I have a DB about renting cars.
I created a CarModels table (ModelID as PK).
I want to create a second table with the same primary key as CarModels have.
This table only contains the number of times this Model was searched on my website.
So lets say you visit my website, you can check a list that contains common cars rented.
"Most popular Cars" table.
It's not about One-to-One relationship, that's for sure.
Is there any SQL code to connect two Primary keys together ?
select m.ModelID, m.Field1, m.Field2,
t.TimesSearched
from CarModels m
left outer join Table2 t on m.ModelID = t.ModelID
but why not simply add the field TimesSearched to table CarModels ?
Then you dont need another table
Easiest is to just use a new primary key on the new table with a foreign key to the CarModels table, like [CarModelID] INT NOT NULL. You can put an index and a unique constraint on the FK.
If you reeeealy want them to be the same, you can jump through a bunch of hoops that will make your life Hell, like creating the table from the CarModels table, then setting that field as the primary key, then whenever you add a new CarModel you'll have to create a trigger that will SET IDENTITY_INSERT ON so you can add the new one, and remember to SET IDENTITY_INSERT OFF when you're done.
Personally, I'd create a CarsSearched table that holds ThisUser selected ThisCarModel on ThisDate: then you can start doing some fun data analysis like [are some cars more popular in certain zip codes or certain times of year?], or [this company rents three cars every year in March, so I'll send them a coupon in January].
You are not extending anything (modifying the actual model of the table). You simply need to make INNER JOIN of the table linking with the primary keys being equal.
It could be outer join as it has been suggested but if it's 1:1 like you said ( the second table with have exact same keys - I assume all of them), inner will be enough as both tables would have the same set of same prim keys.
As a bonus, it will also produce fewer rows if you didn't match all keys as a nice reminder if you fail to match all PKs.
That being said, do you have a strong reason why not to keep the said number in the same table? You are basically modeling 1:1 relationship for 1 extra column (and small one too, by data type)
You could extend (now this is extending tables model) with the additional attribute of integer that keeps that number for you.
Later is preferred for simplicity and lower query times.
Related
I have problem with SQL .
I am trying to create tables from other tables with foreign keys. The table is created normally without problem but when I'm trying to see the data inside the table there is none of the data inside! Anyone who knows?
-- Here is my table with the foreign keys
CREATE TABLE StaffXCustomersXMeals(
--MealID int FOREIGN KEY REFERENCES Meals(MealID),
--CustomersID int FOREIGN KEY REFERENCES Customers(CustomersID),
--StaffID int FOREIGN KEY REFERENCES Staff(StaffID)
)
You might want to consider reading up on the concept of a SQL View - think of it as a "virtual table based on the result-set of an SQL statement." When you create a view based on the JOIN between multiple tables, it saves your query and you can then select from it like you would a table.
For example, you might have the following:
CREATE VIEW StaffCustomerMeals
AS
SELECT
m.MealID,
c.CustomersID,
s.StaffID
FROM
Meals m
LEFT JOIN
Customers c ON
m.SomeIDThatMeansThisCustomer = c.CustomersID
LEFT JOIN
Staff s ON
m.SomeIDThatMeansAStaffMember = s.StaffID
You need to define these relationships in accordance with your own schema - it's your homework assignment, so do your best to figure it out. A couple more questions to consider:
Does a meal always have both a customer and a staff member, or are they customers who might happen to be staff members?
Should you include other information besides the IDs (e.g., CustomerName, StaffMemberDepartment, MealPrice, PurchaseDate)
You didn't enter any data. You only said that whatever data entered must reference the other tables. So you cannot enter invalid meals for instance.
You need INSERT statments to fill the table with whatever data you need.
I have the following database schema:
members_company1(id, name, ...);
members_company2(id, name, ...);
profiles(memberid, membertypeid, ...);
membertypes(id, name, ...)
[
{ id : 1, name : 'company1', ... },
{ id : 2, name : 'company2', ... }
];
So each profile belongs to a certain member either from company1 or company2 depending on membertypeid value
members_company1 ————————— members_company2
———————————————— ————————————————
id ——————————> memberid <——————————— id
name membertypeid name
/|\
|
|
profiles |
—————————— |
memberid ————————+
membertypeid
I am wondering if it's possible to create a foreign key in profiles table for referential integrity based on memberid and membertypeid pair to reference either members_company1 or members_company2 table records?
A foreign key can only reference one table, as stated in the documentation (emphasis mine):
A foreign key (FK) is a column or combination of columns that is used
to establish and enforce a link between the data in two tables.
