Query Alternate Table if record not exist - sql

Question

I have two queries as bellow
SELECT i.IncidentId, rl.Description Category FROM incident i
JOIN IncidentLikelihood l ON i.IncidentId = l.IncidentId
JOIN IncidentSeverity s ON i.IncidentId = s.IncidentId
JOIN LikelihoodSeverity ls ON l.LikelihoodId = ls.LikelihoodId AND s.SeverityId = ls.SeverityId
JOIN RiskLevel rl ON ls.RiskLevelId = rl.riskLevelId
And
SELECT i.IncidentId, rl.Description Category FROM incident i
JOIN incidentreportlikelihood l ON i.IncidentId = l.IncidentId
JOIN IncidentReportSeverity s ON i.IncidentId = s.IncidentId
JOIN LikelihoodSeverity ls ON l.LikelihoodId = ls.LikelihoodId AND s.SeverityId = ls.SeverityId
JOIN RiskLevel rl ON ls.RiskLevelId = rl.riskLevelId
I need to prepare a query with WITH clause such that within the clause either of the two query runs and then this view can be joined with another table.
Can you guys please suggest how I can proceed ?
I have tried UNION but that doesn't work for me. I am looking for something using IFEXISTS or EXISTS where - some like bellow :
WITH IncidentCategory AS (
SELECT i.IncidentId, rl.Description Category FROM incident i
JOIN IncidentLikelihood l ON i.IncidentId = l.IncidentId
JOIN IncidentSeverity s ON i.IncidentId = s.IncidentId
JOIN LikelihoodSeverity ls ON l.LikelihoodId = ls.LikelihoodId AND s.SeverityId = ls.SeverityId
JOIN RiskLevel rl ON ls.RiskLevelId = rl.riskLevelId
WHERE EXISTS (SELECT NULL)
OR
(SELECT i.IncidentId, rl.Description Category FROM incident i
JOIN incidentreportlikelihood l ON i.IncidentId = l.IncidentId
JOIN IncidentReportSeverity s ON i.IncidentId = s.IncidentId
JOIN LikelihoodSeverity ls ON l.LikelihoodId = ls.LikelihoodId AND s.SeverityId = ls.SeverityId
JOIN RiskLevel rl ON ls.RiskLevelId = rl.riskLevelId)
)
A little more to get the idea : for each of the rows [incidentid] if the the first query returns any result - it should show otherwise the second query result should show

Answer 1

If I understand the requirements correctly you have to run first query and if it has no results the second one should be run.
In fact you can union the both queries. The amount of rows in the second one should depend on the first. If count(*) for the first returns >0 the second query rows are included. See the example
SELECT i.IncidentId, rl.Description Category FROM incident i
JOIN IncidentLikelihood l ON i.IncidentId = l.IncidentId
JOIN IncidentSeverity s ON i.IncidentId = s.IncidentId
JOIN LikelihoodSeverity ls ON l.LikelihoodId = ls.LikelihoodId AND s.SeverityId = ls.SeverityId
JOIN RiskLevel rl ON ls.RiskLevelId = rl.riskLevelId
UNION
SELECT i.IncidentId, rl.Description Category FROM incident i
JOIN incidentreportlikelihood l ON i.IncidentId = l.IncidentId
JOIN IncidentReportSeverity s ON i.IncidentId = s.IncidentId
JOIN LikelihoodSeverity ls ON l.LikelihoodId = ls.LikelihoodId AND s.SeverityId = ls.SeverityId
JOIN RiskLevel rl ON ls.RiskLevelId = rl.riskLevelId
join (select count(*) as count
FROM incident i
JOIN IncidentLikelihood l ON i.IncidentId = l.IncidentId
JOIN IncidentSeverity s ON i.IncidentId = s.IncidentId
JOIN LikelihoodSeverity ls ON l.LikelihoodId = ls.LikelihoodId AND s.SeverityId = ls.SeverityId
JOIN RiskLevel rl ON ls.RiskLevelId = rl.riskLevelId) q1_copy on q1_copy.count=0