how to get the current stock up to date by a SQL query or from which tables?

Hi All,

 i want to know the current stock but i don't know  what tables i should use 

 and how to get only the current stock

for example :

if there are 100  items in the inventory

 then 20 items have been issued

 then 10 items have been added to the stock

so my current stock now is 90 items 

how to get these 90 items in a SQL query

reply ASAP please

Thanks