Write a query using INNER JOIN AND OR - sql

Question

I have a form and an insert button and when i click on the button - the fields goes in to these tables (i didn't put here all the fields because they are not important for my question).
The Tables:
CREATE TABLE SafetyAct (
SafetyAct_id int identity(1,1),
Username varchar(50),
SafetyType_id int,
constraint pk_SafetyAct_id
primary key (SafetyAct_id),
constraint fk_Users_SafetyAct
foreign key(Username)
references Users(Username)
on delete cascade
)
CREATE TABLE Product (
Product_id int identity(1,1) primary key,
SafetyAct_id int,
Cause_id int,
constraint fk_SafetyAct_Product
foreign key(SafetyAct_id)
references SafetyAct(SafetyAct_id)
on delete cascade,
constraint fk_Cause_Product
foreign key(Cause_id)
references Cause(Cause_id)
on delete cascade
)
CREATE TABLE SafetyIntervention (
SafetyIntervention_id int identity(1,1) primary key,
SafetyAct_id int,
Cause_id int,
constraint fk_SafetyAct_SafetyIntervention
foreign key(SafetyAct_id)
references SafetyAct(SafetyAct_id)
on delete cascade,
constraint fk_Cause_SafetyIntervention
foreign key(Cause_id)
references Cause(Cause_id)
on delete cascade
)
CREATE TABLE Cause (
Cause_id int primary key,
Cause_name varchar(80)
)
I want to write a query that shows the fields - SafetyAct_id and Cause_name.
in the Cause_name field i have a problem because i want that the query will show me the cause name drom the Product table or from the SafetyIntervension table (of course to connect it to the Cause table because i have only the cause_id - foriegn key in these tables) and i don't know how to write INNER JOIN and OR at the same query.
I am new with this so plase be patient.
Thank you!

Answer 1

SELECT SA.SafetyAct_id,
C.Cause_name
FROM SafetyAct SA
LEFT JOIN Product P
ON SA.SafetyAct_id = P.SafetyAct_id
LEFT JOIN SafetyIntervention SI
ON SA.SafetyAct_id = SI.SafetyAct_id
LEFT JOIN Cause C
ON ISNULL(P.Cause_id,SI.Cause_id) = C.Cause_id

Answer 2

An or is easy. So, based on Lamak's code:
SELECT SA.SafetyAct_id,
C.Cause_name
FROM SafetyAct SA LEFT JOIN
Product P
ON SA.SafetyAct_id = P.SafetyAct_id LEFT JOIN
SafetyIntervention SI
ON SA.SafetyAct_id = SI.SafetyAct_id LEFT JOIN
Cause C
ON C.Cause_id = P.Cause_id OR C.Cause_Id = SI.Cause_id;
--------------------------------^
You can just use OR in the ON condition.
However, it is often challenging for a SQL engine (SQL Server included) to optimize a join when the on condition includes or or functions on the columns used for the join. Hence, the following is often more efficient:
SELECT SA.SafetyAct_id,
COALESCE(Cp.Cause_name, Csi.Cause_Name) as Cause_Name
FROM SafetyAct SA LEFT JOIN
Product P
ON SA.SafetyAct_id = P.SafetyAct_id LEFT JOIN
SafetyIntervention SI
ON SA.SafetyAct_id = SI.SafetyAct_id LEFT JOIN
Cause Cp
ON Cp.Cause_id = P.Cause_id LEFT JOIN
Cause Csi
ON Csi.Cause_Id = SI.Cause_id;
If only some of the records have a cause of either type, then add:
WHERE Cp.Cause_Id IS NOT NULL OR Csi.Cause_Id IS NOT NULL;