I have a situation while database designing, A simple issue but needed a working suggestions
My database tables:
TableAees.
TableBees.
Aees can mapped/contain one or more records of table Bees or also can be without any Bees
Aees can also mapped with one or more records of table Aees itself
Here normal primary key and foreign key relationship/hierarchy won't solve the purpose and also worried that parent/child hierarchy may end up in forming a loop between tables and can give a duplicates records on various joins.
Need a better table mapping for above mentioned tables(a,b) which will satisfy 1 and 2 points.
So to avoid such a situation, how the table relationship/hierarchy will be a better approach?
Database used: SQL Server
Thanks for sharing your knowledge.
You seem to describe a many-to-many relationship. If so, you would create a thrid table to store that relationship, like so:
create table a (
a_id int primary key,
...
);
create table b (
b_id int primary key,
...
);
create table ab (
a_id int references a(a_id),
b_id int references b(b_id),
primary key (a_id, b_id)
)
Each a/b tuple is stored on a separate row in bridge table ab.
I have created a movie database that contain following tables :
1.Film
2.People
3.Genres
4.Role
5.Users
Here is my sql
create table Film ( id INT(30),title varchar(30),images varchar(30)
primary key (id));
create table people(id INT(30),Fname varchar(30),Lname varchar(30),
primary key (id));
create table Role(id INT(30), name varchar(30),
primary key(id));
i want create relation between Film,People and Role table.SO my Question is do i need to create a table to make relation between those table and is it necessary to use auto_increment for id column?
You'd want to create some tables like:
FilmRole( FilmId INT, RoleId INT) these 2 columns would make your PK and they are also FK's to their
FilmPeople (FilmId INT, PeopleId INT) respective source tables.
FilmUsers( FilmId INT, UserId INT)
You could add a single IDENTITY (for SQL Server for example) column to each table if you wanted but in this particular case a 2 column PK is adequate as these tables simply point to other records.
You need to alter your table and add in a foreign key (Primary key in one table and attribute in another). Examples how to do it here! http://www.w3schools.com/sql/sql_foreignkey.asp
do i need to create a table to make relation between those table ?
YES ! to enforce Referential integrity read this
is it necessary to use auto_increment for id column?
Depends but it is most prefered way of creating a PK on a table
Why does add a foreign key to the tblDomare table result in this error?
The ALTER TABLE statement conflicted with the FOREIGN KEY constraint "FK__tblDomare__PersN__5F7E2DAC". The conflict occurred in database "almu0004", table "dbo.tblBana", column 'BanNR'.
Code
CREATE TABLE tblDomare
(PersNR VARCHAR (15) NOT NULL,
fNamn VARCHAR (15) NOT NULL,
eNamn VARCHAR (20) NOT NULL,
Erfarenhet VARCHAR (5),
PRIMARY KEY (PersNR));
INSERT INTO tblDomare (PersNR,fNamn,eNamn,Erfarenhet)
Values (6811034679,'Bengt','Carlberg',10);
INSERT INTO tblDomare (PersNR,fNamn,eNamn,Erfarenhet)
Values (7606091347,'Josefin','Backman',4);
INSERT INTO tblDomare (PersNR,fNamn,eNamn,Erfarenhet)
Values (8508284163,'Johanna','Backman',1);
CREATE TABLE tblBana
(BanNR VARCHAR (15) NOT NULL,
PRIMARY KEY (BanNR));
INSERT INTO tblBana (BanNR)
Values (1);
INSERT INTO tblBana (BanNR)
Values (2);
INSERT INTO tblBana (BanNR)
Values (3);
ALTER TABLE tblDomare
ADD FOREIGN KEY (PersNR)
REFERENCES tblBana(BanNR);
It occurred because you tried to create a foreign key from tblDomare.PersNR to tblBana.BanNR but/and the values in tblDomare.PersNR didn't match with any of the values in tblBana.BanNR. You cannot create a relation which violates referential integrity.
This query was very useful for me. It shows all values that don't have any matches
select FK_column from FK_table
WHERE FK_column NOT IN
(SELECT PK_column from PK_table)
Try this solution:
There is a data item in your table whose associated value doesn't exist in the table you want to use it as a primary key table.
Make your table empty or add the associated value to the second table.
It is possible to create the foreign key using ALTER TABLE tablename WITH NOCHECK ..., which will allow data that violates the foreign key.
"ALTER TABLE tablename WITH NOCHECK ..." option to add the FK -- This solution worked for me.
Remove all existing data from your tables and then make a relation between the tables.
Before You add Foreign key to the table, do the following
Make sure the table must empty or The column data should match.
Make sure it is not null.
If the table contains do not go to design and change, do it manually.
alter table Table 1 add foreign key (Column Name) references Table 2 (Column Name)
alter table Table 1 alter column Column Name attribute not null
I guess, a column value in a foreign key table should match with the column value of the primary key table. If we are trying to create a foreign key constraint between two tables where the value inside one column(going to be the foreign key) is different from the column value of the primary key table then it will throw the message.
So it is always recommended to insert only those values in the Foreign key column which are present in the Primary key table column.
