具有条件的SQL内部联接

Question

I have following SQL Statment

 Select 
      a.A,
      a.B,
      b.C
 FROM table a 
 INNER JOIN table b
 on (a.A =  b.A)
 WHERE a.A = @A

My problem is that when there is no A in table b matches the A in table a, all the A, BC, will not be showing. It works only when there is a value that meet aA = bA How could I get all A, B, and C even if if is not meet aA = bA I tried to use IF statement which is obviously not working.Thanks.

Answer 1

SELECT
      a.A,
      a.B,
      b.C
 FROM table_a  a
 LEFT JOIN table_b b
 ON (a.A =  b.A)
 WHERE a.A = @A

Of course, you won't get anything from bC if the join condition doesn't match but you will get all rows in table a

If your really want ALL rows in both table_a and table_b, use a FULL OUTER JOIN instead.

Answer 2

Using a LEFT JOIN will get everything from your left table (table_a here) and if there is a matching record (dictated by your JOIN criteria) in the right table (table_b here) then the data from that table will be returned. If There is not a corresponding record in the right table for the record in the left table, then NULL will be returned for that/those column/s you are selecting from the right table.

SELECT
    a.A,
    a.B,
    ISNULL(b.C, 'No Match In table_b')
FROM table_a a
LEFT JOIN table_b b
ON (a.A = b.A)
WHERE a.A = @A