Flag if there is a JOIN - SQL

Question

Two tables with same field names >> student.id in student table (a) and student.id in record table (b)

If student.id from student table is also in record table (join on a.student.id = b.student.id) then say "Yes" else "No" as "match"

Answer 1

If there are no duplicates, then you can use a left join :

select s.*, (case when r.student_id is null then 'No' else 'Yes' end)
from students s left join
     records r
     on r.student_id = s.id;

If there can be duplicates, then remove the duplicates before joining:

select s.*, (case when r.student_id is null then 'No' else 'Yes' end)
from students s left join
     (select distinct r.student_id
      from records r
     ) r
     on r.student_id = s.id;

Answer 2

Consider a LEFT JOIN which will show all the records from the first table, plus any matching records from the second table. You could use a CASE statement to display whether they match.

SELECT
    A.StudentID,
    B.StudentID,
    CASE WHEN b.StudentID IS NOT NULL THEN 'Yes' ELSE 'NO' END As Match
FROM
    StudentA a
    LEFT JOIN StudentB b on a.StudentID = b.StudentID

Answer 3

you could use a left join and a case when

select  case when b.student_id is null then 'No' else 'Yes' END match
from  student a
left join  record  b on a.student_id = b.student_id

Answer 4

The following is safe from duplicates and probably faster if there are many duplicates.

select *, 
case when exists (select null from other_students b 
                  where b.id = a.id) then 'Yes' else 'No' end student_match
from students a

See the SQL Fiddle in action here.