Postgresql – Insertion of data violates the foreign key constraint

database-designforeign keyinsertpostgresql

Consider I have two tables Employee and Department with the fields given below:

Employee:

Fname, Minit, Lname, Ssn Bdate, Address, Sex, Salary, Super_ssn, Dno

Department:

Dname, Dnumber, Mgr_ssn, Mgr_start_date

Italics and code block represent the primary key of table.

In above case, Super_Ssn references the SSN of employee, while Dno references the Department's Dnumber.
Creating the sql for them is pretty straightforward by adding columns first and altering them later when both tables are formed. However, while inserting the data like given below:

insert into employee values('James','E','Borg','888665555','1937-11-10','asda','M',55000,NULL,1);
ERROR:  insert or update on table "employee" violates foreign key constraint "dno_ref_dnum_dpt"
DETAIL:  Key (dno)=(1) is not present in table "department".

and

insert into sp0090.department values ('Headquarter',1,'888665555','1981-06-19');
ERROR:  insert or update on table "department" violates foreign key constraint "department_mgr_ssn_fkey"
DETAIL:  Key (mgr_ssn)=(888665555) is not present in table "employee".

I am getting the foreign key constraint violated. This can be traced because to insert Mgr_ssn in department, it has to exists in Employee and to insert dno in Employee it has to exists in Department. However, I am required to fill the tables with maintaining the schema as it is. How do I solve this circular referencing? Thanks.

Best Answer

In Postgres, there are two ways to circumvent the chicken-and-egg problem of inserting into tables with circular references.

a) Defer one of the constraints, either declaring it as DEFERRABLE INITIALLY DEFERRED when created or as DEFERRABLE as created and then defer it when needed.

For an example of using deferrable constraints, see my answer in this question:
Implementation of a many-to-many relationship with total participation constraints in SQL.

b) Use a single statement to insert into the two (or more) tables, using modifying CTEs. Example for your case:

with
  insert_emp as
  ( insert into employee 
        (Fname, Minit, Lname, Ssn, Bdate, Address, Sex, Salary, Super_ssn, Dno)
    values
        ('James', 'E', 'Borg', '888665555', '1937-11-10', 'asda', 'M', 55000, NULL, 1)
    returning 
        *
  ), 
  insert_dep
  ( insert into sp0090.department 
        (Dname, Dnumber, Mgr_ssn, Mgr_start_date) 
    values 
        ('Headquarter', 1, '888665555', '1981-06-19')
    returning 
        *
  ) 
select *
from insert_emp as e
  full join insert_dep as d
  on  e.dno = d.dnumber
  and e.ssn = d.mgr_ssn ;

Related Solutions

Postgresql – Violates foreign key constraint

Basically foreign key constraints are not inherited. If you are working with table inheritance you have a few options.

Stop enforcing foreign keys
Use constraint triggers to enforce foreign keys

In most cases you are better off with a single large table and smaller join tables possibly with deferred foreign keys. Unfortunately these are not possible either to fully do the things that table inheritance can do so typically there is going to be at least some custom coding.

PostgreSQL – insert/update violates foreign key constraints

There are a few problems with your tables. I'll try to address the foreign keys first, since you question asked about them :)

But before that, we should realize that the two sets of tables (the first three you created and the second set, which you created after dropping the first set) are the same. Of course, the definition of Table3 in your second attempt has syntax and logical errors, but the basic idea is:

CREATE TABLE table3 (   
  "ID" bigint NOT NULL DEFAULT '0',   
  "DataID" bigint DEFAULT NULL,   
  "Address" numeric(20) DEFAULT NULL,   
  "Data" bigint DEFAULT NULL,
   PRIMARY KEY ("ID"),   
   FOREIGN KEY ("DataID") REFERENCES Table1("DataID") on delete cascade on update cascade,   
   FOREIGN KEY ("Address") REFERENCES Table2("Address") on delete cascade on update cascade
);

This definition tell PostgreSQL roughly the following: "Create a table with four columns, one will be the primary key (PK), the others can be NULL. If a new row is inserted, check DataID and Address: if they contain a non-NULL value (say 27856), then check Table1 for DataID˙and Table2 for Address. If there is no such value in those tables, then return an error." This last point which you've seen first:

ERROR: insert or update on table "Table3" violates foreign key constraint 
    "Table3_DataID_fkey" DETAIL: Key (DataID)=(27856) is not present in table "Table1".

So simple: if there is no row in Table1 where DataID = 27856, then you can't insert that row into Table3.

If you need that row, you should first insert a row into Table1 with DataID = 27856, and only then try to insert into Table3. If this seems to you not what you want, please describe in a few sentences what you want to achieve, and we can help with a good design.

And now about the other problems.

You define your PKs as

CREATE all_your_tables (
    first_column NOT NULL DEFAULT '0',   
    [...]
    PRIMARY KEY ("ID"),

A primary key means that all the items in it are different from each other, that is, the values are UNIQUE. If you give a static DEFAULT (like '0') to a UNIQUE column, you will experience bad surprises all the time. This is what you got in your third error message.

Furthermore, '0' means a text string, but not a number (bigint or numeric in your case). Use simply 0 instead (or don't use it at all, as I written above).

And a last point (I may be wrong here): in Table2, your Address field is set to numeric(20). At the same time, it is the PK of the table. The column name and the data type suggests that this address can change in the future. If this is true, than it is a very bad choice for a PK. Think about the following scenario: you have an address '1234567890454', which has a child in Table3 like

ID        DataID           Address             Data
123       3216547          1234567890454       654897564134569

Now that address happens to change to something other. How do you make your child row in Table3 follow its parent to the new address? (There are solutions for this, but can cause much confusion.) If this is your case, add an ID column to your table, which will not contain any information from the real world, it will simply serve as an identification value (that is, ID) for an address.

Best Answer

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