So, per Mehrdad's answer to a related question, I get it that a "proper" database table column doesn't store a list. Rather, you should create another table that effectively holds the elements of said list and then link to it directly or through a junction table. However, the type of list I want to create will be composed of unique items (unlike the linked question's fruit example). Furthermore, the items in my list are explicitly sorted - which means that if I stored the elements in another table, I'd have to sort them every time I accessed them. Finally, the list is basically atomic in that any time I wish to access the list, I will want to access the entire list rather than just a piece of it - so it seems silly to have to issue a database query to gather together pieces of the list.
AKX's solution (linked above) is to serialize the list and store it in a binary column. But this also seems inconvenient because it means that I have to worry about serialization and deserialization.
Is there any better solution? If there is no better solution, then why? It seems that this problem should come up from time to time.
... just a little more info to let you know where I'm coming from. As soon as I had just begun understanding SQL and databases in general, I was turned on to LINQ to SQL, and so now I'm a little spoiled because I expect to deal with my programming object model without having to think about how the objects are queried or stored in the database.
Thanks All!
John
UPDATE: So in the first flurry of answers I'm getting, I see "you can go the CSV/XML route... but DON'T!". So now I'm looking for explanations of why. Point me to some good references.
Also, to give you a better idea of what I'm up to: In my database I have a Function table that will have a list of (x,y) pairs. (The table will also have other information that is of no consequence for our discussion.) I will never need to see part of the list of (x,y) pairs. Rather, I will take all of them and plot them on the screen. I will allow the user to drag the nodes around to change the values occasionally or add more values to the plot.
No, there is no "better" way to store a sequence of items in a single column. Relational databases are designed specifically to store one value per row/column combination. In order to store more than one value, you must serialize your list into a single value for storage, then deserialize it upon retrieval. There is no other way to do what you're talking about (because what you're talking about is a bad idea that should, in general, never be done).
I understand that you think it's silly to create another table to store that list, but this is exactly what relational databases do. You're fighting an uphill battle and violating one of the most basic principles of relational database design for no good reason. Since you state that you're just learning SQL, I would strongly advise you to avoid this idea and stick with the practices recommended to you by more seasoned SQL developers.
The principle you're violating is called first normal form, which is the first step in database normalization.
At the risk of oversimplifying things, database normalization is the process of defining your database based upon what the data is, so that you can write sensible, consistent queries against it and be able to maintain it easily. Normalization is designed to limit logical inconsistencies and corruption in your data, and there are a lot of levels to it. The Wikipedia article on database normalization is actually pretty good.
Basically, the first rule (or form) of normalization states that your table must represent a relation. This means that:
You must be able to differentiate one row from any other row (in other words, you table must have something that can serve as a primary key. This also means that no row should be duplicated.
Any ordering of the data must be defined by the data, not by the physical ordering of the rows (SQL is based upon the idea of a set, meaning that the only ordering you should rely on is that which you explicitly define in your query)
Every row/column intersection must contain one and only one value
The last point is obviously the salient point here. SQL is designed to store your sets for you, not to provide you with a "bucket" for you to store a set yourself. Yes, it's possible to do. No, the world won't end. You have, however, already crippled yourself in understanding SQL and the best practices that go along with it by immediately jumping into using an ORM. LINQ to SQL is fantastic, just like graphing calculators are. In the same vein, however, they should not be used as a substitute for knowing how the processes they employ actually work.
Your list may be entirely "atomic" now, and that may not change for this project. But you will, however, get into the habit of doing similar things in other projects, and you'll eventually (likely quickly) run into a scenario where you're now fitting your quick-n-easy list-in-a-column approach where it is wholly inappropriate. There is not much additional work in creating the correct table for what you're trying to store, and you won't be derided by other SQL developers when they see your database design. Besides, LINQ to SQL is going to see your relation and give you the proper object-oriented interface to your list automatically. Why would you give up the convenience offered to you by the ORM so that you can perform nonstandard and ill-advised database hackery?
You can just forget SQL all together and go with a "NoSQL" approach. RavenDB, MongoDB and CouchDB jump to mind as possible solutions. With a NoSQL approach, you are not using the relational model..you aren't even constrained to schemas.
