DEMO, PostgreSQL - rextester

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--PostgreSQL 9.6
--'\\' is a delimiter

--drop table ASSEMBLYLINES;
--drop table ASSEMBLYLINEISSUES;

create table ASSEMBLYLINES(ORDERNUMBER varchar(20), LINECODE int, ORDERQTY int, SYSUNIQUEID int);

insert into ASSEMBLYLINES  values ('13088N03', 810044, 3, 12345678);

create table ASSEMBLYLINEISSUES(ORDERNUMBER varchar(20), COMPONENTCODE int, ISSUEQTY int, SYSUNIQUEID int, HEADERSYSUNIQUEID int);

insert into ASSEMBLYLINEISSUES values ('13088N03', 810044, 1, 11234567, 12345678);
insert into ASSEMBLYLINEISSUES values ('13088N03', 810044, 1, 12234567, 12345678);
insert into ASSEMBLYLINEISSUES values ('13088N03', 810044, 1, 13234567, 12345678);

SELECT a.ORDERNUMBER,
       a.LINECODE,
       CASE
           WHEN row_number() over(partition BY a.ORDERNUMBER, a.LINECODE,b.ORDERNUMBER, b.COMPONENTCODE, b.ISSUEQTY
                                  ORDER BY a.ORDERNUMBER) = 1 THEN a.ORDERQTY
           ELSE NULL
       END AS Order_Quantity,
       b.ORDERNUMBER,
       b.COMPONENTCODE,
       b.ISSUEQTY
FROM ASSEMBLYLINES a
LEFT JOIN ASSEMBLYLINEISSUES b ON a.SYSUNIQUEID = b.HEADERSYSUNIQUEID;


absolute service time: 0,45 sec