what is this code doing can not understand it - sql

Question

I am trying to figure out what a query is doing and I just do not understand why it would join its self to its self on multiple occassions highlighted bit is the piece i am talking about?
its the part that starts with the [SupplyStatusUpdated] = COALESCE( gas supply and gas supply history
USE [CRM];
SELECT gs.GasSupplyID,
[Account ID] = acc.AccountID,
[GMSReference] = cast(gms.SiteRefNum as varchar),
[AccountNumber] = cast(gms.AccountNum as varchar),
[Organisation Name] = prf.[Name] ,
con.DeclaredDate,
[Contract Date] = CAST(con.ContractDate AS DATE),
[Contract Version] = cv.Name,
[Contract Status] = cs.Name,
loa.ContractEndDate [LOA CED],
gs.CurrentSupplierEndDate [PrevSupplierEndDate],
loa.ContractEndDate,
con.StartDate,
[Supply Status] = gss.Name,
[SupplyStatusUpdated] = COALESCE(
(
SELECT TOP 1 MAX(gsh2.CreatedDate)
FROM GasSupply gs2
INNER JOIN GasSupplyHistory gsh2
ON gsh2.GasSupplyFK = gs2.GasSupplyID
WHERE gsh2.EventFK = 2
AND gsh2.Notes LIKE '%Gas Supply Status (%'
AND gsh2.GasSupplyFK = gs.GasSupplyID
GROUP BY gsh2.GasSupplyFK
),
(
SELECT TOP 1 MAX(gsh3.CreatedDate)
FROM GasSupplyHistory gsh3
INNER JOIN (
SELECT gsh4.GasSupplyFK, MAX(gsh4.EventFK) AS [MaxEvent]
FROM GasSupplyHistory gsh4
WHERE gsh4.GasSupplyFK = gs.GasSupplyID
GROUP BY gsh4.GasSupplyFK
HAVING MAX(gsh4.EventFK) = 1
) dt
ON dt.GasSupplyFK = gsh3.GasSupplyFK
)
),
[EAC] = gs.EstimatedAnnualConsumption,
[PreviousSupplier] = r.name,
[Branch] = b.name,
[LeadSource] = ls.name,
gs.UnitPrice,
gs.StandingCharge,
gs.WholesalePrice,
COALESCE(deal.weeknumber,DATEPART(ISOWK, con.[ContractDate])) AS [Week]
FROM acc.Account acc
INNER JOIN [Profile] prf
ON acc.ProfileFK = prf.ProfileID
INNER JOIN [Contract] con
ON acc.AccountID = con.AccountFK
INNER JOIN [loacontract] lo
ON lo.ContractFK = con.ContractID
LEFT join [LeadSource] ls
ON ls.LeadSourceID = con.LeadSourceFK
INNER JOIN [ContractStatus] cs
ON cs.ContractStatusID = con.ContractStatusFK
INNER JOIN [ContractVersion] cv
ON cv.ContractVersionID = con.ContractVersionFK
INNER JOIN GasSupply gs
ON gs.ContractFK = con.ContractID
INNER JOIN GasSupplyStatus gss
ON gss.GasSupplyStatusID = gs.GasSupplyStatusFK
LEFT JOIN Deal deal
ON deal.ContractFK = con.ContractID
OUTER APPLY GetGMSReferenceNumbers(con.ContractID) gms
LEFT JOIN Branch b
ON b.BranchID = deal.BranchFK
LEFT JOIN DealBroker bro
ON bro.DealFK = deal.DealID
AND bro.BrokerTypeFK = 36
LEFT JOIN Person p
ON p.PersonID = bro.EmployeeFK
LEFT JOIN Reseller r
ON r.ResellerID = gs.ResellerFK
LEFT JOIN (
SELECT l.contractfk,
MIN(l.contractenddate)[ContractEndDate]
FROM CRM.[contract].LOAContract l
GROUP BY l.ContractFK
) loa
ON loa.ContractFK = con.ContractID
WHERE acc.AccountID not in (
select AccountFK
from AccountOption
where OptionFK=9
)
AND cast(gms.SiteRefNum as varchar) IS NULL

Answer 1

COALESCE(Something, SomethingElse) says if the first argument is NULL, return the second argument (note that you can have more than 2 args and it'll just keep going down the list).
As such it's running the first sub-query, and if the result is NULL, returning the result of the second query. Why exactly is your business logic, which we can't answer :-)
(MSDN link on Coalesce)

Answer 2

COALESCE returns the first non-null value it finds, so if the GasSupply query returns a result it will use that: if that returns null it will see if the GasSupplyHistory query returns a result, and if so use that. If both queries return null then SupplyStatusUpdated will be null.

Answer 3

It joins to itself to get historical records from the same source as where it gets the current record.
Representation of historical data is is one common reason for using a BigData/NoSQL database instead of a SQL/Relational database.