But if you want to start cleaning things up you could create a members table as #KevinCrowell suggested, populate it from the two members_company tables and replace them with views. You can use INSTEAD OF triggers on the views to 'redirect' updates to the new table. This is still some work, but it would be one way to fix your data model without breaking existing applications (if it's feasible in your situation, of course)
Operating under the fact that you can't change the table structure:
Option 1
How important is referential integrity to you? Are you only doing inner joins between these tables? If you don't have to worry too much about it, then don't worry about it.
Option 2
Ok, you probably have to do something about this. Maybe you do have inner joins only, but you have to deal with data in profiles that doesn't relate to anything in the members tables. Could you create a job that runs once per day or week to clean it out?
Option 3
Yeah, that one may not work either. You could create a trigger on the profiles table that checks the reference to the members tables. This is far from ideal, but it does guarantee instantaneous checks.
My Opinion
I would go with option 2. You're obviously dealing with a less-than-ideal schema. Why make this worse than it has to be. Let the bad data sit for a week; clean the table every weekend.
No. A foreign key can reference one and only one primary key and there is no way to spread primary keys across tables. The kind of logic you hope to achieve will require use of a trigger or restructuring your database so that all members are based off a core record in a single table.
Come on you can create a table but you cannot modify members_company1 nor members_company2?
Your idea of a create a members table will require more actions when new records are inserted into members_company tables.
So you can create triggers on members_company1 and members_company2 - that is not modify?
What are the constraints to what you can do?
If you just need compatibility on selects to members_company1 and members_company2 then create a real members table and create views for members_company1 and members_company2.
A basic select does not know it is a view or a table on the other end.
CREATE VIEW dbo.members_company1
AS
SELECT id, name
FROM members
where companyID = 1
You could possible even handle insert, updates, and deletes with instead-of
INSTEAD OF INSERT Triggers
A foreign key cannot reference two tables. Assuming you don't want to correct your design by merging members_company1 and members_company2 tables, the best approach would be to:
Add two columns called member_company1_id and member_company2_id to your profiles table and create two foreign keys to the two tables and allow nulls. Then you could add a constraint to ensure 1 of the columns is null and the other is not, at all times.
I'm dealing with this situation. I've got three tables in SQL SERVER called Movies, Series and Orders.
Orders has an ItemId where this could be a MovieId (Movies PK) or SerieId (Series PK). I mean, in Orders table could have records where are from movies of series.
I don't know how to maintain this relationship or which could be the best way to implement it. Until I know, I only can create 1 to 1 or 1 to many relationships between 2 tables, not for 3.
Thanks in advance.
In this case I think it would be better to store Movies and Series in the same table with the common attributes incl. a column which indicates the type (Movie or Serie) and then have the additional attributes in seperate tables (if you want to normalize) or even in the same table (in order to avoid joins).
You should learn about implement table inheritance in sql-server.
Do you have a product and this product may be a Movie or a Serie.
In linked sample a person may be a student or a teacher.
The best way is:
create a generic table for movies and series with the fields that both entities should share (like ItemId).
create a table for movies that references the table created on step 1, containing the fields that must be compiled only for movies. This new table will be in relation one-to-one with the previous one.
same thing for series.
create the orders table and set ItemId foreign key to point to the ItemId primary key of the table created on step 1.
I am dealing with a table that contains both cars and owners (table CO). I am creating another table to contain attributes for an owner (table OwnerAttributes), that a user can assign to through a GUI. My problem lies in the fact that owners are not unique and since I am using SQL Server I cannot create a foreign key on them. There is an id in the table, but it identifies the car and owner as a whole.
The idea I had to get around this problem is to create a new table (table Owners) that contains distinct owners, and then adding a trigger to table CO that would update the Owners with any changes. I can then use table Owners for my OwnerAttributes table and solve my problem.
The question I want answered is if there is a better way to do this?
I am using a preexisting database, that is heavily used by an old application. The application is hooked up to use the table CO for owners and cars. There also exists several other tables that use the CO table. I wish I could split the table into Owners and Cars, but the company doesn't want me to spend all my time doing it as there are several more features I need to add to the application.
Your thoughts on the Owners table are on the right track! Your problem is because your schema is not normalized. It's the fact you're storing two things (cars, and owners) in one table (your table CO).
You are correct that you should make an Owner table, but you should then remove the Owner information from the CO table entirely, and replace it with a foreign key to the Owners table.
