I have a scenario with a table RegisteredUsers and a primary key on userid.
Here is code:
create table RegisteredUsers
(
userId INT not null IDENTITY(1,1),
userName varchar(255) not null,
userpassword varchar(255) not null,
primary key (userId)
);
A second table Answers has a foreign key reference to RegisteredUsers:
create table answers (
surveyid int not null
foreign key (userId) references RegisteredUsers (userId)
);
Whenever I insert a row into the first table, I want to also insert a row into table Answers. When I insert value now, it only inserts into the first table. What do I need to do?
I am using SQL Server 2008. Please help. Thanks
Related
CREATE TABLE Comments(
Id INT PRIMARY KEY IDENTITY(0,1),
TEXT NOT NULL,
Date Date NOT NULL ,
Point INT NOT NULL DEFAULT(0),
ID_User INT FOREIGN KEY REFERENCES Users(Id) ON DELETE CASCADE NOT NULL,
ID_Post INT FOREIGN KEY REFERENCES Posts(Id) NOT NULL
)
When I delete User from Users table it show me error that Comments table has
other Reference Key. What i have to do ?
The DELETE statement conflicted with the REFERENCE constraint "FK__Comments__ID_Pos__76969D2E". The conflict occurred in database "Facebook", table "dbo.Comments", column 'ID_Post'.
If you want to delete a user record, you need to delete the records in the foreign key tables.
In this case, you need to delete records in Comments table.
DELETE from dbo.Commnts
Where ID_User = "userid"
Then, you can remove the user record from Users table
I did some work on this, there is no error in the foreign keys you referenced here. you may probably have reference to comment id in some other tables.
this is what I tried
CREATE TABLE Users(
Id int primary key
)
CREATE TABLE posts(
Id int primary key
)
insert into Users values(1);
insert into Users values(2);
insert into posts values(3);
insert into posts values(4);
CREATE TABLE Comments(
Id INT PRIMARY KEY IDENTITY(0,1),
ID_User INT FOREIGN KEY REFERENCES Users(Id) ON DELETE CASCADE NOT NULL,
ID_Post INT FOREIGN KEY REFERENCES Posts(Id) NOT NULL
)
insert into Comments values(1,3);
insert into Comments values(2,4);
DELETE
FROM Users
WHERE id = 1 --this works fine
In my database, I have the following code
CREATE TABLE [Users]
(
userID INT NOT NULL IDENTITY(1,1) PRIMARY KEY,
Username VARCHAR(255),
Password VARCHAR(255)
);
CREATE TABLE [Image]
(
ImageID INT NOT NULL IDENTITY(1,1) PRIMARY KEY,
Imagename VARCHAR,
userID INT,
FOREIGN KEY(userID) REFERENCES Users (userID)
);
However, here on stackOverflow and multiple other sites, people suggest writing it like this:
CREATE TABLE [Users]
(
userID INT NOT NULL IDENTITY(1,1) PRIMARY KEY,
Username VARCHAR(255),
Password VARCHAR(255)
);
CREATE TABLE [Image]
(
ImageID INT NOT NULL IDENTITY(1,1) PRIMARY KEY,
Imagename VARCHAR,
userID INT,
CONSTRAINT fk_Users
FOREIGN KEY(userID) REFERENCES Users (userID)
);
I've tried executing both statements, and it seems to do the same..
What am I missing, what's the trick when writing CONSTRAINT fk_Users?
Writing constraint allows you to name the constraint.
In general, you notice this when the constraint is violated. If you have an opportunity to name the constraint, the error message will make more sense.
For instance, this:
The INSERT statement conflicted with the FOREIGN KEY constraint
"fk_image_userId". The conflict occurred in database "rextester",
table "dbo.Users", column 'userID'.
The statement has been terminated.
is clearer than this:
The INSERT statement conflicted with the FOREIGN KEY constraint
"FK__Image__userID__09211CD4". The conflict occurred in database
"rextester", table "dbo.Users", column 'userID'.
The statement has been terminated.
In addition, the constraint name is used in the alter table when you drop or disable the constraint, so having a reasonably named constraint makes that easier.
