I have two tables
Department
Professor
in which Department has an attribute called HeadID referencing Professor
and Professor has an attribute called DeptID referencing Department
They form a circular relationship.
But the problem is that, how to insert a row to any of these tables?
Oracle complained "parent key not found" after I tried insert a row.
You can define one of the foreign key constraints as DEFERRABLE and defer constraint checking until the end of your transaction (instead of checking at the end of statement which ends with "parent key not found"). Read here
The other solutions described here are simpler.
But if you really want the DB to describe your buisiness (which is not necessarily the best approach) then you can have another table, lets say DEPT_HEAD_POSITIONS. the Department table will have the FK (HeadID) refer to this table, and the Professor table will have another nullable field as a FK to this new table.
Now, what you have is:
departments head positions
departments (that must have a head position)
professors (which must belong to a department and may be head of the department)
It is possible for a foreign key consisting of multiple columns to allow one of the columns to contain a value for which there is no matching value in the referenced columns, per the SQL-92 standard. To avoid this situation, create NOT NULL constraints on all of the foreign key's columns
for reference
so I think you can insert data in one of the row without giving value in foreign key column and then insert row into second row referring value of primary key in the first table and then you can proceed ...
If you have the authority to redesign the schema you should. If not I think the simplest and best approach is described in deathApril's comment.
In the use case where you want to add a new department and a new professor who heads it, you're best of:
Adding the Professor under a different department
Adding the Department with the Professor from Step 1 as head
Updating the Professor record from Step 1 to refer to his new Department created in Step 2
Oracle and SQL Server do not allow circular referencing because there is always a problem when deleting a row from a table having dependencies to another row from another table (foreign key) which refers to the row being deleted.....
For more Info: Click here
Related
This ERD is part of my school work and something doesn't seem right. The ERD table 'Course' looks like its referencing 2 tables. Is the column titled 'Qual_Code' in table 'Course' from the 'Prerequisite' table, the 'Qualification' table or both? I don't think its both because you cannot have a single column with a foreign key that references two different tables.
Help because I have to write the SQL codes for this!
At the first glance, it appears that the column qual_code of the COURSE table is redundant. When writing the DDL code for this, you can include or exclude this column - see the following examples:
course table with qual_code column
course table without qual_code column
The PREREQUISITE table maps courses to qualifications. The PK constraint in this table will not allow NULLs in either of the columns ie if a row gets INSERTed, it must contain values for both course_num and qual_code.
I don't think its both because you cannot have a single column with a
foreign key that references two different tables.
It seems that you are misinterpreting this. The PK column of the QUALIFICATION table is referenced twice (FK columns reference a PK or UNIQUE column, not the other way round -> see the DDL code on DBfiddle).
Very new to SQL and have spent a day on this already.
Here are my two tables:
Centre(cid, name, location, nurse_supervisor)
Nurse(nid, name, centre_id, certificate)
I have a big problem. The (nurse_supervisor) in Centre is a foreign key to Nurse (nid).
The (centre_id) in Nurse is a foreign key to (Centre cid).
I can't figure out how to populate these tables. I have tried:
INSERT ALL, which produces "A foreign key value has no matching primary key value"
I have tried removing the foreign key constraints and adding them after populating the tables but when I do that it says I can't add a constraint to tables with preexisting data.
I tried removing NOT NULL - but realized that was silly as the constraints will be enforced anyways.
Everything I look through says populate the parent table first and then the child, but these tables are linked to each other.
I am using SQL developer.
This is a poor schema design, but one way to get around it would be to:
Make both centre_id and nurse_supervisor columns NULL in the two table definitions
Insert all rows into both tables, but with NULL for those two columns
Update centre_id to the correct value for each row in the Nurse table
Update nurse_supervisor to the correct value for each row in the Centre table
I have the following situation. My table is:
Table: CompanyEmployees
EmployeeID
Date of Birth
Date Joined
I also want to store the sales information for each employee. I have this:
Table: DealsCompleted
ID
EmployeeID
Deal Name
Deal Amount
My question is this- should there be a column in CompanyEmployees called "DealsCompletedID" which directly refers to the ID column in DealsCompleted, or is it acceptabe to just create a foreign key between the two Employee ID columns? Does this disadvantage the design or potentially distort the normalization?