For ex. If the Primary table column has values 1, 2, 3 and in Foreign key column the values inserted are different, then the query would not be executed as it expects the values to be between 1 & 3.
In very simple words your table already has data present in it and the table you are trying to create relationship with does have that Primary key set for the values that are already present.
Either delete all the values of the existing table.
Add all the values of foreign key reference in the new table.
Try DELETE the current datas from tblDomare.PersNR . Because the values in tblDomare.PersNR didn't match with any of the values in tblBana.BanNR.
When you define a Foreign Key in table B referencing the Primary Key of table A it means that when a value is in B, it must be in A. This is to prevent unconsistent modifications to the tables.
In your example, your tables contain:
tblDomare with PRIMARY KEY (PersNR):
PersNR |fNamn |eNamn |Erfarenhet
-----------|----------|-----------|----------
6811034679 |'Bengt' |'Carlberg' |10
7606091347 |'Josefin' |'Backman' |4
8508284163 |'Johanna' |'Backman' |1
---------------------------------------------
tblBana:
BanNR
-----
1
2
3
-----
This statement:
ALTER TABLE tblDomare
ADD FOREIGN KEY (PersNR)
REFERENCES tblBana(BanNR);
says that any line in tblDomare with key PersNR must have a correspondence in table tblBana on key BanNR. Your error is because you have lines inserted in tblDomare with no correspondence in tblBana.
2 solutions to fix your issue:
either add lines in tblBana with BanNR in (6811034679, 7606091347, 8508284163)
or remove all lines in tblDomare that have no correspondence in tblBana (but your table would be empty)
General advice: you should have the Foreign Key constraint before populating the tables. Foreign keys are here to prevent the user of the table from filling the tables with inconsistencies.
i had this error too
as Smutje reffered make sure that you have not a value in foreign key column of your base foreign key table that is not in your reference table i.e(every value in your base foreign key table(value of a column that is foreign key) must also be in your reference table column)
its good to empty your base foreign key table first then set foreign keys
the data you have entered a table(tbldomare) aren't match a data you have assigned primary key table. write between tbldomare and add this word (with nocheck) then execute your code.
for example you entered a table tbldomar this data
INSERT INTO tblDomare (PersNR,fNamn,eNamn,Erfarenhet)
Values (6811034679,'Bengt','Carlberg',10);
and you assigned a foreign key table to accept only 1,2,3.
you have two solutions one is delete the data you have entered a table then execute the code. another is write this word (with nocheck) put it between your table name and add
like this
ALTER TABLE tblDomare with nocheck
ADD FOREIGN KEY (PersNR)
REFERENCES tblBana(BanNR);
Smutje is correct and Chad HedgeCock offered a great layman's example.
Id like to build on Chad's example by offering a way to find/delete those records.
We will use Customer as the Parent and Order as the child. CustomerId is the common field.
select * from Order Child
left join Customer Parent on Child.CustomerId = Parent.CustomerId
where Parent.CustomerId is null
if you are reading this thread... you will get results. These are orphaned children. select * from Order Child
left join Customer Parent on Child.CustomerId = Parent.CustomerId
where Parent.CustomerId is null Note the row count in the bottom right.
Go verify w/ whomever you need to that you are going to delete these rows!
begin tran
delete Order
from Order Child
left join Customer Parent on Child.CustomerId = Parent.CustomerId
where Parent.CustomerId is null
Run the first bit.
Check that row count = what you expected
commit the tran
commit tran
Be careful. Someone's sloppy programming got you into this mess. Make sure you understand the why before you delete the orphans. Maybe the parent needs to be restored.
From our end, this is the scenario:
We have an existing table in the database with records.
Then I introduces a NOT nullable foreign key
After executing the update i got this error.
How did i solve you ask?
SOLUTION: I just removed all the records of the table, then tried to update the database and it was successful.
This happens to me, since I am designing my database, I notice that I change my seed on my main table, now the relational table has no foreign key on the main table.
So I need to truncate both tables, and it now works!
You should see if your tables has any data on the rows. If "yes" then you should truncate the table(s) or else you can make them to have the same number of data at tblDomare.PersNR to tblBana.BanNR and vise-verse.
In my scenario, using EF, upon trying to create this new Foreign Key on existing data, I was wrongly trying to populate the data (make the links) AFTER creating the foreign key.
The fix is to populate your data before creating the foreign key since it checks all of them to see if the links are indeed valid. So it couldn't possibly work if you haven't populated it yet.
I encounter some issue in my project.
In child table, there isn't any record Id equals 1 and 11
I inserted DEAL_ITEM_THIRD_PARTY_PO table which Id equals 1 and 11 then I can create FK
Please first delete data from that table and then run the migration again. You will get success
I had the same problem.
My issue was having nullable: true in column (migration file):
AddColumn("dbo.table", "column", c => c.Int(nullable: true));
Possible Solutions:
Change nullable 'false' to 'true'. (Not Recommended)
Change property type from int to int? (Recommended)
And if required, change this later after adding column > then missing field data in previous records
If you've changed an existing property from nullable to non-nullable:
3) Fill the column data in database records
A foreign key constraint in a child table must have a parent table with a primary key. The primary key must be unique. The foreign key value must match a value in the patent table primary key
When you alter table column from nullable to not nullable column where this column is foreign key, you must :
Firstly, initialize this column with value (because it is foreign
key not nullable).
