people agree with it, it makes those HAYUGE files just a little smaller and easier to manage. At least for me, fewer characters to convey the same info seems to go a little easier on my brain.
I like long explicit table names (it's not uncommon to be more than 100 characters) because I use many tables and if the names aren't explicit, I might get confused as to what each table stores.
So when I write a query, I tend to use shorter aliases that make sense within the scope of the query and that makes the code much more readable.
I always use aliases in my queries and it is part of the code guidebook in my company. First of all you need aliases or table names when there are columns with identical names in the joining tables. In my opinion the aliases improve readability in complex queries and allow me to see quickly the location of each columns. We even use aliases with single table queries, because experience has shown that single table queries don´t stay single table for long.
IMHO, it doesn't really matter with short table names that make sense, I have on occasion worked on databases where the table name could be something like VWRECOFLY or some other random string (dictated by company policy) that really represents users, so in that case I find aliases really help to make the code FAR more readable. (users.username makes a lot more sence then VWRECOFLY.username)
I always use aliases, since to get proper performance on MSSQL you need to prefix with schema at all times. So you'll see a lot of
Select
Person.Name
From
dbo.Person As Person
I always use aliases when writing queries. Generally I try and abbreviate the table name to 1 or 2 representative letters. So Users becomes u and debtor_transactions becomes dt etc...
It saves on typing and still carries some meaning.
The shorter names makes it more readable to me as well.
If you do not use an alias, it's a bug in your code just waiting to happen.
SELECT Description -- actually in a
FROM
table_a a,
table_b b
WHERE
a.ID = b.ID
What happens when you do a little thing like add a column called Description to Table_B. That's right, you'll get an error. Adding a column doesn't need to break anything. I never see writing good code, bug free code, as a necessary evil.
Aliases are required when joining tables with columns that have identical names.