I am unclear what the rule is as to whether I should include an extra column in CompanyEmployees or not.
EDIT Please assume there will only be one row in the deal table, per employee.
A FOREIGN KEY should point from one table to its referenced row in a parent table, the two tables should generally not reference each other (with foreign keys defined in both).
The FOREIGN KEY is most appropriately defined in the DealsCompleted table, which points back to CompanyEmployees.EmployeeID. Think about it this way - The CompanyEmployees table stores information about employees. Deals they completed do not really count as information about employees. However, the employee who completed a deal is a part of the information about a deal, so the key belongs there.
Having DealsCompleted.EmployeeID will allow for a proper one to many relationship between employees and deals. That is, one employee can have as many related rows in DealsCompleted as needed. Including a DealsCompleted column in the CompanyEmployees table on the other hand, would require you to either duplicate rows about employees, which breaks normalization, or include multiple DealCompletedID values in one column which is also incorrect.
Update after edit above Even if you plan for only a one-to-one relationship (one deal per employee), it is still more appropriate to reference the EmployeeID in DealsCompleted rather than the other way around (or both ways). ...And it allows you to expand to one-to-many, when the need arises.
Assuming that the relationship will always be one-to-one, as you state, then the answer depends on what is the primary entity within the Domain Model. If this database is at its core a database about Deals, and employee data is ancillary, then I would add an EmployeeId FK column in the Deal table. If otoh, this is a database about Employees, and Deals are ancillary, then eliminate the EmployeeId column in the Deal table, and add a DealId FK column to the Employeee table.
I have 2 tables - EmpDetails & ChangeLog
EmpDetails stores details of employees - it has ID, Name etc.
ChangeLog is used to log changes to employee details - it has ID, DateOfChange, ChangeDescription, etc.
I wanted to make sure that ChangeLog.ID is a value contained in EmpDetails.ID column.
So, I put a CHECK constraint using a user-defined function for ChangeLog.ID column in (the UDF checks if ID exists in EmpDetails.ID or not)
My question is - if a particular ID's row is deleted from EmpDetails, will an error be raised if there are rows for that ID in ChangeLog ?
This does not seem to be the case... And I don't understand why.
So, how would I get such a functionality ? One way I can think of is to create a trigger for delete operations on EmpDetails..
Any other solution to the above problem ?
EDIT -
I tried to specify a Foreign Key relationship. But ID in ChangeLog is not a key as the ChangeLog table can contain multiple records for the same ID (I mean , employees can change their details more than once, hence there will be more than 1 record for the same ID in ChangeLog). Should I be able to specify a Foreign Key relationship even in that case ?
What you are describing is a foreign key relationship. To enforce it:
ALTER TABLE ChangeLog
ADD CONSTRAINT FK_EmpDetailsId FOREIGN KEY (ID)
REFERENCES EmpDetails (ID);
SQL Server will then maintain the relationship for you, without the need for your UDF. Inserts to ChangeLog will fail if the corresponding row in EmpDetails does not exist and deletes from EmpDetails will fail if there is a matching row in ChangeLog.
The check constraint in on the ChangeLog table, not on the EmpDetails, so when you modify EmpDetails, it simply does not get checked. SQL server is not smart enough to figure out that you would want the check to run when some other table is changed.
But, if you just want to ensure that there is a row in the EmpDetails table, why not use a simple referential integrity rule (a.k.a. a foreign key)?
What kind of sql tricks you use to enter data into two tables with a circular reference in between.
Employees
EmployeeID <PK>
DepartmentID <FK> NOT NULL
Departments
DepartmentID <PK>
EmployeeID <FK> NOT NULL
The employee belongs to a department, a department has to have a manager (department head).
Do I have to disable constraints for the insert to happen?
I assume your Departments.EmployeeID is a department head. What I'd do is make that column nullable; then you can create the department first, then the employee.
Q: Do I have to disable constraints for the insert to happen?
