CODEWITHSUNDEEP
sqloracle-database
Cosmin Oprea
Cosmin Oprea

Reputation: 595

SQL add constraint

I have an table employees , and I want to add an constraint which makes me able to have ONLY one employee with id_manager null.. I have a sample from my code , but I think it's wrong..

ALTER TABLE employees ADD CONSTRAINT u_t UNIQUE (id_manager);

And this is the error I get.

SQL Error: ORA-02299: (OPREAC_IDD.U_T)
02299. 00000 - "cannot validate (%s.%s) - duplicate keys found"
*Cause:    an alter table validating constraint failed because the table has
       duplicate key values.

Upvotes: 1

Views: 618

Answers (1)

sstan
sstan

Reputation: 36523

You say:

I want to add a constraint which makes me able to have ONLY one employee with id_manager null.

So if I understand this correctly, you're ok with having duplicates in the id_manager column. The only thing you're trying to restrict is making sure that no more than one row can have null as the value for id_manager.

If that's the case, you'll want to create a unique index with an expression so that only null values are considered and validated:

create unique index emp_idx
on employees (case when id_manager is null then 1 end)

Upvotes: 4

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