I am trying to enforce the property that table Match should have all unique tuples (Team 1, Team 2). However, let Team 1 = Detroit Pistons and Team 2 = Chicago Bulls. I do not want to allow (Detroit Pistons, Chicago Bulls) to be inserted into the table if (Chicago Bulls, Detroit Pistons) already exists.
How can I enforce this constraint?
A) The tuples are semantically identical. (I think this is your case.)
That means the tuple {Chicago Bulls, Detroit Pistons} means exactly the same thing as the tuple {Detroit Pistons, Chicago Bulls}. Use a CHECK constraint to impose an order on the two columns.
CHECK (column_1 < column_2)
That kind of constraint would allow {Chicago Bulls, Detroit Pistons}, but it would reject {Detroit Pistons, Chicago Bulls}. This is kind of like imposing a canonical form on otherwise free-form data.
B) The tuples are semantically distinct.
That means the tuple {Chicago Bulls, Detroit Pistons} means one thing, and the tuple {Detroit Pistons, Chicago Bulls} means something else. For example, the first attribute might mean "home team", and the second might mean "visiting team". In this case, all you need is some kind of unique constraint on the pair of columns.
You can create a unique function-based index:
CREATE UNIQUE INDEX unq_match ON match ( LEAST(team1,team2), GREATEST(team1,team2) );
LEAST() will get the "lesser" of the two teams (whether by ID or name, it doesn't matter) while GREATEST will get the "greater" of the two. Unfortunately this particular solution doesn't scale up to 3-or-more-tuples.
Related
I'm building a database schema for users of my app, and I am thinking of setting the userid value according to user type. So,
buyers: 10001 to 19999
sellers: 20001 to 29999
shippers: 30001 to 39999
Next, I assign unique email addresses to the userid:
Login_table
Email.......password.......userid
aaaaa#yy.com....... password.......10005 ---> this email belong to user 10005 (a buyer)
bbbbb#yy.com.......password.......20008 ---> this email belongs to user 20008 (a seller)
ccccc#yy.com.......password.......30187 ---> this email belongs to user 30187 (a shipper)
I then have 3 tables for buyers, sellers, and shippers because each may have different attributes:
buyer_table
buyerid.......name....... mother
10005....... John....... Mary
10006 ....... Chris....... Nancy
seller_table
sellerid....... name....... pet
20008 ....... Adam....... Dog
20018 ....... Tony ....... cat
shipper_table
shipperid....... name....... car
30187....... George....... GMC
30188 ....... Larry ....... Honda
The advantage here is that I have a single login_table for all user types. I do not want to have 3 login tables for each type. Based on the userid value I know what type of user it is. Keeping three tables for each user (buyer_table, seller_table, and shipper_table) is good for making the schema more understandable, in addition to being able to assign different attributes to each user type.
Sounds good? Maybe.
However, I have a problem in that the login_table refers to “userid” while the three user tables each has a different id name for the user, so in the buyer_table I have buyerid as primary key, in the seller_table it is sellerid as primary key, and finally in the shipper_table, the shipperid is the primary key.
How can I link these three primary keys to the login_table? The login_table has userid as a foreign key to one of those three tables, but it is called “userid”, not buyerid, or sellerid, or shipperid!
1) Is it a good idea to classify the userid value according to ranges?
2) If so, how can I resolve the PK-FK issue as described above?
3) Am I off completely?
Having ranges of values for different kinds of similar objects is not bad. If you feel like doing so, you could use sequences wich support value ranges. This way, you could have a buyer sequence wich goes from 0-1000, a seller one from 1001 to 2000 and so on. That would also help you keeping track of the increasing index of the different kinds!
Summery: I need any combination of [Field_1] and [Field_2] to be unique and for that uniqueness to be enforced. Note: This is not for permutations - and that's the difficulty.
In Depth:
I'm trying to track contacts for vendor software. I've set my DB up in the time old fashion such that a Vendor record may have many contacts. The trick is that contacts may be related to each other and may not be related to the parent vendor record. An example:
1. SuperBrokenSoftware is a tool who's vendor I need to contact all the time.
2. WeMakeBadSoftware is the Vendor
3. Fred works for WeMakeBadSoftware
4. Gale works for WeHelpPeopleWhenOthersWont
Let's say Gale is the appropriate contact to fix my issue with the SuperBrokenSoftware.
There is no way using the current hierarchy to track Gales relationship to SuperBrokenSoftware.
My solution is to keep track of these relationships in a table like so:
Field1 Field2 Field3
Fred Gale Gale handles specific issues for Fred
However given this solution Field_1 and Field_2 must be unique in combination. That is to say the records:
Field1 Field2 Field3
Fred Gale "Gale handles specific issues for Fred"
Gale Fred "Gale is awesome - Fred sucks"
Should be viewed as the same. Record 2 should not be allowed in the database because it is not unique.
What I have Tried:
Using the bijective - Szudzik's function: a >= b ? a * a + a + b : a + b * b; where a, b >= 0
I can calculate a unique identifier for every combination - but access cannot enforce uniqueness on a calculated field.
What is the best way to enforce a combination in Access?
Thanks in advance!!!
