How supply of goods and work contract differs bcoz for both different rate is there 18 and 12

It seems you have mixed up two different concepts. 

Works contract is basically the Supply of service where contractor under takes supply both goods as well as labour work. It includes both supply of goods as well as service. Hence SAC code is applicable.

If supplier only supplies goods and not labour then it is covered under Supply of Goods and HSN code is applied.

Generally, rates applicable to the Goods supplied as separately indicated in HSN are applicable to the services in relation to supply of such goods, if rates for such service are not specifically mentioned any where.

 

Thanks for your reply..
I got your point but I have one more confusion..
I work in state PWD ..here we normally have either work contract or supply contract... work contract have 12% gst and supply contract have 18% gst irrespective of goods and the rate of goods.
so I what to know where this 18% for supply is mentioned