CREATE DATABASE Company;
USE Company;
CREATE TABLE Employee(
EmployeeID INT PRIMARY KEY,
EmployeeName CHAR(25),
DID INT NOT NULL
);
CREATE TABLE Department(
DepartmentID INT NOT NULL,
DpartmentName CHAR(25),
FOREIGN KEY (DepartmentID) REFERENCES Employee(DID)
);
This is a simple SQL query for creating 2 tables and adding a foreign key,
I made sure to--
Have same data type
Different names in table.
Correct syntax.
But still, I get the error for
0 23 16:48:05 CREATE TABLE Department(
DepartmentID INT NOT NULL,
DpartmentName CHAR(25),
FOREIGN KEY (DepartmentID) REFERENCES Employee(DID)
) Error Code: 1822. Failed to add the foreign key constraint. Missing index for constraint 'department_ibfk_1' in the referenced table 'employee' 0.329 sec
How do I correct this? Thanks in advance
Missing index for constraint 'department_ibfk_1' in the referenced table 'employee'
You need an index on the DID column of the Employee table if you are creating a foreign key referencing the Employee.DID column.
But as suggested in the comments, you likely want it the other way around: the Employee.DID column sounds like a reference to Department.Id.
Thus, the foreign key should be on the Employee table, referencing the Department table. (And this way, there wouldn't be any error as Department.id is already indexed).
Related
My problem is that i have two tables with each table having a foreign key to the other table.
Each time , i execute the SQL file containing the creation of the two tables, it gives me an error that he doesn't find the other table. I'm working with sqlplus to execute the sql file.
Here's an example of SQL file i tried with :
create table A(
Age number(3),
name number(3) constraint A_FK references B(name))
/
create table B(
Age number(3) constraint B_FK references A(Age),
name number(3))
And even if i reverse the order, it gives the same error.
Thanks for help.
This is a problem of cycles in foreign keys. One method is to add all foreign keys after table creation (as I think the other answers propose).
You can also just do that for the first table:
create table A (
Age number(3) primary key,
name number(3)
);
create table B (
name number(3) primary key,
Age number(3),
constraint B_FK foreign key (age) references A(Age)
);
alter table B add constraint A_FK foreign key (name) references B(name);
Here is a db<>fiddle.
Notes:
Foreign keys should reference primary keys, so I added that declaration as well.
I recommend making the primary key the first column in the table.
You can also define the constraint inline for one of the tables (i.e. age number(3) constraint b_fk references a(age)).
The table column(s) that is referred by a foreign key must exist at the time when the constraint is created. Since you have some kind of cyclic reference between the tables, you need to do this in three steps:
first create one table without the foreign key
create the second table (with its foreign key)
finally add the foreign key to the first table with an alter table statement
You also need the referred column to have a unique or primary key constraint set up, otherwise you would get error ORA-02270: no matching unique or primary key for this column-list.
create table A(
age number(3) primary key,
name number(3)
);
create table B(
age number(3) constraint B_FK references A(Age),
name number(3) primary key
);
alter table A add constraint A_FK foreign key (name) references B(name);
Demo on DB Fiddle
Side note: I am quite suspicious about your sample structure, but this could be because your oversimplified it in the question.
It fails because the reference table doesn't exist yet.
Create the tables without the key first. Then drop one and recreated it with the reference. Then drop the 2nd and recreate it with the reference.
Create table first and then ADD the CONSTRAINT
ALTER TABLE A
ADD FOREIGN KEY (name) REFERENCES B(name);
ALTER TABLE B
ADD FOREIGN KEY (age) REFERENCES A(age);
I have created PERSON table in Oracle by this SQL syntax:
Create table person
(
p_id int not null,
personName char(5) not null );
Then I am trying to create ORDERS table with the following syntax:
CREATE TABLE Orders
(
O_Id int NOT NULL,
OrderNo int NOT NULL,
P_Id int,
PRIMARY KEY (O_Id),
CONSTRAINT fk_PerOrders FOREIGN KEY (P_Id)
REFERENCES person(p_id) );
But I am getting the following error .