Create new field for unique identifier with unique index and create Before Change data macro, which should insert/change calculated identifier in new field.
Unique key can be just sorted concatenation of field1 and field2
I'm using ssas tabular (powerpivot) and need to design a data-model and write some DAX.
I have 4 tables in my relational database-model:
Orders(order_id, order_name, order_type)
Spots (spot_id,order_id, spot_name, spot_time, spot_price)
SpotDiscount (spot_id, discount_id, discount_value)
Discounts (discount_id, discount_name)
One order can include multiple spots but one spot (spot_id 1) can only belong to one order.
One spot can include different discounts and every discount have one discount_value.
Ex:
Order_1 has spot_1 (spot_price 10), spot_2 (spot_price 20)
Spot_1 has discount_name_1(discount_value 10) and discount_name_2 (discount_value 20)
Spot_2 has discount_name_1(discount_value 15) and discount_name_3 (discount_value 30)
I need to write two measures: price(sum) and discount_value(average)
How do I correctly design a star schema with fact table (or maybe two fact tables) so that I in my powerpivot cube can get:
If i choose discount_name_1 I should get
order_1 with spot_1 and spot_2 and price on order_1 level will have value 50 and discount_value = 12,5
If I choose discount_name_3 I should get
order_1 with only spot_2 and price on order level = 20 and discount_value = 30
Fact(OrderKey, SpotKey, DiscountKey, DateKey, TimeKey Spot_Price, Discount_Value,...)
DimOrder, DimSpot, DimDiscount, etc....
TotalPrice:=
SUMX(
SUMMARIZE(
Fact
,Fact[OrderKey]
,Fact[SpotKey]
,Fact[Spot_Price]
)
,Fact[Spot_Price]
)
AverageDiscount:=
AVERAGE(Fact[Discount_Value])
Fact table is denormalized and you end up with the simplest star schema you can have.
First measure deserves some explanation. [Spot_Price] is duplicated for any spot with multiple discounts, and we would get wrong results with a simple SUM(). SUMMARIZE() does a group by on all the columns passed to it, following relationships (if necessary, we're looking at a single table here so nothing to follow).
SUMX() iterates over this table and accumulates the value of the expression in its second argument. The SUMMARIZE() has removed our duplicate [Spot_Price]s so we accumulate the unique ones (per unique combination of [OrderKey] and [SpotKey]) in a sum.
You say
One order can include multiple spots but one spot (spot_id 1) can only
belong to one order.
That's is not supported in the table definitions you give just above that statement. In the table definitions, one order has only one spot but (unless you've added a unique index to Orders on spot_id) each Spot can have multiple orders. Each Spot can also have multiple discounts.
If you want to have the relationship described in your words, the table definitions should be:
Orders(order_id, order_name, order_type)
OrderSpot(order_id, spot_id) -- with a Unique index on spot_id)
Spots (spot_id, spot_name, spot_time, price)
or:
Orders(order_id, order_name, order_type)
Spots (spot_id, spot_name, spot_time, order_id, price)
You can create the ssas cube with Order as the fact table, with one dimention in the Spot Table. If you then add the SpotDiscount and Discount tables with their relations (SpotDiscount to Spot, Discount to SpotDiscount) you have a 1 dimentional.
EDIT as per comments
Well, the Fact table would have order_id, order_name, order_type
The Dimension would be made up of the other 3 tables and have the columns you're interested in: probably spot_name, spot_time, spot_price, discount_name, discount_value.
I have an application where the database back-end has around 15 lookup tables. For instance there is a table for Counties like this:
CountyID(PK) County
49001 Beaver
49005 Cache
49007 Carbon
49009 Daggett
49011 Davis
49015 Emery
49029 Morgan
49031 Piute
49033 Rich
49035 Salt Lake
49037 San Juan
49041 Sevier
49043 Summit
49045 Tooele
49049 Utah
49051 Wasatch
49057 Weber
The UI for this app has a number of combo boxes in various places for these lookup tables, and my client has asked that the boxes list in this case:
CountyID(PK) County
49035 Salt Lake
49049 Utah
49011 Davis
49057 Weber
49045 Tooele
'The Rest Alphabetically
The best plan I have for accomplishing this is to add a column to each lookup table for SortOrder(numeric). I had a colleague tell me he thought that would cause the tables to violate 3rd-Normal-Form, but I think the sort order still depends on the key and only the key (even though the rest of the list is alphabetical).
Is adding the SortOrder column the best way to do this, or is there a better way I am just not seeing?
I agree with #cletus that a sort order column is a good way to go and it does not violate 3NF (because, as you said, the sort order column entries are functionally dependent on the candidate keys of the table).
I'm not sure I agree that alphanumeric is better than numeric. In the specific case of counties, there are seldom new ones created. But there is no requirement that the numbers assigned are sequential; you can allocate them with numbers that are a multiple of a hundred, for example, leaving ample room for insertions.
Yes I agree a sort order column is the best solution when the requirements call for a custom sort order like the one you cite. I wouldn't go with a numeric column however. If the data is alphanumeric, the sort order should be alphanumeric. That way you can seed the value with whatever is in the county field.
