It is not always obvious what request is actually executed on SQL Server. The most confusing is probably exists join in X++. Let's analyze how joins in select statement in X++ are translated into T-SQL statement sent to SQL Server.


1. join in X++:
select AccountNum from custTable
join TaxGroupId from custGroup
where custGroup.CustGroup == custTable.CustGroup;
CROSS JOIN in T-SQL:
SELECT T1.ACCOUNTNUM, T1.RECID, T2.TAXGROUPID, T2.RECID
FROM CUSTTABLE T1 CROSS JOIN CUSTGROUP T2
WHERE ((T1.PARTITION=?) AND (T1.DATAAREAID=?))
AND (((T2.PARTITION=?) AND (T2.DATAAREAID=?))
AND (T2.CUSTGROUP=T1.CUSTGROUP))

2. outer join in X++:
select AccountNum from custTable
outer join AccountID from custBankAccount
where custBankAccount.CustAccount == custTable.AccountNum;
LEFT OUTER JOIN in T-SQL:
SELECT T1.ACCOUNTNUM, T1.RECID, T2.ACCOUNTID, T2.RECID
FROM CUSTTABLE T1 LEFT OUTER JOIN CUSTBANKACCOUNT T2
ON (((T2.PARTITION=?) AND (T2.DATAAREAID=?))
AND (T1.ACCOUNTNUM=T2.CUSTACCOUNT))
WHERE ((T1.PARTITION=?) AND (T1.DATAAREAID=?))

3. exists join in X++:
select AccountNum from custBankAccount
exists join custTable
where custBankAccount.CustAccount == custTable.AccountNum;
EXISTS (SELECT 'x'...) in T-SQL:
SELECT T1.ACCOUNTNUM, T1.RECID
FROM CUSTBANKACCOUNT T1
WHERE ((T1.PARTITION=?) AND (T1.DATAAREAID=?))
AND EXISTS (SELECT 'x'
FROM CUSTTABLE T2
WHERE (((T2.PARTITION=?) AND (T2.DATAAREAID=?))
AND (T1.CUSTACCOUNT=T2.ACCOUNTNUM)))

4. notexists join in X++:
select AccountNum from custBankAccount
notexists join custTable
where custBankAccount.CustAccount == custTable.AccountNum;
NOT (EXISTS (SELECT 'x'...)) in T-SQL:
SELECT T1.ACCOUNTNUM, T1.RECID
FROM CUSTBANKACCOUNT T1
WHERE ((T1.PARTITION=?) AND (T1.DATAAREAID=?))
AND NOT (EXISTS (SELECT 'x'
FROM CUSTTABLE T2
WHERE (((T2.PARTITION=?) AND (T2.DATAAREAID=?))
AND (T1.CUSTACCOUNT=T2.ACCOUNTNUM))))

5. join after exists join in X++:
select AccountNum from custBankAccount
exists join custTable
where custBankAccount.CustAccount == custTable.AccountNum
join TaxGroupId from custGroup
where custGroup.CustGroup == custTable.CustGroup;
EXISTS (SELECT 'x'...CROSS JOIN...) in T-SQL:
SELECT T1.ACCOUNTNUM, T1.RECID
FROM CUSTBANKACCOUNT T1
WHERE ((T1.PARTITION=?) AND (T1.DATAAREAID=?))
AND EXISTS (SELECT 'x'
FROM CUSTTABLE T2 CROSS JOIN CUSTGROUP T3
WHERE (((T2.PARTITION=?) AND (T2.DATAAREAID=?))
AND (T1.CUSTACCOUNT=T2.ACCOUNTNUM))
AND (((T3.PARTITION=?) AND (T3.DATAAREAID=?))
AND (T3.CUSTGROUP=T2.CUSTGROUP)))


6. exists join after exists join in X++:
select AccountNum from custBankAccount
exists join custTable
where custBankAccount.CustAccount == custTable.AccountNum
exists join custGroup
where custGroup.CustGroup == custTable.CustGroup
&& custGroup.TaxGroupId == "Std";
EXISTS (SELECT 'x'... EXISTS (SELECT 'x'...)) in T-SQL:
SELECT T1.ACCOUNTNUM, T1.RECID
FROM CUSTBANKACCOUNT T1
WHERE ((T1.PARTITION=?) AND (T1.DATAAREAID=?))

AND EXISTS (SELECT 'x' FROM CUSTTABLE T2
WHERE (((T2.PARTITION=?) AND (T2.DATAAREAID=?))
AND (T1.CUSTACCOUNT=T2.ACCOUNTNUM))
AND EXISTS (SELECT 'x'
FROM CUSTGROUP T3
WHERE (((T3.PARTITION=?) AND (T3.DATAAREAID=?))
AND ((T3.CUSTGROUP=T2.CUSTGROUP) AND (T3.TAXGROUPID=?)))))

Conclusion
each exists join is considered as a separate statement, but any join after exists join is coupled or nested.