No matching unique or primary key for this column-list.
What is the problem ? How can I solve this ?
Add primary key to person table:
CREATE TABLE person(
p_id int not null,
personName char(5) not null,
PRIMARY KEY (p_ID)
);
SqlFiddleDemo
Foreign keys enforce a one-to-many relationship. That is, however many records there are in the dependent table they can only reference a single record in the parent table. This means the referenced column(s) in the parent table must be constrained by a PRIMARY or UNIQUE key.
The error message is telling you that there is no such constraint on person(p_id). And lo! if we compare the two DDL statements you have posted we can see that you have created a primary key for ORDERS but not for PERSON.
The solution is simple: constrain P_ID by adding a primary key to PERSON. You can either drop and re-create the table, or you can use an alter table statement to add a primary key.
You should add primary key to person table.
try this:
ALTER TABLE Person
ADD CONSTRAINT p_id PRIMARY KEY (p_id);
I created two tables How to delete relationship between them in sql code;
Course table:
create table course
(
course_id int primary key identity (1,1),
course_name varchar(40)
);
Employee table:
create table employee
(
emp_id int identity(1,1) primary key,
fname varchar(30),
course_id int
foreign key references course (course_id)
);
If this is for SQL Server - then you can find out the name of the FK constraint using this statement:
SELECT name
FROM sys.foreign_keys
WHERE parent_object_id = OBJECT_ID('employee')
AND referenced_object_id = OBJECT_ID('course')
and once you have that FK constraint name, you can use the usual
ALTER TABLE dbo.employee DROP CONSTRAINT (name of the FK constraint)
This will be an arbitrary, system-generated name (something like FK__employee__cours__7EB7AD3A or similar) - and this is the reason I'd recommend to always explicitly name your constraints - like this:
create table employee
(
emp_id int identity(1,1) primary key,
fname varchar(30),
course_id int
constraint fk_employee_course
foreign key references course (course_id)
);
Now, you know what the name of that FK constraint is - you named it fk_employee_course. This is also beneficial if you get any error messages about FK constraint violations - if that name is intuitive and obvious, then you'll know what went wrong
I have not seen any clear, concise examples of this anywhere online.
With an existing table, how do I add a foreign key which references this table? For example:
CREATE TABLE dbo.Projects(
ProjectsID INT IDENTITY(1,1) PRIMARY KEY,
Name varchar(50)
);
How would I write a command to add a foreign key which references the same table? Can I do this in a single SQL command?
I'll show you several equivalent ways of declaring such a foreign key constraint. (This answer is intentionally repetitive to help you recognise the simple patterns for declaring constraints.)
Example: This is what we would like to end up with:
Case 1: The column holding the foreign keys already exists, but the foreign key relationship has not been declared / is not enforced yet:
In that case, run this statement:
ALTER TABLE Employee
ADD FOREIGN KEY (ManagerId) REFERENCES Employee (Id);
Case 2: The table exists, but it does not yet have the foreign key column:
ALTER TABLE Employee
ADD ManagerId INT, -- add the column; everything else is the same as with case 1
FOREIGN KEY (ManagerId) REFERENCES Employee (Id);
or more succinctly:
ALTER TABLE Employee
ADD ManagerId INT REFERENCES Employee (Id);
Case 3: The table does not exist yet.