If you use a numeric field you'll have to resequence the entire table (potentially) whenever you add a new entry. So:
Columns: ID, County, SortOrder
Seed:
UPADTE County SET SortOrder = CONCAT('M-', County)
and for the special cases:
UPDATE County
SET SortOrder = CONCAT('E-' . County)
WHERE County IN ('Salt Lake', 'Utah', 'Davis', 'Weber', 'Tooele')
Arguably you may want to put another marker column in to indicate those entries are special.
I went with numeric and large multiples.
Even with the CONCAT('E-'.. example, I don't get the required sort order. That would give me Davis, SL, Tooele... and Salt Lake needs to be first.
I ended up using multiples of 10 and assigned the non-special-sort entries a value like 10000. That way the view for each lookup can have
ORDER BY SortOrder ASC, OtherField ASC
Another programmer suggested using DECODE in Oracle, or CASE statements in SQL Server, but this is a more general solution. YMMV.
I am looking for a way to retrieve the "surrounding" rows in a NHibernate query given a primary key and a sort order?
E.g. I have a table with log entries and I want to display the entry with primary key 4242 and the previous 5 entries as well as the following 5 entries ordered by date (there is no direct relation between date and primary key). Such a query should return 11 rows in total (as long as we are not close to either end).
The log entry table can be huge and retrieving all to figure it out is not possible.
Is there such a concept as row number that can be used from within NHibernate? The underlying database is either going to be SQlite or Microsoft SQL Server.
Edited Added sample
Imagine data such as the following:
Id Time
4237 10:00
4238 10:00
1236 10:01
1237 10:01
1238 10:02
4239 10:03
4240 10:04
4241 10:04
4242 10:04 <-- requested "center" row
4243 10:04
4244 10:05
4245 10:06
4246 10:07
4247 10:08
When requesting the entry with primary key 4242 we should get the rows 1237, 1238 and 4239 to 4247. The order is by Time, Id.
Is it possible to retrieve the entries in a single query (which obviously can include subqueries)? Time is a non-unique column so several entries have the same value and in this example is it not possible to change the resolution in a way that makes it unique!
"there is no direct relation between date and primary key" means, that the primary keys are not in a sequential order?
Then I would do it like this:
Item middleItem = Session.Get(id);
IList<Item> previousFiveItems = Session.CreateCriteria((typeof(Item))
.Add(Expression.Le("Time", middleItem.Time))
.AddOrder(Order.Desc("Time"))
.SetMaxResults(5);
IList<Item> nextFiveItems = Session.CreateCriteria((typeof(Item))
.Add(Expression.Gt("Time", middleItem.Time))
.AddOrder(Order.Asc("Time"))
.SetMaxResults(5);
There is the risk of having several items with the same time.
Edit
This should work now.
Item middleItem = Session.Get(id);
IList<Item> previousFiveItems = Session.CreateCriteria((typeof(Item))
.Add(Expression.Le("Time", middleItem.Time)) // less or equal
.Add(Expression.Not(Expression.IdEq(middleItem.id))) // but not the middle
.AddOrder(Order.Desc("Time"))
.SetMaxResults(5);
IList<Item> nextFiveItems = Session.CreateCriteria((typeof(Item))
.Add(Expression.Gt("Time", middleItem.Time)) // greater
.AddOrder(Order.Asc("Time"))
.SetMaxResults(5);
This should be relatively easy with NHibernate's Criteria API:
List<LogEntry> logEntries = session.CreateCriteria(typeof(LogEntry))
.Add(Expression.InG<int>(Projections.Property("Id"), listOfIds))
.AddOrder(Order.Desc("EntryDate"))
.List<LogEntry>();
Here your listOfIds is just a strongly typed list of integers representing the ids of the entries you want to retrieve (integers 4242-5 through 4242+5 ).
Of course you could also add Expressions that let you retrieve Ids greater than 4242-5 and smaller than 4242+5.
Stefan's solution definitely works but better way exists using a single select and nested Subqueries:
ICriteria crit = NHibernateSession.CreateCriteria(typeof(Item));
DetachedCriteria dcMiddleTime =
DetachedCriteria.For(typeof(Item)).SetProjection(Property.ForName("Time"))
.Add(Restrictions.Eq("Id", id));
DetachedCriteria dcAfterTime =
DetachedCriteria.For(typeof(Item)).SetMaxResults(5).SetProjection(Property.ForName("Id"))
.Add(Subqueries.PropertyGt("Time", dcMiddleTime));
DetachedCriteria dcBeforeTime =
DetachedCriteria.For(typeof(Item)).SetMaxResults(5).SetProjection(Property.ForName("Id"))
.Add(Subqueries.PropertyLt("Time", dcMiddleTime));
crit.AddOrder(Order.Asc("Time"));
crit.Add(Restrictions.Eq("Id", id) || Subqueries.PropertyIn("Id", dcAfterTime) ||
Subqueries.PropertyIn("Id", dcBeforeTime));
return crit.List<Item>();
This is NHibernate 2.0 syntax but the same holds true for earlier versions where instead of Restrictions you use Expression.
I have tested this on a test application and it works as advertised