What I have seen many people do is this (it may not be the best approach, correct me if I am wrong):
The table which I am using in the example is given below(the table includes nicknames that you have given to your specific girlfriends. Each girlfriend has a unique id):
nicknames(id,seq_no,names)
Suppose, you want to store many nicknames under an id. This is why we have included a seq_no field.
Now, fill these values to your table:
(1,1,'sweetheart'), (1,2,'pumpkin'), (2,1,'cutie'), (2,2,'cherry pie')
If you want to find all the names that you have given to your girl friend id 1 then you can use:
select names from nicknames where id = 1;
Simple answer: If, and only if, you're certain that the list will always be used as a list, then join the list together on your end with a character (such as '\0') that will not be used in the text ever, and store that. Then when you retrieve it, you can split by '\0'. There are of course other ways of going about this stuff, but those are dependent on your specific database vendor.
As an example, you can store JSON in a Postgres database. If your list is text, and you just want the list without further hassle, that's a reasonable compromise.
Others have ventured suggestions of serializing, but I don't really think that serializing is a good idea: Part of the neat thing about databases is that several programs written in different languages can talk to one another. And programs serialized using Java's format would not do all that well if a Lisp program wanted to load it.
If you want a good way to do this sort of thing there are usually array-or-similar types available. Postgres for instance, offers array as a type, and lets you store an array of text, if that's what you want, and there are similar tricks for MySql and MS SQL using JSON, and IBM's DB2 offer an array type as well (in their own helpful documentation). This would not be so common if there wasn't a need for this.
What you do lose by going that road is the notion of the list as a bunch of things in sequence. At least nominally, databases treat fields as single values. But if that's all you want, then you should go for it. It's a value judgement you have to make for yourself.
In addition to what everyone else has said, I would suggest you analyze your approach in longer terms than just now. It is currently the case that items are unique. It is currently the case that resorting the items would require a new list. It is almost required that the list are currently short. Even though I don't have the domain specifics, it is not much of a stretch to think those requirements could change. If you serialize your list, you are baking in an inflexibility that is not necessary in a more-normalized design. Btw, that does not necessarily mean a full Many:Many relationship. You could just have a single child table with a foreign key to the parent and a character column for the item.
If you still want to go down this road of serializing the list, you might consider storing the list in XML. Some databases such as SQL Server even have an XML data type. The only reason I'd suggest XML is that almost by definition, this list needs to be short. If the list is long, then serializing it in general is an awful approach. If you go the CSV route, you need to account for the values containing the delimiter which means you are compelled to use quoted identifiers. Persuming that the lists are short, it probably will not make much difference whether you use CSV or XML.
If you need to query on the list, then store it in a table.
If you always want the list, you could store it as a delimited list in a column. Even in this case, unless you have VERY specific reasons not to, store it in a lookup table.
Many SQL databases allow a table to contain a subtable as a component. The usual method is to allow the domain of one of the columns to be a table. This is in addition to using some convention like CSV to encode the substructure in ways unknown to the DBMS.
When Ed Codd was developing the relational model in 1969-1970, he specifically defined a normal form that would disallow this kind of nesting of tables. Normal form was later called First Normal Form. He then went on to show that for every database, there is a database in first normal form that expresses the same information.
Why bother with this? Well, databases in first normal form permit keyed access to all data. If you provide a table name, a key value into that table, and a column name, the database will contain at most one cell containing one item of data.
If you allow a cell to contain a list or a table or any other collection, now you can't provide keyed access to the sub items, without completely reworking the idea of a key.
Keyed access to all data is fundamental to the relational model. Without this concept, the model isn't relational. As to why the relational model is a good idea, and what might be the limitations of that good idea, you have to look at the 50 years worth of accumulated experience with the relational model.
I'd just store it as CSV, if it's simple values then it should be all you need (XML is very verbose and serializing to/from it would probably be overkill but that would be an option as well).
Here's a good answer for how to pull out CSVs with LINQ.
Only one option doesn't mentioned in the answers. You can de-normalize your DB design. So you need two tables. One table contains proper list, one item per row, another table contains whole list in one column (coma-separated, for example).
