Oracle sql约束问题

Question

I'm having problems with creating a oracle sql script. How do I create these two constraints?

1. If VID is null then FID has to be null as well (VID = null -> FID = null)
2. There has to be exactly one row where VID is null which also means that FID has to be null because of 1.

Here is what I have so far:

create table Employee(
Id int primary key,
Name varchar(15) not null,
VID int,
FID int

);

Employee

Answer 1

You can come very close to what you want without a trigger.

You can use a check constraint for the first:

alter table Employee add constraint chk_vid_fid
    check (vid is not null or fid is null);

You can do the second using a unique constraint:

create unique index unq_Employee_vid on
    Employee(case when vid is null then -1 else id end);

This formulation assumes that id is non-negative, as most ids usually are. If you do use the full range of integer values, then I would be explicit with a string:

create unique index unq_Employee_vid on
    Employee(case when vid is null then 'null vid' else cast(id as varchar2(255)) end);

This guarantees at most one row where vid is null, rather than exactly one row. You cannot easily have a constraint that guarantees that exactly one row has a value, because empty tables do not match the condition -- and tables are empty when they are created.

Answer 2

Here is how you can enforce your requirement - to have exactly one row with VID is NULL . As I said in my Comment under your original question (and Gordon said as well in his accepted Answer), you cannot do this just with constraints. However, you can do it with a materialized view. Note that you must create the MV with refresh fast on commit , and that requires certain things for the materialized view log on the base table.

Also: If you try everything exactly as written, the constraint on the MV will fail (of course, because the base table will be empty, so there will be no row where VID is NULL ). Add rows to the base table, of which exactly one has NULL for VID , then commit the transaction, and then execute the alter table command to add the constraint to the MV. From that point on, a transaction on the base table (consisting of one or more insert , delete , update or merge statements, followed by one commit ) will go through if and only if it leaves exactly one row with NULL in the VID column.

One oddity to keep in mind: even though there is an ALTER MATERIALIZED VIEW statement in Oracle SQL, to add a constraint on a MV we must use the ALTER TABLE statement (with the MV's name), not ALTER MATERIALIZED VIEW .

Note that I used the name t_Employee for the base table, as I already had a table EMPLOYEE and I don't want to mess with my existing objects.

create table t_Employee(
Id   int primary key,
Name varchar(15) not null,
VID  int,
FID  int
);

alter table t_Employee add constraint chk_vid_fid
    check (vid is not null or fid is null)
;

create materialized view log on t_Employee 
with rowid
(VID)
including new values
;

create materialized view mv_Employee
refresh fast on commit
as select count(*) ct
   from   t_Employee
   where  VID is null
;

alter table mv_Employee add constraint chk_vid_ct
    check (ct = 1)
;