SELECT Table1.Col1, Table1.Col2, Table1.Col3, Table2.Col1 FROM Table2 
LEFT OUTER JOIN Table1 ON Table1.SONumber  like '%' + Table2.SONumber + '%' WHERE ...

Table2.SONumber  like '%' + Table1.SONumber  + '%'

create table #Table1
(
	SONumber varchar(10)
)

create table #Table2
(
	SONumber varchar(10)
)

insert #Table1
select '123'

insert #Table2
select '123/1'

select * 
from #Table1 t1
left join #Table2 t2 on CHARINDEX(t1.SONumber, t2.SONumber) > 0 

drop table #Table1
drop table #Table2

create table #Table1
(
	SONumber varchar(10)
)

create table #Table2
(
	SONumber varchar(10)
)

insert #Table1
select '123'

insert #Table2
select '123/1'

select * 
from #Table1 t1
left join #Table2 t2 on t1.SONumber = SUBSTRING(t2.SONumber, 1, len(t1.SONumber)) 

drop table #Table1
drop table #Table2

select * 
from #Table1 t1
left join #Table2 t2 on t2.SONumber like t1.SONumber + '%'

Create Table Ex 
(Id Int Identity(1,1), 
 Value NVarchar(5) ) 
 
Create Table Ex1 
(Id Int Identity(1,1), 
 Value NVarchar(5) ) 

Insert Into Ex 
Select '123'
 
Insert Into Ex1 
Select '123/1'

Select * From Ex As a JOIN Ex1 As b ON b.Value LIKE Left(a.Value, CHARINDEX('/', a.Value)) + '%'