Flag missing record between 2 datasets - sql-server

Question

The Title doesn't quite explain the issue, so I will give an example:
Table 1 (OrderNum, OrderLine) - All OrderLines ever created.
Table 2 (OrderNum, OrderLin) - Order Lines to be printed right now.
Table 2 always has matches in Table 1 as it is populated from table 1, but sometimes is missing a Line from an order in Table 1. For instance:
Table 1:
OrderNum, OrderLine
100, 1
100, 2
100, 3
100, 4
101, 1
102, 1
102, 2
Table 2:
OrderNum, OrderLine
100, 1
100, 2
100, 4
101, 1
In this instance I need to print all 4 lines of Order 100, with Line 3 flagged as missing, and all of Order 101, but none of order 102. Obviously my tables are much larger than this, but this sums up my dilemma. I cannot control how either table is populated.
If I LEFT Join Table 2 to Table 1 on OrderNum and OrderLine, then i get OrderNum 102 as well and I don't want that. If I Join on OrderNum Only then I get dupes of Order 100 for every line in Table 2, how do i find the 1 record not-duped?
Hope that makes sense as to my issue. I have tried Googling, but the terms are so generic I get way to many irrelevant results.

Answer 1

using a left join and exists():
select
t1.OrderNum
, t1.OrderLine
, Missing = case when t2.OrderLine is null then 'Flag' else '' end
from t1
left join t2
on t1.OrderNum = t2.OrderNum
and t1.OrderLine = t2.OrderLine
where exists (
select 1
from t2 i
where t1.OrderNum = i.OrderNum
)
rextester demo: http://rextester.com/SZPX33524
returns:
+----------+-----------+---------+
| OrderNum | OrderLine | Missing |
+----------+-----------+---------+
| 100 | 1 | |
| 100 | 2 | |
| 100 | 3 | Flag |
| 100 | 4 | |
| 101 | 1 | |
+----------+-----------+---------+