A: In Oracle, no, not if the foreign key constraints are DEFERRABLE (see example below)
For Oracle:
SET CONSTRAINTS ALL DEFERRED;
INSERT INTO Departments values ('foo','dummy');
INSERT INTO Employees values ('bar','foo');
UPDATE Departments SET EmployeeID = 'bar' WHERE DepartmentID = 'foo';
COMMIT;
Let's unpack that:
(autocommit must be off)
defer enforcement of the foreign key constraint
insert a row to Department table with a "dummy" value for the FK column
insert a row to Employee table with FK reference to Department
replace "dummy" value in Department FK with real reference
re-enable enforcement of the constraints
NOTES: disabling a foreign key constraint takes effect for ALL sessions, DEFERRING a constraint is at a transaction level (as in the example), or at the session level (ALTER SESSION SET CONSTRAINTS=DEFERRED;)
Oracle has allowed for foreign key constraints to be defined as DEFERRABLE for at least a decade. I define all foreign key constraints (as a matter of course) to be DEFERRABLE INITIALLY IMMEDIATE. That keeps the default behavior as everyone expects, but allows for manipulation without requiring foreign keys to be disabled.
see AskTom: http://www.oracle.com/technology/oramag/oracle/03-nov/o63asktom.html
see AskTom: http://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:10954765239682
see also: http://www.idevelopment.info/data/Oracle/DBA_tips/Database_Administration/DBA_12.shtml
[EDIT]
A: In Microsoft SQL Server, you can't defer foreign key constraints like you can in Oracle. Disabling and re-enabling the foreign key constraint is an approach, but I shudder at the prospect of 1) performance impact (the foreign key constraint being checked for the ENTIRE table when the constraint is re-enabled), 2) handling the exception if (when?) the re-enable of the constraint fails. Note that disabling the constraint will affect all sessions, so while the constraint is disabled, other sessions could potentially insert and update rows which will cause the reenable of the constraint to fail.
With SQL Server, a better approach is to remove the NOT NULL constraint, and allow for a NULL as temporary placeholder while rows are being inserted/updated.
For SQL Server:
-- (with NOT NULL constraint removed from Departments.EmployeeID)
insert into Departments values ('foo',NULL)
go
insert into Employees values ('bar','foo')
go
update Departments set EmployeeID = 'bar' where DepartmentID = 'foo'
go
[/EDIT]
This problem could be solved with deferable constraints. Such constraints are checked when the whole transaction is commited, thus allowing you to insert both employee and department in the same transaction, referring to each other. (Assuming the data model makes sense)
Refactor the schema by removing the circular reference.
Delete an ID column from either of the table schema.
Departments.EmployeeID doesn't seem to belong there in my opinion.
I can't think of a non hackish way to do this. I think you will need to remove the constraint or do some type of silly dummy values that get updated after all the inserts.
I'd recommend refactoring the DB schema. I can't think of any reasons why you would want it to work this way.
Maybe something like, Employee, EmployeeDepartment (EmployeeId, DepartmentId) and Department would be a better way to accomplish the same goal.
You could create a row in the Department table for 'Unassigned'
To create a new department with a new Employee you then would
Create the Employee (EmployeeA) in the 'Unassigned' Department
Create the new department (DepartmentA) with the employee EmployeeA
Update EmployeeA to be in DepartmentA
This wouldn't invalidate your current schema, and you could set up a task to be run regularly to check there are no members of the Unassigned department.
You would also need to create a default employee to be the Employee of Unassigned
EDIT:
The solution proposed by chaos is much simpler though
There are a few good designs I've used. All involve removing the "manager" EmployeeID from the Department table and removing the DepartmentID from the Employee table. I've seen a couple answers which mention it, but I'll clarify how we used it:
I typically end up with an EmployeeDepartment relationship link table - many to many, usually with flags like IsManager, IsPrimaryManager, IsAdmin, IsBackupManager etc., which clarify the relationship Some may be constrained so that there is only one Primary Manager allowed per department (although a person can be a PrimaryManager of multiple departments). If you don't like the single table, then you can have multiple tables: EmployeeDepartment, ManagerDepartment, etc. but then you could have situations where a person is a manager but not an employee, etc.
We also typically allowed people to be members of multiple departments.
For simplified access, you can provide views which perform the join appropriately.
Yes, in this instance you will have to disable a foreign key.
You need to get rid of one or the other reference permanently . This is not a viable design structure. Which has to be entered first? Department or Employee? Unless your departments are all one employee big, the structure doesn't make sense anyway as each employee would have to have a distinct departmentid.