sqlite INNER JOIN only if data - sql

Question

Example with tables :
Promotion(idPromo, nameUser)
Company(idCompany, nameCompany)
PromoCompany(idPromo, idCompany)
I try to get with only one query, all promotions who have a company (idCompany = 1 for example) OR who have 0 company.
To describe :
PromoCompany is a restriction table... if data exist for a promotion, promotion is valable for only this companies, if no data, promotion valable for all companies.
Example :
Promo[{
idPromo:1
namePromo:"promo test"
},
{
idPromo:2
namePromo:"promo test 2"
}]
Company[{
idCompany:10
nameCompany:"CompanyPloof"
},{
idCompany:12
nameCompany:"CompanyPaf"
}
]
PromoCompany[{
idPromo:1
idCompany:10
},{
idPromo:1
idCompany:12
}
]
If my company is CompanyPloof, promos are idPromo 1 et 2
If my company is CompanyPaf, promos are idPromo 2 (because not restricted)

Answer 1

Hmmm . . . this sounds like two conditions:
select p.*
from promotions p
where exists (select 1 from promocompany pc where pc.idpromo = p.idpromo and p.idcompany = 1) or
not exists (select 1 from promocompany pc where pc.idpromo = p.idpromo);
I would suggest a small change to your data model. Add a flag in promotions that says whether the promotion is available for all companies or if it is restricted. Having to search through a table is a little awkward -- and potentially confusing. Adding one row to the table could invalidate a promotion for everyone else.