Show Zero if there is no record count - ORACLE SQL query - sql

Question

The below Oracle query gives if there are any different errors with error_message and Serial_num.
If there is ZERO or No Different error count instead of showing Blank/Null result. How can i see the output like this? I tried with NVL(error_message,0) and COALESCE (Sum(total),0) but not getting the desired output.
Expected output:
1 Different Errors: 0
Oracle SQL Query:
SELECT
1 as Index_Num,
CONCAT('Different Errors: ', error_message || '# ' || serial_num),
SUM(total)
FROM (
SELECT error_message, serial_num, COUNT(*) total
FROM Table1
WHERE error_message NOT LIKE '%INVALID%'
GROUP BY error_message, serial_num
)
GROUP BY error_message, serial_num

Answer 1

Create a CTE for the subquery and use UNION ALL with NOT EXISTS to cover the case that the CTE does not return any rows:
WITH cte AS (
SELECT error_message, serial_num, COUNT(*) total
FROM Table1
WHERE error_message NOT LIKE '%INVALID%'
GROUP BY error_message, serial_num
)
SELECT
1 as Index_Num,
CONCAT(
'Different Errors: ',
list_agg(error_message || '# ' || serial_num) within group (order by error_message)
),
SUM(total)
FROM cte
UNION ALL
SELECT 1, 'Different Errors: ', 0
FROM dual
WHERE NOT EXISTS (SELECT 1 FROM cte)

Answer 2

D'oh! Looks like I took too long. Here's another option for posterity:
SELECT
1,
CONCAT(
'Different Errors: ',
CASE
WHEN src.error_message IS NULL THEN ''
ELSE src.error_message || ' # ' || src.serial_num
END
) Summary,
COALESCE(src.total, 0) AS total
FROM dual -- Get a seed row (in case there are no rows in error table)
LEFT JOIN (
SELECT error_message, serial_num, COUNT(*) total
FROM Table1
WHERE error_message NOT LIKE '%INVALID%'
GROUP BY error_message, serial_num
) src ON 0=0
SQL Fiddle

Answer 3

It is not exactly what you are asking for, but might prove useful. You can easily add a row with the total number of errors, using grouping sets:
SELECT 1 as Index_Num,
('Different Errors: ' || error_message || '# ' || serial_num),
COUNT(*) as total
FROM Table1
WHERE error_message NOT LIKE '%INVALID%'
GROUP BY GROUPING SETS ( (error_message, serial_num), () );
Alas, this produces the summary row even when there are errors. It occurs to me that you might find this useful.