After that you can alter your table column normally.
Please try below query:
CREATE TABLE tblBana
(BanNR VARCHAR (15) NOT NULL PRIMARY KEY,
);
CREATE TABLE tblDomare
(PersNR VARCHAR (15) NOT NULL PRIMARY KEY,
fNamn VARCHAR (15) NOT NULL,
eNamn VARCHAR (20) NOT NULL,
Erfarenhet VARCHAR (5),
FK_tblBana_Id VARCHAR (15) references tblBana (BanNR)
);
INSERT INTO tblBana (BanNR)
Values (3);
INSERT INTO tblDomare (PersNR,fNamn,eNamn,Erfarenhet,FK_tblBana_Id)
Values (8508284173,'Johanna','Backman',1,3);
or you can use this
SELECT fk_id FROM dbo.tableA
Except
SELECT fk_id From dbo.tableB
and just FYI, in case you do all of your data reference checks and find no bad data...apparently it is not possible to create a foreign key constraint between two tables and fields where those fields are the primary key in both tables! Do not ask me how I know this.
This isn't a big deal, but my OCD is acting up with the following problem in the database I'm creating. I'm not used to working with databases, but the data has to be stored somewhere...
Problem
I have two tables A and B.
One of the datafields is common to both tables - segments. There's a finite number of segments, and I want to write queries that connect values from A to B through their segment values, very much asif the following table structure was used:
However, as you can see the table Segments is empty. There's nothing more I want to put into that table, rather than the ID to give other table as foreign keys. I want my tables to be as simple as possible, and therefore adding another one just seems wrong.
Note also that one of these tables (A, say) is actually master, in the sense that you should be able to put any value for segment into A, but B one should first check with A before inserting.
EDIT
I tried one of the answers below:
create table A(
id int primary key identity,
segment int not null
)
create table B(
id integer primary key identity,
segment int not null
)
--Andomar's suggestion
alter table B add constraint FK_B_SegmentID
foreign key (segment) references A(segment)
This produced the following error.
Maybe I was somehow unclear that segments is not-unique in A or B and can appear many times in both tables.
Msg 1776, Level 16, State 0, Line 11 There are no primary or candidate
keys in the referenced table 'A' that match the referencing column
list in the foreign key 'FK_B_SegmentID'. Msg 1750, Level 16, State 0,
Line 11 Could not create constraint. See previous errors.
You can create a foreign key relationship directly from B.SegmentID to A.SegmentID. There's no need for the extra table.
Update: If the SegmentIDs aren't unique in TableA, then you do need the extra table to store the segment IDs, and create foreign key relationships from both tables to this table. This however is not enough to enforce that all segment IDs in TableB also occur in TableA. You could instead use triggers.
You can ensure the segment exists in A with a foreign key:
alter table B add constraint FK_B_SegmentID
foreign key (SegmentID) references A(SegmentID)
To avoid rows in B without a segment at all, make B.SegmentID not nullable:
alter table B alter column SegmentID int not null
There is no need to create a Segments table unless you want to associate extra data with a SegmentID.
As Andomar and Mark Byers wrote, you don't have to create an extra table.
You can also CASCADE UPDATEs or DELETEs on the master. Be very carefull with ON DELETE CASCADE though!
For queries use a JOIN:
SELECT *
FROM A
JOIN B ON a.SegmentID = b.SegmentID
Edit:
You have to add a UNIQUE constraint on segment_id in the "master" table to avoid duplicates there, or else the foreign key is not possible. Like this:
ALTER TABLE A ADD CONSTRAINT UNQ_A_SegmentID UNIQUE (SegmentID);
If I've understood correctly, a given segment cannot be inserted into table B unless it has also been inserted into table A. In which case, table A should reference table Segments and table B should reference table A; it would be implicit that table B ultimately references table Segments (indirectly via table A) so an explicit reference is not required. This could be done using foreign keys (e.g. no triggers required).
Because table A has its own key I assume a given segment_ID can appear in table A more than once, therefore for B to be able to reference the segment_ID value in A then a superkey would need to be defined on the compound of A_ID and segment_ID. Here's a quick sketch:
CREATE TABLE Segments
(
segment_ID INTEGER NOT NULL UNIQUE
);
CREATE TABLE A
(
A_ID INTEGER NOT NULL UNIQUE,
segment_ID INTEGER NOT NULL
REFERENCES Segments (segment_ID),
A_data INTEGER NOT NULL,
UNIQUE (segment_ID, A_ID) -- superkey
);
CREATE TABLE B
(
B_ID INTEGER NOT NULL UNIQUE,
A_ID INTEGER NOT NULL,
segment_ID INTEGER NOT NULL,
FOREIGN KEY (segment_ID, A_ID)
REFERENCES A (segment_ID, A_ID),
B_data INTEGER NOT NULL
);