如何在 SQL 中加入 4 个表？

Question

I just started using SQL and I need some help. I have 4 tables in a database. All four are connected with each other. I need to find the amount of unique transactions but can't seem to find it.

Transactions

• transaction_id pk
• name

Partyinvolved

• transaction.id pk
• partyinvolved.id
• type (buyer, seller)

PartyCompany

• partyinvolved.id
• Partycompany.id

Companies

• PartyCompany.id pk
• sector

pk = primary key

The transaction is unique if the conditions are met. I only need a certain sector out of Companies, this is condition1. Condition2 is a condition inside table Partyinvolved but we first need to execute condition1. I know the conditions but do not know where to put them.

SELECT *
FROM group
INNER JOIN groupB ON groupB.group_id = group.id
INNER JOIN companies ON companies.id = groupB.company_id
WHERE condition1 AND condition2 ;

I want to output the amount of unique transactions with the name.

Answer 1

It is a bit unclear what you are asking as your table definitions look like your hinting at column meanings more than names such as partycompany.id you are probably meaning the column that stores the relationship to PartyCompany column Id ......

Anyway, If I follow that logic and I look at your questions about wanting to know where to limit the recordsets during the join. You could do it in Where clause because you are using an Inner Join and it wont mess you your results, but the same would not be true if you were to use an outer join . Plus for optimization it is typically best to add the limiter to the ON condition of the join.

I am also a bit lost as to what exactly you want eg a count of transactions or the actual transactions associated with a particular sector for instance. Anyway, either should be able to be derived from a basic query structure like:

SELECT
    t.*
FROM
    Companies co
    INNER JOIN PartyCompancy pco
    ON co.PartyCompanyId = pco.PartyCompanyId
    INNER JOIN PartyInvolved pinv
    ON pco.PartyInvolvedId = pinv.PartyInvolvedId
    AND pinv.[type] = 'buyer'
    INNER JOIN Transactions t
    ON ping.TransactionId = t.TransactionId
WHERE
    co.sector = 'some sector'