Here it is 'traditional' DB design:
List(ListID, ListName)
Item(ItemID,ItemName)
List_Item(ListID, ItemID, SortOrder)
Here it is de-normalized table:
Lists(ListID, ListContent)
The idea here - you maintain Lists table using triggers or application code. Every time you modify List_Item content, appropriate rows in Lists get updated automatically. If you mostly read lists it could work quite fine. Pros - you can read lists in one statement. Cons - updates take more time and efforts.
I was very reluctant to choose the path I finally decide to take because of many answers. While they add more understanding to what is SQL and its principles, I decided to become an outlaw. I was also hesitant to post my findings as for some it's more important to vent frustration to someone breaking the rules rather than understanding that there are very few universal truthes.
I have tested it extensively and, in my specific case, it was way more efficient than both using array type (generously offered by PostgreSQL) or querying another table.
Here is my answer:
I have successfully implemented a list into a single field in PostgreSQL, by making use of the fixed length of each item of the list. Let say each item is a color as an ARGB hex value, it means 8 char. So you can create your array of max 10 items by multiplying by the length of each item:
ALTER product ADD color varchar(80)
In case your list items length differ you can always fill the padding with \0
NB: Obviously this is not necessarily the best approach for hex number since a list of integers would consume less storage but this is just for the purpose of illustrating this idea of array by making use of a fixed length allocated to each item.
The reason why:
1/ Very convenient: retrieve item i at substring i*n, (i +1)*n.
2/ No overhead of cross tables queries.
3/ More efficient and cost-saving on the server side. The list is like a mini blob that the client will have to split.
While I respect people following rules, many explanations are very theoretical and often fail to acknowledge that, in some specific cases, especially when aiming for cost optimal with low-latency solutions, some minor tweaks are more than welcome.
"God forbid that it is violating some holy sacred principle of SQL": Adopting a more open-minded and pragmatic approach before reciting the rules is always the way to go. Else you might end up like a candid fanatic reciting the Three Laws of Robotics before being obliterated by Skynet
I don't pretend that this solution is a breakthrough, nor that it is ideal in term of readability and database flexibility, but it can certainly give you an edge when it comes to latency.
What I do is that if the List required to be stored is small then I would just convert it to a string then split it later when required.
example in python -
for y in b:
if text1 == "":
text1 = y
else:
text1 = text1 + f"~{y}"
then when I required it I just call it from the db and -
out = query.split('~')
print(out)
this will return a list, and a string will be stored in the db. But if you are storing a lot of data in the list then creating a table is the best option.
If you really wanted to store it in a column and have it queryable a lot of databases support XML now. If not querying you can store them as comma separated values and parse them out with a function when you need them separated. I agree with everyone else though if you are looking to use a relational database a big part of normalization is the separating of data like that. I am not saying that all data fits a relational database though. You could always look into other types of databases if a lot of your data doesn't fit the model.
I think in certain cases, you can create a FAKE "list" of items in the database, for example, the merchandise has a few pictures to show its details, you can concatenate all the IDs of pictures split by comma and store the string into the DB, then you just need to parse the string when you need it. I am working on a website now and I am planning to use this way.
you can store it as text that looks like a list and create a function that can return its data as an actual list. example:
database:
_____________________
| word | letters |
| me | '[m, e]' |
| you |'[y, o, u]' | note that the letters column is of type 'TEXT'
| for |'[f, o, r]' |
|___in___|_'[i, n]'___|
And the list compiler function (written in python, but it should be easily translatable to most other programming languages). TEXT represents the text loaded from the sql table. returns list of strings from string containing list. if you want it to return ints instead of strings, make mode equal to 'int'. Likewise with 'string', 'bool', or 'float'.
def string_to_list(string, mode):
items = []
item = ""
itemExpected = True
for char in string[1:]:
if itemExpected and char not in [']', ',', '[']:
item += char
elif char in [',', '[', ']']:
itemExpected = True
items.append(item)
item = ""
newItems = []
if mode == "int":
for i in items:
newItems.append(int(i))
elif mode == "float":
for i in items:
newItems.append(float(i))
elif mode == "boolean":
for i in items:
if i in ["true", "True"]:
newItems.append(True)
elif i in ["false", "False"]:
newItems.append(False)
else:
newItems.append(None)
elif mode == "string":
return items
else:
raise Exception("the 'mode'/second parameter of string_to_list() must be one of: 'int', 'string', 'bool', or 'float'")
return newItems
Also here is a list-to-string function in case you need it.
def list_to_string(lst):
string = "["
for i in lst:
string += str(i) + ","
if string[-1] == ',':
string = string[:-1] + "]"
else:
string += "]"
return string
Imagine your grandmother's box of recipes, all written on index cards. Each of those recipes is a list of ingredients, which are themselves ordered pairs of items and quantities. If you create a recipe database, you wouldn't need to create one table for the recipe names and a second table where each ingredient was a separate record. That sounds like what we're saying here. My apologies if I've misread anything.