CREATE TABLE Employee -- create the table; everything else is the same as with case 1
(
Id INT NOT NULL PRIMARY KEY,
ManagerId INT
);
ALTER TABLE Employee
ADD FOREIGN KEY (ManagerId) REFERENCES Employee (Id);
or, declare the constraint inline, as part of the table creation:
CREATE TABLE Employee
(
Id INT NOT NULL PRIMARY KEY,
ManagerId INT,
FOREIGN KEY (ManagerId) REFERENCES Employee (Id)
);
or even more succinctly:
CREATE TABLE Employee
(
Id INT NOT NULL PRIMARY KEY,
ManagerId INT REFERENCES Employee (Id)
);
P.S. regarding constraint naming: Up until the previous revision of this answer, the more verbose SQL examples contained CONSTRAINT <ConstraintName> clauses for giving unique names to the foreign key constraints. After a comment by #ypercube I've decided to drop these clauses from the examples, for two reasons: Naming a constraint is an orthogonal issue to (i.e. independent from) putting the constraint in place. And having the naming out of the way allows us to focus on the the actual adding of the constraints.
In short, in order to name a constraint, precede any mention of e.g. PRIMARY KEY, REFERENCES, or FOREIGN KEY with CONSTRAINT <ConstraintName>. The way I name foreign key constraints is <TableName>_FK_<ColumnName>. I name primary key constraints in the same way, only with PK instead of FK. (Natural and other alternate keys would get the name prefix AK.)
You can add the column and constraint in one operation
ALTER TABLE dbo.Projects ADD
parentId INT NULL,
CONSTRAINT FK FOREIGN KEY(parentid) REFERENCES dbo.Projects
Optionally you could specify the PK column in brackets after the referenced table name but it is not needed here.
If the table already exists: Assuming you don't already have a column to store this data. If you do then skip this step.
ALTER TABLE [dbo].[project]
ADD [fkProjectsId] INT;
GO
ALTER TABLE [dbo].[projects]
ADD CONSTRAINT [FK_Projects_ProjectsId] FOREIGN KEY ([fkProjectsId]) REFERENCES [dbo].[Projects] ([ProjectsID])
GO
A similar question is asked here multiple foreign keys referencing single column in other table
but the syntax is not shown in the answer. I would like to know how this can be accomplished in SQL server. The following syntax gives error
ALTER TABLE ItemIssue ADD CONSTRAINT FK_ItemIssue_Person
FOREIGN KEY (PersonID, AdvisorID) REFERENCES Person (PersonID)
;
ERROR: Number of referencing columns in foreign key differs from number of referenced columns, table 'ItemIssue'.
-- Create Tables
CREATE TABLE ItemIssue (
ItemIssueID int identity(1,1) NOT NULL,
PersonID int,
AdvisorID int,
)
;
CREATE TABLE Person (
PersonID int NOT NULL,
Name nvarchar(500),
)
;
You need to define two foreign keys, one for each column:
ALTER TABLE ItemIssue ADD CONSTRAINT FK_ItemIssue_Person
FOREIGN KEY (PersonID) REFERENCES Person (PersonID)
;
ALTER TABLE ItemIssue ADD CONSTRAINT FK_ItemAdvisor_Person
FOREIGN KEY (AdvisorID) REFERENCES Person (PersonID)
;
It is impossible to create one foreign key for two columns referencing one column. Create them seperate:
ALTER TABLE ItemIssue
ADD CONSTRAINT FK_ItemIssue_Person_Person FOREIGN KEY (PersonID) REFERENCES Person (PersonID),
ADD CONSTRAINT FK_ItemIssue_Advisor_Person FOREIGN KEY (AdvisorID) REFERENCES Person (PersonID);
To define two foreign keys, one for each column-
Table
Contract - HospidPharmacyId Column
Hospice- HospiceID PK
Pharmacy PharmacyId Pk
Using Following Query we can apply 2 Foreign Key for 1 column.
Alter Table Contract
Add Constraint fk_pharmacyID Foreign Key ([HospIDPharmID]) references Pharmacy([PharmacyID])
Alter TAble contract
Add Constraint Fk_hospId Foreign key ([HospIDPharmID]) references Hospice(HospiceID)
In the Contract Table for column-HospidPharmacyId we can insert common value in both the
tables. those which are present in hospice & not in Pharmacy then we cant insert that value in
contract table & vice versa.