Re: Subject: Re: constrain with MATCH full and NULL values inreferenced table - Mailing list pgsql-general

On Mon, Aug 12, 2019 at 10:16:41AM -0700, Adrian Klaver wrote:
> On 8/12/19 10:06 AM, stan wrote:
> > Cc: pgsql-general.lists.postgresql.org@panix.com
> > Subject: Re: constrain with MATCH full and NULL values in referenced table
> > User-Agent: Mutt/1.12.1 (2019-06-15)
> > X-Editor: gVim
> > 
> > On Mon, Aug 12, 2019 at 06:22:54PM +0200, Francisco Olarte wrote:
> > > Stan:
> > > 
> > > On Mon, Aug 12, 2019 at 5:11 PM stan <stanb@panix.com> wrote:
> > > > 
> > > > I am creating a table that has 2 values in it which are keys pointing to 2
> > > > other tables. I need for the UNIQUE combination of these 2 keys to exist in
> > > > a fourth table. It has been recommended to use a foreign key constraint with
> > > > the MATCH FULL parameter.
> > > > 
> > > > Here is my question, does this deal with NULLS in the 4th table? I am
> > > > concerned that this constraint might fail to reject an entry if one, or both
> > > > of the 2 key values being inserted in the table are NULLS,.
> > > 
> > > If you have:
> > > 
> > > Table TA (a: PK)
> > > Table TB (b: PK)
> > > Table TAB( a, b,....)  PK(A,B), FK(a ref TA), FK(b ref TB)
> > > Table FOURTH(a,b,...) FK((A,B) ref TAB mach full)
> > > 
> > > Note TAB cannot have nulls in A,B as it is the PK.
> > > 
> > > And you insert (null, null) in FOURTH it will be treated as in single
> > > column, allowed by the fk ( but you may have non null constraints on
> > > either a or b).
> > > If you try to insert (a1, null) or (null, b1), it will ber rejected,
> > > MATCH FULL does not allow null/non-null mix.
> > > 
> > > OTOH, if you use MATCH SIMPLE the partial-null cases will be not
> > > checked at all, as if they where not null. As stated in the docs, you
> > > can use extra single column FK in a and/or b to  get them checked in
> > > TA/TB, and also you can put non-null constraints on either on them.
> > > 
> > > The exact combo depends on what you are trying to model, which gives
> > > you what you want. I.e., say I want to:
> > > 1.- check a,b combos.
> > > 2.- Allow (a,null) but have it checked against ta.
> > > 3.- Forbid (null,b)
> > > 4.- Aloow (null, null)
> > > You can use MATCH simple FK(a,b) against TAB for (1,4), single column
> > > FK(a) against TA for(2)  and a check constraint (A is not null OR B is
> > > null , If I'm not confused ) for (3,4).
> > > ( Note you do not have to check b against tb, because if b is present,
> > > a is present, a,b is checked against TAB and TAB.b is checked against
> > > TB ).
> > > 
> > > (match simple checks 1 and allows 2,3,4, FK(a) checks 2, and the check
> > > constraint forbids 3)
> > > 
> > > The DB deals with nulls in many way, you just have to enumerate your
> > > conditions and elaborate on that.
> > > Note in this case it FAILS to reject an entry if b is null, because I
> > > dessigned it that way, but DOES REJECT if a is null and B is not.
> > > 
> > 
> > Thank you.
> > 
> > Testing seems to verify that I have this correct.
> > 
> > I thought I would include what I came up with, so it gets in the archive.
> > Some fields eliminated for clarity.
> > 
> > The task_instance table is the one the original question was in reference
> > to.
> > 
> > CREATE TABLE employee (
> >      employee_key            integer DEFAULT nextval('employee_key_serial')
> >      PRIMARY KEY ,
> >      id                     varchar(5)  NOT NULL UNIQUE ,
> >      first_name             varchar  NOT NULL,
> > );
> > 
> > CREATE TABLE work_type (
> >      work_type_key      integer DEFAULT nextval('work_type_key_serial')
> >      PRIMARY KEY ,
> >      type               smallint UNIQUE ,
> >      descrip            varchar UNIQUE ,
> >      modtime            timestamptz DEFAULT current_timestamp
> > );
> > 
> > CREATE TABLE rate (
> >      employee_key       integer NOT NULL,
> >      work_type_key      integer NOT NULL,
> >      rate            numeric (5, 2) NOT NULL,
> >      descrip            varchar ,
> >      modtime            timestamptz DEFAULT current_timestamp ,
> >      FOREIGN KEY (employee_key) references employee(employee_key) ,
> >      FOREIGN KEY (work_type_key) references work_type(work_type_key) ,
> >      CONSTRAINT rate_constraint UNIQUE (employee_key , work_type_key)
> > );
> > 
> > 
> > CREATE TABLE task_instance (
> >      task_instance      integer DEFAULT nextval('task_instance_key_serial')
> >      PRIMARY KEY ,
> >      project_key        integer NOT NULL ,
> >      employee_key       integer NOT NULL ,
> >      work_type_key      integer NOT NULL ,
> >      hours            numeric (5, 2) NOT NULL ,
> >      work_start         timestamptz ,
> >      work_end           timestamptz ,
> >      modtime            timestamptz DEFAULT current_timestamp ,
> >      descrip            varchar ,
> 
> Aren't the marked ones below redundant?:
> 
> >      FOREIGN KEY (employee_key) references employee(employee_key) ,
>        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> >      FOREIGN KEY (project_key) references project(project_key) ,
> >      FOREIGN KEY (work_type_key) references work_type(work_type_key) ,
>        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> >      FOREIGN KEY (work_type_key , employee_key) REFERENCES rate (work_type_key , employee_key) MATCH FULL
> 
> They are covered above.
> 
> > );

OK, looks like learning time for me, again. Which is alwasy a good thing. My
thought here was that I needed to specify these on the dreivative table
(task_instnce). Are you teaching me that, since these constraints exist on
the tables that rate is derived from, I do not need to specify thmm for the
rate table?

The purpose of those is to verify that the key being inserted already exists
in the parent (eg employee) table.
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