Using IFNULL in where clause - mysql

Question

I have a problem with an SQL statement not returning any result from a specific result.
SELECT statementbalance AS 'BringForwardFromPreviousDay',
IFNULL((SELECT SUM(statementdebit) FROM statement WHERE merchantid = '4' AND statementdate = '2018-01-08'),0) AS 'TotalDebit',
IFNULL((SELECT SUM(statementcredit) FROM statement WHERE merchantid = '4' AND statementdate = '2018-01-08'),0) AS 'TotalCredit',
IFNULL((SELECT statementbalance FROM statement WHERE merchantid = '4' AND statementdate <= '2018-01-08' ORDER BY transactionid DESC LIMIT 1),0) AS 'TotalBalance'
FROM statement
WHERE merchantid = '4' and statementdate <= '2018-01-07'
ORDER BY transactionid DESC LIMIT 1
The SQL statement is to capture data dated from today '2018-01-08'and from previous days '2018-01-07' and grab the last recorded data hence the '<='
Because merchantid = '4' is a newly added merchant, it does not have any data to grab from <= 2018-01-07 hence i want it to return 0 instead of null and prevent other data from returning null.
I tried adding ifnull on the statementnbalance but it still returns null and i can only think of including ifnull on the where clause but i tried to no avail.
Here is the sqlfiddle of using merchantid '2' that works fine.
http://sqlfiddle.com/#!9/7cae3e0/1

Answer 1

I think this is what you want:
select
x.merchantid as merchantid,
ifnull(c.previous_days_balance, 0) as BringForwardFormPreviousDay,
ifnull(a.latest_total_debit, 0) as TotalDebit,
ifnull(a.latest_total_credit, 0) as TotalCredit,
ifnull(b.latest_balance, 0) as LatestBalance
from
(
select distinct merchantid from statement
) x
left outer join
(
select
merchantid,
sum(statementdebit) as latest_total_debit,
sum(statementcredit) as latest_total_credit
from
statement
where
statementdate = '2018-01-08'
group by
merchantid
) a
on x.merchantid = a.merchantid
left outer join
(
select
merchantid,
statementbalance as latest_balance
from
statement
where
(merchantid, transactionid) in
(
select
merchantid,
max(transactionid)
from
statement
where
statementdate = '2018-01-08'
group by
merchantid
)
) b
on x.merchantid = b.merchantid
left outer join
(
select
merchantid,
statementbalance as previous_days_balance
from
statement
where
(merchantid, transactionid) in
(
select
merchantid,
max(transactionid)
from
statement
where
statementdate <= '2018-01-07'
group by
merchantid
)
) c
on x.merchantid = c.merchantid;
I added another row to illustrate the extra case:
INSERT INTO statement VALUES ('99', '5', '131', 'Purchase: TopUp Cheezy', '2018-01-05', '23:35:31', '38.20', '0.00', '5000.00');
The results are:
+------------+-----------------------------+------------+-------------+----------------+
| merchantid | BringForwardFormPreviousDay | TotalDebit | TotalCredit | LatestBalance |
+------------+-----------------------------+------------+-------------+----------------+
| 1 | 35 | 15 | 0 | 5 |
| 2 | 182.33 | 4.9 | 0 | 177.43 |
| 4 | 0 | 95.48 | 200 | 104.52 |
| 5 | 5000 | 0 | 0 | 0 |
+------------+-----------------------------+------------+-------------+----------------+
4 rows in set (0.00 sec)
This assumes that transactionId keeps on increasing with time. That is not an entirely safe assumption. It would be better to use timestamps rather than dates for the transaction so you can find the latest one (or the latest one that is before today). I see that you do have statementtime but a separate column...

Answer 2

The query doesn't return anything because there are no data to return that fit your where clause. What you could do is using an IFNULL on your whole select statement:
SELECT ifnull((SELECT statementbalance AS 'BringForwardFromPreviousDay'
FROM statement
WHERE merchantid = '4'
AND statementdate <= '2018-01-07'
ORDER BY transactionid DESC LIMIT 1), 0)
and work from there. However, your statement can only contain one column, so you either have to work this into a stored procedure, or you have to deal with this issue at a different place (maybe in your code?)

Answer 3

You need to use a LEFT JOIN to allow for rows that don't exist.
SELECT IFNULL(t3.statementbalance, 0) AS BringForwardFromPreviousDay,
t1.TotalDebit, t1.TotalCredit, t2.TotalBalance
FROM (SELECT SUM(statementdebit) AS TotalDebit,
SUM(statementcredit) AS TotalCredit
FROM statement
WHERE merchantId = '4' AND statementdate = '2018-01-08') AS t1
CROSS JOIN (
SELECT statementbalance AS TotalBalance
FROM statement
WHERE merchantId = '4' AND statementdate <= '2018-01-08'
ORDER BY statementdate DESC
LIMIT 1) AS t2
LEFT JOIN (
SELECT statementbalance
FROM statement
WHERE t3.merchantID = '4' AND t3.statementdate <= '2018-01-07'
ORDER BY statementdate DESC
LIMIT 1) ON 1=1