From Microsoft's T-SQL Fundamentals:
Atomicity of attributes is subjective in the same way that the
definition of a set is subjective. As an example, should an employee
name in an Employees relation be expressed with one attribute
(fullname), two (firstname and lastname), or three (firstname,
middlename, and lastname)? The answer depends on the application. If
the application needs to manipulate the parts of the employee’s name
separately (such as for search purposes), it makes sense to break them
apart; otherwise, it doesn’t.
So, if you needed to manipulate your list of coordinates via SQL, you would need to split the elements of the list into separate records. But is you just wanted to store a list and retrieve it for use by some other software, then storing the list as a single value makes more sense.
What I'm doing
I am creating an SQL table that will provide the back-end storage mechanism for complex-typed objects. I am trying to determine how to accomplish this with the best performance. I need to be able to query on each individual simple type value of the complex type (e.g. the String value of a City in an Address complex type).
I was originally thinking that I could store the complex type values in one record as an XML, but now I am concerned about the search performance of this design. I need to be able to create variable schemas on the fly without changing anything about the database access layer.
Where I'm at now
Right now I am thinking to create the following tables.
TABLE: Schemas
COLUMN NAME DATA TYPE
SchemaId uniqueidentifier
Xsd xml //contains the schema for the document of the given complex type
DeserializeType varchar(200) //The Full Type name of the C# class to which the document deserializes.
TABLE: Documents
COLUMN NAME DATA TYPE
DocumentId uniqueidentifier
SchemaId uniqueidentifier
TABLE: Values //The DocumentId+ValueXPath function as a PK
COLUMN NAME DATA TYPE
DocumentId uniqueidentifier
ValueXPath varchar(250)
Value text
from these tables, when performing queries I would do a series of self-joins on the value table. When I want to get the entire object by the DocumentId, I would have a generic script for creating a view mimics a denormalized datatable of the complex-type.
What I want to know
I believe there are better ways to accomplish what I am trying to, but I am a little too ignorant about the relative performance benefits of different SQL techniques. Specifically I don't know the performance cost of:
1 - comparing the value of a text field versus of a varchar field.
2 - different kind of joins versus nested queries
3 - getting a view versus an xml document from the sql db
4 - doing some other things that I don't even know I don't know would be affecting my query but, I am experienced enough to know exist
I would appreciate any information or resources about these performance issues in sql as well as a recommendation for how to approach this general issue in a more efficient way.
For Example,
Here's an example of what I am currently planning on doing.
I have a C# class Address which looks like
public class Address{
string Line1 {get;set;}
string Line2 {get;set;}
string City {get;set;}
string State {get;set;}
string Zip {get;set;
}
An instance is constructed from new Address{Line1="17 Mulberry Street", Line2="Apt C", City="New York", State="NY", Zip="10001"}
its XML value would be look like.
<Address>
<Line1>17 Mulberry Street</Line1>
<Line2>Apt C</Line2>
<City>New York</City>
<State>NY</State>
<Zip>10001</Zip>
</Address>
Using the db-schema from above I would have a single record in the Schemas table with an XSD definition of the address xml schema. This instance would have a uniqueidentifier (PK of the Documents table) which is assigned to the SchemaId of the Address record in the Schemas table. There would then be five records in the Values table to represent this Address.
They would look like:
DocumentId ValueXPath Value
82415E8A-8D95-4bb3-9E5C-AA4365850C70 /Address/Line1 17 Mulberry Street
82415E8A-8D95-4bb3-9E5C-AA4365850C70 /Address/Line2 Apt C
82415E8A-8D95-4bb3-9E5C-AA4365850C70 /Address/City New York
82415E8A-8D95-4bb3-9E5C-AA4365850C70 /Address/State NY
82415E8A-8D95-4bb3-9E5C-AA4365850C70 /Address/Zip 10001
Just Added a Bounty...