I have a question about foreign keys.
How does it work when I want to add a foreign key to the first table that I make that references to the primary key of the second table I create?
CREATE TABLE table1
(
name_id INT NOT NULL,
team TEXT REFERENCES table2(team_id),
PRIMARY KEY(name_id)
);
CREATE TABLE table2
(
team_id INT NOT NULL,
teamname TEXT,
PRIMARY KEY(team_id)
);
If I try the code above I get the following error:
ERROR: relation "" does not exist
Thanks in advance.
Either create the second table first. Or use alter table. That is, create the first table without the reference and then do:
alter table table1 add constraint fk_table1_team
foreign key (team_id) REFERENCES table2(team_id);
The declaration for table1 would be:
CREATE TABLE table1 (
name_id INT NOT NULL,
team_id INT,
PRIMARY KEY(name_id)
);
The reference between the tables should be on the primary key and certainly not on a character column, if an integer is available.
here's the syntax of creating a table with Foreign key:
CREATE TABLE table11
(
name_id INT NOT NULL,
team INT,
PRIMARY KEY(name_id),
foreign key(team) references table22(team_id)
);
CREATE TABLE table22
(
team_id INT NOT NULL,
teamname TEXT,
PRIMARY KEY(team_id)
);
but there was another problem. a foreign key from a child table cannot reference to a primary key from a parent folder if they do not contain the same type. in your code team was of TEXT and team_id was of INT which cannot be.
I have 3 tables:
Create TABLE Subjects
(
SubjectID INT PRIMARY KEY NOT NULL IDENTITY(1,1),
SubjectName VARCHAR(20) NOT NULL,
ClassID VARCHAR(10) FOREIGN KEY REFERENCES Classes(ClassID) NOT NULL
);
Create TABLE Topic
(
TopicID INT PRIMARY KEY NOT NULL IDENTITY(1,1),
TopicName VARCHAR(100),
SubjectID INT FOREIGN KEY REFERENCES Subjects(SubjectID)
);
Create Table Worksheet
(
WorksheetName varchar(100) PRIMARY KEY,
TopicID INT Foreign KEY References Topic(TopicID),
Num_Q INT NOT NULL,
W_Type varchar(30)
);
Each one is a one to many relationship. When I try to delete from Subjects I get a foreign key constraint which is fine. What I want to know is how to get around this and perform a query to delete all relating aspects in a cascading style. I looked it up and there's but I am not sure how it works there seems to be multiple queries. Would it be better to create a trigger or is there a basic cascading function to do it all? I'm using visual studio to perform queries but not sure where the options to perform tasks like this are?
You can add the ON DELETE CASCADE right after the foreign key definition:
Create TABLE Subjects (
SubjectID INT PRIMARY KEY NOT NULL IDENTITY(1, 1),
SubjectName VARCHAR(20) NOT NULL,
ClassID VARCHAR(10) NOT NULL
FOREIGN KEY REFERENCES Classes(ClassID) ON DELETE CASCADE
);
You can also define it as a separate constraint, if you like, either within the CREATE TABLE statement or using ALTER TABLE ADD CONSTRAINT.
Here is the DDL for your Topic table with a CASCADE for delete. Its just a matter of defining it in your FK but using a slightly different syntax. This is for MS Sql Server.
CREATE TABLE Topic
(
TopicID INT PRIMARY KEY NOT NULL IDENTITY(1,1),
TopicName VARCHAR(100),
SubjectID INT,
CONSTRAINT FK_Subjects_Topic FOREIGN KEY (SubjectID)
REFERENCES Subjects (SubjectID)
ON DELETE CASCADE
ON UPDATE NO ACTION
)
EDIT - added DELETE CASCADE on Worksheet table based on comment feedback.
Create Table Worksheet
(
WorksheetName varchar(100) PRIMARY KEY,
TopicID INT,
Num_Q INT NOT NULL,
W_Type varchar(30),
CONSTRAINT FK_Topic_Worksheet FOREIGN KEY (TopicID)
REFERENCES Topic (TopicID)
ON DELETE CASCADE
ON UPDATE NO ACTION
);
With this updated definition a delete on table Subjects will also delete child records in table Topics.