So you want something like this:
CREATE TABLE Owner (
ownerID int not null primary key indentity(1,0),
FirstName varchar(255),
LastName varchar(255),
/* other fields here */
)
GO
CREATE TABLE Car
carID int not null primary key identity(1,0),
ownerID int not null references Owner(ownerID),
/* other fields go here */
GO
/* a convenience, read only view to replace your old CAR OWNER table */
CREATE VIEW Car_Owner AS
SELECT c.*, o.FirstName, o.LastName FROM Car c INNER JOIN Owner o ON c.ownerID = o.ownerID
Now, you have everything properly normalized in SQL. A view has given you back the car_owner as one thing in a pseudo-table.
But the real answer is, normalize your schema. Let SQL do what it does best (relate things to other things). Combining the two things on one table will just lead to more problems like you're encountering downstream.
Hopefully this answer seems helpful and not condescending, which is what I was going for! I have learned the hard way that this approach (normalize everything, let the database do some extra work to retrieve/display/insert it) is the only one that works out in the end.
You should create Owner table, Car table, OwnerCar table(if person can has a few cars). Owner table contains fields, that describe owner(owner properties)
I have a table which has employee relationship defined within itself.
i.e.
EmpID Name SeniorId
-----------------------
1 A NULL
2 B 1
3 C 1
4 D 3
and so on...
Where Senior ID is a foreign key whose primary key table is same with refrence column EmpId
I want to clear all rows from this table without removing any constraint. How can i do this?
Deletion need to be performed like this
4, 3 , 2 , 1
How can I do this
EDIT:
Jhonny's Answer is working for me but which of the answers are more efficient.
I don't know if I am missing something, but maybe you can try this.
UPDATE employee SET SeniorID = NULL
DELETE FROM employee
If the table is very large (cardinality of millions), and there is no need to log the DELETE transactions, dropping the constraint and TRUNCATEing and recreating constraints is by far the most efficient way. Also, if there are foreign keys in other tables (and in this particular table design it would seem to be so), those rows will all have to be deleted first in all cases, as well.
Normalization says nothing about recursive/hierarchical/tree relationships, so I believe that is a red herring in your reply to DVK's suggestion to split this into its own table - it certainly is viable to make a vertical partition of this table already and also to consider whether you can take advantage of that to get any of the other benefits I list below. As DVK alludes to, in this particular design, I have often seen a separate link table to record self-relationships and other kinds of relationships. This has numerous benefits:
have many to many up AND down instead of many-to-one (uncommon, but potentially useful)
track different types of direct relationships - manager, mentor, assistant, payroll approver, expense approver, technical report-to - with rows in the relationship and relationship type tables instead of new columns in the employee table
track changing hierarchies in a temporally consistent way (including terminated employee hierarchy history) by including active indicators and effective dates on the relationship rows - this is only fully possible when normalizing the relationship into its own table
no NULLs in the SeniorID (actually on either ID) - this is a distinct advantage in avoiding bad logic, but NULLs will usually appear in views when you have to left join to the relationship table anyway
a better dedicated indexing strategy - as opposed to adding SeniorID to selected indexes you already have on Employee (especially as the number of relationship types grows)
And of course, the more information you relate to this relationship, the more strongly is indicated that the relationship itself merits a table (i.e. it is a "relation" in the true sense of the word as used in relational databases - related data is stored in a relation or table - related to a primary key), and thus a normal form for relationships might strongly indicate that the relationship table be created instead of a simple foreign key relationship in the employee table.
Benefits also include its straightforward delete scenario:
DELETE FROM EmployeeRelationships;
DELETE FROM Employee;
You'll note a striking equivalence to the accepted answer here on SO, since, in your case, employees with no senior relationship have a NULL - so in that answer the poster set all to NULL first to eliminate relationships and then remove the employees.
There is a possibly appropriate usage of TRUNCATE depending upon constraints (EmpployeeRelationships is typically able to be TRUNCATEd since its primary key is usually a composite and not a foreign key in any other table).
Try this
DELETE FROM employee;
Inside a loop, run a command that deletes all rows with an unreferenced EmpID until there are zero rows left. There are a variety of ways to write that inner DELETE command:
DELETE FROM employee WHERE EmpID NOT IN (SELECT SeniorID FROM employee)
DELETE FROM employee e1 WHERE NOT EXISTS
(SELECT * FROM employee e2 WHERE e2.SeniorID = e.EmpID
and probably a third one using a JOIN, but I'm not familiar with the SQL Server syntax for that.
One solution is to normalize this by splitting out "senior" relationship into a separate table. For the sake of generality, make that second table "empID1|empID2|relationship_type".
Barring that, you need to do this in a loop. One way is to do it:
declare #count int
select #count=count(1) from table
while (#count > 0)
BEGIN
delete employee WHERE NOT EXISTS
(select 1 from employee 'e_senior'
where employee.EmpID=e_senior.SeniorID)
select #count=count(1) from table
END