My objective is to obtain the resources I need in order to give my application a data access layer that is fully searchable and has a data-schema generated from the application layer that does not require direct database configuration (i.e. creating a new SQL table) in order to add a new aggregate root to the domain model.
I am open to the possibility of using .NET compatible technologies other than SQL, but I will require that any such suggestions be adequately substantiated in order to be considered.
How about looking for a solution at the architectural level? I was also breaking my head on complex graphs and performance until I discovered CQRS.
[start evangelist mode]
You can go document-based or relational as storage. Even both! (Event Sourcing)
Nice separation of concerns: Read Model vs Write Model
Have your cake and eat it too!
Ok, there is an initial learning / technical curve to get over ;)
[end evangelist mode]
As you stated: "I need to be able to create variable schemas on the fly without changing anything about the database access layer." The key benefit is that your read model can be very fast since it's made for reading. If you add Event Sourcing to the mix, you can drop and rebuild your Read Model to whatever schema you want... even "online".
There are some nice opensource frameworks out there like nServiceBus which saves lots of time and technical challenges. All depends on how far you want to take these concepts what you're willing/can spend time on. You can even start with just basics if you follow Greg Young's approach. See the info in the links below.
See
CQRS Examples and Screencasts
CQRS Questions
Intro (Also see the video)
Somehow what you want sounds like a painful thing to do in SQL. Basically, you should treat the inside of a text field as opaque as when querying an SQL database. Text fields were not made for efficient queries.
If you just want to store serialized objects in a text field, that is fine. But do not try to build queries that look inside the text field to find objects.
Your idea sounds like you want to perform some joins, XML parsing, and XPath application to get to a value. This doesn't strike me as the most efficient thing to do.
So, my advise:
Either just store serialized objects in the db, and do nothing more than load them and perform all other operations in memory
Or, if you need to query complex data structures, you may really want to look into document stores/databases like CouchDB or MongoDB; you can also check Wikipedia on the subject. There are even databases specifically designed for storing XML, even though I personally don't like them very much.
Addendum, per your explanations above
Simply put, don't go over the top with this thing:
If you just want to persist C#/.NET objects, just use the XML Serialization already built into the framework, a single table and be done with it.
If you, for some reason, need to store complex XML, use a dedicated XML store
If you have a fixed database schema, but it is too complex for efficient queries, use a Document Store in memory where you keep a denormalized version of your data for faster queries (or just simplify your database schema)
If you don't really need a fixed schema, use just a Document Store, and forget about having any "schema definition" at all
As for your solution, yes, it could work somehow. As could a plain SQL schema if you set it up right. But for applying an XPath, you'll probably parse the whole XML document each time you access a record, which wouldn't be very efficient to begin with.
If you want to check out Document databases, there are .NET drivers for CouchDB and MongoDB. The eXist XML database offers a number of Web protocols, and you can probably create a client class easily with VisualStudio's point-and-shoot interface. Or just google for someone who already did.
I need to be able to create variable
schemas on the fly without changing
anything about the database access
layer.
You are re-implementing the RDBMS within an RDBMS. The DB can do this already - that is what the DDL statements like create table and create schema are for....
I suggest you look into "schemas" and SQL security. There is no reason with the correct security setup you cannot allow your users to create their own tables to store document attributes in, or even generate them automatically.
Edit:
Slightly longer answer, if you don't have full requirements immediately, I would store the data as XML data type, and query them using XPath queries. This will be OK for occasional queries over smallish numbers of rows (fewer than a few thousand, certainly).
Also, your RDBMS may support indexes over XML, which may be another way of solving your problem. CREATE XML INDEX in SqlServer 2008 for example.
However for frequent queries, you can use triggers or materialized views to create copies of relevant data in table format, so more intensive reports can be speeded up by querying the breakout tables.
I don't know your requirements, but if you are responsible for creating the reports/queries yourself, this may be an approach to use. If you need to enable users to create their own reports that's a bigger mountain to climb.
I guess what i am saying is "are you sure you need to do this and XML can't just do the job".
In part, it will depend of your DB Engine. You're using SQL Server, don't you?
