execute sql task error

hi,

i am using merger in the execute sql task,

which is like

merge [dbo].[emp] a
using  [dbo].[emp1] b
on a.id=b.id
when matched and a.number =0   then
update set
a.emp=b.emp,

 a.[Address_1] = b.[Address_1] ,
 a.[Address_2] = b.[Address_2] ,
 a.[Address_3] = b.a.[Address_3] ,
 a.[Address_4]  = a.[Address_4] ,

when not matched then

insert 

            -------------------

go

getting error