Unable to get the data with multiple ID's

It seems like your rule input is just a single integer. Make it a list and this should work.

The a!flatten and the curly brackets in line 12 are not necessary.

In the below code on the if condition that no value is passing in the ri!caseIds i mean 0 then i should get null value but iam not getting anything.

{
a!localVariables(
local!data: a!queryEntity(
entity: cons!BEMS_ENTITY_PAYABLE_CASE_VIEW,
query: a!query(
/*selection: a!querySelection(a!queryColumn(field: "clientName")),*/
aggregation: a!queryAggregation(
a!queryAggregationColumn(field: "clientName", isGrouping: true)
),
filter: a!queryFilter(
field: "caseId",
operator: "in",
value: ri!caseIds
),
pagingInfo: rule!TCO_defaultPagingInfo()
)
),
{
if(
local!data.totalCount > 0,
if(
local!data.totalCount = 1,
if(
count(ri!caseIds) = 1,
left(
"Standard Template - " & proper(
joinarray(local!data.data.clientName, " & ")
) & " - " & "CaseId:" & joinarray(ri!caseIds, ", "),
80
),
left(
"Standard Template - " & proper(
joinarray(local!data.data.clientName, " & ")
) & " - " & "Multiple Case Id ",
80
)
),
if(
local!data.totalCount >= 2,
left(
"Standard Template - " & proper(
joinarray(local!data.data.clientName, " & ")
) & " - " & "Multiple Case Id ",
80
),
{}
)
),
{}
)
}
)
}

In the below code on the if condition that no value is passing in the ri!caseIds i mean 0 then i should get null value but iam not getting anything.

{
a!localVariables(
local!data: a!queryEntity(
entity: cons!BEMS_ENTITY_PAYABLE_CASE_VIEW,
query: a!query(
/*selection: a!querySelection(a!queryColumn(field: "clientName")),*/
aggregation: a!queryAggregation(
a!queryAggregationColumn(field: "clientName", isGrouping: true)
),
filter: a!queryFilter(
field: "caseId",
operator: "in",
value: ri!caseIds
),
pagingInfo: rule!TCO_defaultPagingInfo()
)
),
{
if(
local!data.totalCount > 0,
if(
local!data.totalCount = 1,
if(
count(ri!caseIds) = 1,
left(
"Standard Template - " & proper(
joinarray(local!data.data.clientName, " & ")
) & " - " & "CaseId:" & joinarray(ri!caseIds, ", "),
80
),
left(
"Standard Template - " & proper(
joinarray(local!data.data.clientName, " & ")
) & " - " & "Multiple Case Id ",
80
)
),
if(
local!data.totalCount >= 2,
left(
"Standard Template - " & proper(
joinarray(local!data.data.clientName, " & ")
) & " - " & "Multiple Case Id ",
80
),
{}
)
),
{}
)
}
)
}

You can make use of the defaultValue() for the totalCount in case of no caseIds

Please explain in more detail what you are doing here, and what you expect to happen.

In the below code that totalCount of ri!caseIds is 0 then it should write null value but its writing no value 

{
if(
local!data.totalCount > 0,
if(
local!data.totalCount = 1,
if(
count(ri!caseIds) = 1,
left(
"Standard Template - " & proper(
joinarray(local!data.data.clientName, " & ")
) & " - " & "CaseId:" & joinarray(ri!caseIds, ", "),
80
),
left(
"Standard Template - " & proper(
joinarray(local!data.data.clientName, " & ")
) & " - " & "Multiple Case Id ",
80
)
),
if(
local!data.totalCount >= 2,
left(
"Standard Template - " & proper(
joinarray(local!data.data.clientName, " & ")
) & " - " & "Multiple Case Id ",
80
),
{}
)
),
{}
)
}