Answering your topics:
1 - Comparing the value of a text field versus of a varchar field: if you're comparing two db fields, varchar fields are smarter. Nvarchar(max) stores data in unicode with 2*l+2 bytes, where "l" is the lengh. For performance issues, you will need consider how much larger tables will be, for selecting the best way to index (or not) your table fields. See the topic.
2 - Sometimes nested queries are easily created and executed, also serving as a way to reduce query time. But, depending of the complexity, would be better to use different kind of joins. The best way is try to do in both ways. Execute two or more times each query, for the DB engine "compiles" a query on first executing, then the subsequent are quite faster. Measure the times for different parameters and choose the best option.
"Sometimes you can rewrite a subquery to use JOIN and achieve better performance. The advantage of creating a JOIN is that you can evaluate tables in a different order from that defined by the query. The advantage of using a subquery is that it is frequently not necessary to scan all rows from the subquery to evaluate the subquery expression. For example, an EXISTS subquery can return TRUE upon seeing the first qualifying row." - link
3- There's no much information in this question, but if you will get the xml document directly from the table, would be a good idea insted a view. Again, it will depends of the view and the document.
4- Other issues is about the total records expected for your table; the indexing of the columns, in wich you need to consider sorting, joining, filtering, PK's and FK's. Each situation could demmand different aproaches. My sugestion is to invest some time reading about your database engine and queries functioning and relating to your system.
I hope I've helped.
Interesting question.
I think you may be asking the wrong question here. Broadly speaking, as long as you have a FULLTEXT index on your text field, queries will be fast. Much faster than varchar if you have to use wild cards, for instance.
However, if I were you, I'd concentrate on the actual queries you're going to be running. Do you need boolean operators? Wildcards? Numerical comparisons? That's where I think you will encounter the real performance worries.
I would imagine you would need queries like:
"find all addresses in the states of New York, New Jersey and Pennsylvania"
"find all addresses between house numbers 1 and 100 on Mulberry Street"
"find all addresses where the zipcode is missing, and the city is New York"
At a high level, the solution you propose is to store your XML somewhere, and then de-normalize that XML into name/value pairs for querying.
Name/value pairs have a long and proud history, but become unwieldy in complex query situations, because you're not using the built-in optimizations and concepts of the relational database model.
Some refinements I'd recommend is to look at the domain model, and at least see if you can factor out separate data types into the "value" column; you might end up with "textValue", "moneyValue", "integerValue" and "dateValue". In the example you give, you might factor "address 1" into "housenumber" (as an integer) and "streetname".
Having said all this - I don't think there's a better solution other than completely changing tack to a document-focused database.
I've got an SQL table that I use to keep product data. Some products have other attached data to them (be it: books have number of pages, cover type; movies have their time-length; etc).
I could use a separate table in SQL to keep those, keeping (name, value) pairs.
I can also just keep an XML-packed data in a single field in a table. It's not a normalized approach, but seems more-natural for me.
I did a similar thing in a shopping basket application. We needed to attach meta data to the products without creating too much of a schema, which would have restricted the format of the meta-data in the future. We kept the meta-data as XML.
The only reason I would not do it is if you're going to end up performing queries on the data. Just make sure you won't have some daft person wanting reports by Publisher meta-data or something (which has happened to me) and you should be fine.
If you were intending to use XML as a way of not properly defining database tables that would indeed an architectural cop-out. I'm not sure about your scenario, it seems dangerously close to that. But key-value pairs are probably worse.
The best thing is to use a specialist XML datatype, if your database has one. In addition to RageZ's list, Oracle as had an XMLType for ten years now (since 9i). The advantage of using XMLType is two-fold. It announces to the casual observer that the documents in this column are XML. It also gives you access to built-in functionality, such as validation with XML Schemas, should you want it. Other features could prove handy if you subsequently have to start referring to the contents of the XML. For instance, Oracle's XDB supports an XML index type which can dramatically improve the performance of XPath queries.
It depends!
If you expect the 'shape' of your products to vary greatly then XML is a good way to go. [If you are using SQL Server you can index an XML field.]
I don't htink it's an architectural misconception. Just make sure you don't want to use those data in a query because it's gonna be complex.
Plus recent RDBM have function to handle XML (MSSQL, Postgres, Mysql) so you would still be able to use those data.