How can a SQL query have two from clauses?

Question

This just looks so odd to me:

delete from GearsDev.dbo.Products 
from GearsDev.dbo.Products as C
inner join #Common as M
    on M.item = C.ItemNumber

#Common is a temp table, but the rest of it makes no sense to me.

How can you have two from clauses?

Answer 1

As can be seen from the documentation of DELETE, it can take two FROM clauses.

The first FROM:

FROM: Is an optional keyword that can be used between the DELETE keyword and the target table_or_view_name, or rowset_function_limited.

The second FROM:

FROM <table_source>: Specifies an additional FROM clause. This Transact-SQL extension to DELETE allows specifying data from and deleting the corresponding rows from the table in the first FROM clause.

This extension, specifying a join, can be used instead of a subquery in the WHERE clause to identify rows to be removed.

So, the SQL will delete records from the Products table that have a matching item when it is joined with #common.

This is equivalent (in meaning) to the following query:

delete from [GearsDev].[dbo].[Products]
where ItemNumber in
(
  select item from #common
)