SQL：其中列不等于返回结果，否则结果等于空白值

Question

So I am coming across an issue with a sql query (mssql) I have been working on that has multi-statement where clause, and one of the where statements may or may not return a value. If the where condition is not met how can I have it return an empty value and still return the rest of my results? I am also using multiple CTEs.

Here is my select clause:

select cte_devinfo.SerialNumber,
    cte_devinfo.DeviceName,
    cte_devinfo.DeviceID, 
    dev_CTE.concurrencies,
    (cte_slots.LocationIndex +1) as 'Total Media',
    cte_changer.SlotCount, cte_changer.TotalMountErrors, cte_changer.TotalMounts,
    cte_mismatch.MismatchSerialNumber
from cte_devinfo, dev_CTE, cte_slots, cte_changer, cte_mismatch

Here is my where clause:

where cte_devinfo.DeviceID = dev_CTE.DeviceParentID 
and cte_slots.LocationID = dev_CTE.DeviceParentID
and cte_changer.ChangerID = dev_CTE.DeviceParentID
and cte_mismatch.LocationID = dev_CTE.DeviceParentID 

I want to add something like this to my where clause:

and cte_mismatch.MismatchSerialNumber != cte_devinfo.SerialNumber 

but this condition may never occur, and if it doesn't how can I ignore the condition and just return ' ' so the rest of the query will run?

Answer 1

First, rewrite your query using ANSI joins:

select cte_devinfo.SerialNumber,
    cte_devinfo.DeviceName,
    cte_devinfo.DeviceID, 
    dev_CTE.concurrencies,
    (cte_slots.LocationIndex +1) as 'Total Media',
    cte_changer.SlotCount, cte_changer.TotalMountErrors, cte_changer.TotalMounts,
    cte_mismatch.MismatchSerialNumber
from cte_devinfo
inner join dev_CTE on cte_devinfo.DeviceID = dev_CTE.DeviceParentID
left outer join cte_slots on cte_slots.LocationID = dev_CTE.DeviceParentID
left outer join cte_changer on cte_changer.ChangerID = dev_CTE.DeviceParentID
left outer join cte_mismatch on cte_mismatch.LocationID = dev_CTE.DeviceParentID 

I changed all joins except the first one to outer, to allow for missing records for slots, changers, and mismatches. dev_CTE remains required, though, because all tables are joined to the record from it.

Now you can add a where clause like this:

WHERE cte_mismatch.MismatchSerialNumber IS NULL OR cte_mismatch.MismatchSerialNumber != cte_devinfo.SerialNumber

This condition allows for NULL in MismatchSerialNumber , or even for missing cte_mismatch record.