如何从一张表中选择关系表中不存在的所有记录？

Question

ObjectClass1

OBID | Name

0001 | ob01
0002 | ob02

Relation1

RELID|LEFTOBID|RIGHTOBID|CLASS1|CLASS2

R001 |0001    |000X     |ObjectClass1|ObjectClass2

Now What I want to find out is how to write a query to find out that relation does not exists for OBID 0002

Answer 1

Use not exists as :

select *
  from ObjectClass1 o
 where not exists
   ( select 1 from Relation1 r where r.LEFTOBID = o.OBID );

Rextester Demo

Answer 2

SQL
SELECT  *
FROM    ObjectClass1    obj
LEFT JOIN
        Relation1       rel
  on    obj.OBID = rel.LEFTOBID
WHERE   rel.LEFTOBID is null