Left join with result of COALESCE - sql

Question

SELECT *
FROM TBDA temp
LEFT JOIN (
SELECT
(
COALESCE (
(
SELECT sum_payment
FROM TBDA_REPAYMENT
WHERE isdn = temp.isdn
AND tr_month = temp.tr_month
AND tr_year = temp.tr_year
),
(
SELECT sum_payment
FROM TBDA_PAYNEW
WHERE isdn = temp.isdn
AND tr_month = temp.tr_month
AND tr_year = temp.tr_year
)
)
)
)
TBDA table has isdn,tr_month,tr_year
TBDA_REPAYMENT table has isdn,tr_month,tr_year,sum_payment
TBDA_PAYNEW table has isdn,tr_month,tr_year,sum_payment
Expected result :
A table off all fields of TBDA with sum_payment, look for sum_payment from two tables (TBDA_REPAYMENT & TBDA_PAYNEW) by ISDN (TBDA table) by comparing isdn,tr_month,tr_year, if sum_payment exists in one of two tables (TBDA_REPAYMENT & TBDA_PAYNEW) then add it to the results, otherwise it's null.
I get the error :
[Err] 42000 - [SQL Server]Incorrect syntax near ')'.

Answer 1

SELECT
*
FROM
TBDA temp
Cross Apply ( SELECT COALESCE (
(
SELECT
sum_payment
FROM
TBDA_REPAYMENT
WHERE
isdn = temp.isdn
AND tr_month = temp.tr_month
AND tr_year = temp.tr_year
),
(
SELECT
sum_payment
FROM
TBDA_PAYNEW
WHERE
isdn = temp.isdn
AND tr_month = temp.tr_month
AND tr_year = temp.tr_year
)
) as First_Null_Value
) as value_table

Answer 2

Unless a CROSS JOIN, CROSS APPLY or OUTER APPLY is used, the ON clause is required.
Hence the error.
Although you could still do something like ... LEFT JOIN othertable ON (1=1).
But you could re-write this query using 2 left joins.
select
temp.*,
coalesce(repay.sum_payment, paynew.sum_payment) as value_table
from TBDA temp
left join TBDA_REPAYMENT repay on (repay.isdn = temp.isdn and repay.tr_month = temp.tr_month and repay.tr_year = temp.tr_year)
left join TBDA_PAYNEW paynew on (paynew.isdn = temp.isdn and paynew.tr_month = temp.tr_month and paynew.tr_year = temp.tr_year);
Note that there's an assumption that the tables TBDA_REPAYMENT & TBDA_PAYNEW have a uniqueness on the 3 fields used in the join (isdn, tr_month, tr_year).
Or the OUTER APPLY syntax could be used.
And if there's no uniqueness on (isdn, tr_month, tr_year) then you can use that to SUM the total at the same time.
select
temp.*, repay.sum_payment as repay_sum_payment, paynew.sum_payment as paynew_sum_payment,
coalesce(repay.sum_payment, paynew.sum_payment) as value_table
from TBDA temp
outer apply
(
select nullif(sum(rp.sum_payment),0) as sum_payment
from TBDA_REPAYMENT rp
where rp.isdn = temp.isdn
and rp.tr_month = temp.tr_month
and rp.tr_year = temp.tr_year
) as repay
outer apply
(
select nullif(sum(pn.sum_payment),0) as sum_payment
from TBDA_PAYNEW pn
where pn.isdn = temp.isdn
and pn.tr_month = temp.tr_month
and pn.tr_year = temp.tr_year
) as paynew;
Example Snippet:
declare #TBDA table (isdn int, tr_month int, tr_year int);
declare #TBDA_REPAYMENT table (isdn int, tr_month int, tr_year int, sum_payment int);
declare #TBDA_PAYNEW table (isdn int, tr_month int, tr_year int, sum_payment int);
insert into #TBDA (isdn, tr_month, tr_year) values (1,6,2018),(2,6,2018);
insert into #TBDA_REPAYMENT (isdn, tr_month, tr_year, sum_payment) values (1,6,2018, 100);
insert into #TBDA_PAYNEW (isdn, tr_month, tr_year, sum_payment) values (1,6,2018, 200),(2,6,2018,100),(2,6,2018,200);
--
-- Using left join
--
select
temp.*, repay.sum_payment, paynew.sum_payment,
coalesce(repay.sum_payment, paynew.sum_payment) as value_table
from #TBDA temp
left join #TBDA_REPAYMENT repay on (repay.isdn = temp.isdn and repay.tr_month = temp.tr_month and repay.tr_year = temp.tr_year)
left join #TBDA_PAYNEW paynew on (paynew.isdn = temp.isdn and paynew.tr_month = temp.tr_month and paynew.tr_year = temp.tr_year);
--
-- Using outer apply
--
select
temp.*, repay.sum_payment as repay_sum_payment, paynew.sum_payment as paynew_sum_payment,
coalesce(repay.sum_payment, paynew.sum_payment) as value_table
from #TBDA temp
outer apply
(
select nullif(sum(rp.sum_payment),0) as sum_payment
from #TBDA_REPAYMENT rp
where rp.isdn = temp.isdn
and rp.tr_month = temp.tr_month
and rp.tr_year = temp.tr_year
) as repay
outer apply
(
select nullif(sum(pn.sum_payment),0) as sum_payment
from #TBDA_PAYNEW pn
where pn.isdn = temp.isdn
and pn.tr_month = temp.tr_month
and pn.tr_year = temp.tr_year
) as paynew;

Answer 3

Error is due to not specifying ON condition on LEFT JOIN.
LEFT JOIN without ON will throw an error.
If you don't want to specify any condition, try CROSS JOIN

Answer 4

Just use correlated subqueries:
SELECT t.*,
IFNULL ( (SELECT r.sum_payment
FROM TBDA_REPAYMENT r
WHERE r.isdn = t.isdn AND
r.tr_month = t.tr_month AND
r.tr_year = t.tr_year
),
(SELECT p.sum_payment
FROM TBDA_PAYNEW p
WHERE p.isdn = t.isdn AND
p.tr_month = t.tr_month AND
p.tr_year = t.tr_year
)
)
)
FROM TBDA t;
Although you can use APPLY, the use of SELECT is already insisting that at most one row be returned from the subqueries.
Also note a couple of things. First, I replaced COALESCE() with IFNULL(). Although I much prefer COALESCE() (because it is the standard and more flexible), the code is more efficient with IFNULL(). COALESCE() causes SQL Server to evaluate the first argument twice when it is not NULL.
Second, table aliases and qualified column names should be used everywhere in the query.