Re: Outer Right Join?
| От | Mark Wallace |
|---|---|
| Тема | Re: Outer Right Join? |
| Дата | |
| Msg-id | 97CC8DF1-ED65-4C25-B7C8-0B4F676330E5@dataxdesign.com обсуждение исходный текст |
| Ответ на | Outer Right Join? ("Bee.Lists" <bee.lists@gmail.com>) |
| Список | pgsql-novice |
First problem is the database design is no good.
1. If companies <—> products is many-to-many, you need a cross-reference table between them.
2. If companies <—> contracts is one-to-one, optional, then contracts should be a list of contracts (not of companies), or, if there are no significant attributes of a contract, merged into companies as a status flag or boolean (is there a contract with this company or not?).
It is almost never a good idea to have the same entity (in this case, companies) stored in more than one table solely because of varying attribute or relationship values.
It’s not a good idea to try to twist SQL into compensating for a confused database design.
Note: There is no way that SELECT DISTINCT needs to be part of your solution.
You might want to have a look at a book such as this one:
Cheers,
Mark
On Feb 20, 2020, at 03:04, Bee.Lists <bee.lists@gmail.com> wrote:I’m looking for a three-table join. The SQL that I have isn’t working.
companies table - list of companies
products table - list of products sold by those companies
contracts table - list of companies that hold ad contracts
I need to ask:
"Show me the queried product (i.e.: Screwdriver) listings with their company names that DO NOT have contracts"
SELECT DISTINCT ON ("listings"."product")
"listings"."product", "companies"."name"
FROM "listings"
RIGHT OUTER JOIN "contracts" ON ("contracts"."companyid" = "listings"."companyid")
LEFT JOIN "companies" ON ("companies"."scid" = "listings"."companyid")
WHERE (("listings"."product" ILIKE '%screwdriver%' ESCAPE '\'))
The result works without the RIGHT OUTER JOIN in there. When the RIGHT OUTER JOIN is in there, I get a hitlist of zero. Currently I have no contracts in that table, so those two queries should be the same. They are not.
To repeat, I want any company’s products that are in the contracts table, to not show up. “products with no contract”.
Any inside as to how I can get this to work, appreciated.
Cheers, Bee
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