PostgreSQL joining multiple tables - sql

Question

Hi guys I am trying to join tables here, I want to add table called Table2 so that students from that table are displayed too. The thing which is confusing is I already have joins in this query, so how should add another join so that students from that Table2 are displayed as well, without breaking the existing code . Also, there is no way I can check output
The ID varables are stsidno in stTable 1, sidno in Table2, scsidno in table schedprd
Here's the code:
String selS = "select distinct stdistrict,stschool,stsidno,"
+ " sname as name,stgrade,"
+ "S.recnum as recnum, S.stldate as stldate,scsec,sctea,sccor,scgrade,scclsprd,scgrdprd,"
+ "case when P.scchangestartdate is null then C.clstart else "
+ "P.scchangestartdate end as scchangestartdate, "
+ "case when S.stedate is null or S.stedate<C.clstart "
+ "then C.clstart else S.stedate end as stedate "
+ "from stTable1 as S join schedprd as P on "
+ "(scyear=styear and scdistrict=stdistrict and scschool=stschool "
+ "and stsidno=scsidno and (scsec is not null and scsec<>'')) "
+ "left outer join calendar as C on (C.clyear=S.styear and "
+ "C.cldistrict=S.stdistrict and C.clschool=S.stschool and C.cltype='10') "
+ "where styear=? and stdistrict=? ";

Answer 1

You could UNION Table1 and Table2 together in a Common Table Expression (CTE) and then reference the CTE instead of Table1 in your query, like below:
WITH CTE
AS
(
SELECT *
FROM Table1
UNION
SELECT *
FROM Table2
)
select distinct
stdistrict,
stschool,
stsidno,
sname as name,
stgrade,
S.recnum as recnum,
S.stldate as stldate,
scsec,
sctea,
sccor,
scgrade,
scclsprd,
scgrdprd,
case when P.scchangestartdate is null then C.clstart else P.scchangestartdate end as scchangestartdate,
case when S.stedate is null or S.stedate<C.clstart then C.clstart else S.stedate end as stedate
from CTE as S
join schedprd as P on (scyear=styear and scdistrict=stdistrict and scschool=stschool and stsidno=scsidno and (scsec is not null and scsec<>''))
left outer join calendar as C on (C.clyear=S.styear and C.cldistrict=S.stdistrict and C.clschool=S.stschool and C.cltype='10')
where styear=? and stdistrict=?;