Question

I have to create a table, with either first name and last name of a person, or a name of an organization. There has to be exactly one of them. For example one row of the table is –

first_name  last_name  organization
----------  ---------  ------------
John        Smith      null

or another row can be –

first_name  last_name  organization
----------  ---------  --------------------
null        null       HappyStrawberry inc.

Is there a way to define this in SQL language? Or should I just define all three columns being able to get null values?

Answer 1

You could use a check constraint, like:

create table YourTable (
    col1 varchar(50)
,   col2 varchar(50)
,   col3 varchar(50)
,   constraint TheConstraint check ( 1 =
        case when col1 is null then 1 else 0 end +
        case when col2 is null then 1 else 0 end +
        case when col3 is null then 1 else 0 end )
)

Another way is to add a type column (EAV method):

create table YourTable (
    type varchar(10) check (type in ('FirstName', 'LastName', 'Organisztion')
,   value varchar(50))
insert YourTable ('LastName', 'Obama')
insert YourTable ('FirstName', 'Barrack')
insert YourTable ('Orginazation', 'White House')