On fly data validation

I have two tables with one table having SLA days,date and another with inventory qty, date

My requirement is to check for the inventory qty availability for missed SLA days

for e.g.: if the SLA days missed is 2 i need to check for the qty avialability on inventiry table for shippeddate-2 and above. If inventory available shows null need to flag that

Can i get any suggestions to acheive this.

Thanks in advance.