Table A
(
Table_A_ID int
)
Table B
(
Table_B_ID int
Value int
)
Say I want to insert data into Table B, where 'Value' would be the same as a Table_A_ID.
How would I make a constraint or check that the data actually exists in the table on insertion?
You probably need to enforce data integrity not only on INSERT into Table B, but also on UPDATE and DELETE in both tables.
Anyway options are:
FOREIGN KEY CONSTRAINT on Table B
TRIGGERs on both tables
As a last resort if for some reason 1 and 2 is not an option STORED PROCEDUREs for all insert, delete update operations for both tables
The preferred way to go in most cases is FOREIGN KEY CONSTRAINT.
Yap, I agree with #peterm.
Cause, if your both Table_A_ID and Table_B_Id are primary keys for both tables, then you don't even need two tables to store the value. Since, your two tables are seems to be on 'one-to-one' relationship. It's one of the database integrity issues.
I think you didn't do proper normalisation for this database.
Just suggesting a good idea!
I found this example which demonstrates how to setup a foreign key constraint.
Create employee table
CREATE TABLE employee (
id smallint(5) unsigned NOT NULL,
firstname varchar(30),
lastname varchar(30),
birthdate date,
PRIMARY KEY (id),
KEY idx_lastname (lastname)
) ENGINE=InnoDB;
Create borrowed table
CREATE TABLE borrowed (
ref int(10) unsigned NOT NULL auto_increment,
employeeid smallint(5) unsigned NOT NULL,
book varchar(50),
PRIMARY KEY (ref)
) ENGINE=InnoDB;
Add a constraint to borrowed table
ALTER TABLE borrowed
ADD CONSTRAINT FK_borrowed
FOREIGN KEY (employeeid) REFERENCES employee(id)
ON UPDATE CASCADE
ON DELETE CASCADE;
NOTE: This tells MySQL that we want to alter the borrowed table by adding a constraint called ‘FK_borrowed’. The employeeid column will reference the id column in the employee table – in other words, an employee must exist before they can borrow a book.
The final two lines are perhaps the most interesting. They state that if an employee ID is updated or an employee is deleted, the changes should be applied to the borrowed table.
NOTE: See the above URL for more details, this is just an excerpt from that article!
Create a foreign key constraint on the column 'Value' on table B that references the 'Table_A_ID' column.
Doing this will only allow values that exist in table A to be added into the 'Value' field of table B.
To accomplish this you first need to make Table_A_ID column the primary key for table A, or it at least has to have some sort of unique constraint applied to it to be a foreign key candidate.
BEGIN TRANSACTION -- REMOVE TRANSACTION AND ROLLBACK AFTER DONE TESTING
--PUT A PRIMARY KEY ON TABLE A
CREATE TABLE A
( Table_A_ID int CONSTRAINT PK_A_Table_A_ID PRIMARY KEY)
--ON VALUE ADD A FOREIGN KEY CONSTRAINT THAT REFERENCEs TABLE A
CREATE TABLE B
( Table_B_ID int,
[Value] int CONSTRAINT FK_B_Value_A REFERENCES A(Table_A_ID)
)
-- TEST VALID INSERT
INSERT A (Table_A_ID) VALUES (1)
INSERT B (Table_B_ID, [Value]) VALUES (1,1)
--NOT ALLOW TO INSERT A VALUE THAT DOES NOT EXIST IN A
--THIS WILL THROW A FOREIGN KEY CONSTRAINT ERROR
INSERT B (Table_B_ID, [Value]) VALUES (1,2) -- 2 DNE in table A
ROLLBACK
Note: there is no magic to 'FK_B_Value_A' or 'PK_A_Table_A_ID' it simply a naming convention and be called anything. The syntax on the foreign key and primary key lines work like this:
column-definition CONSTRAINT give-the-constraint-a-name REFERENCES table-name ( table-column )
column-definition CONSTRAINT give-the-constraint-a-name PRIMARY KEY