左外连接的SQL where子句

Question

I have a problem with a view I want to create. I have two tables joined in a left outer join, say tableA and tableB , where tableB is left outer joined.

I want to select only those rows from table B where state equals 4, so I add WHERE state = 4 to my query. Now the result set is trimmed quite a bit because all rows without a matching row in tableB are removed from the result (since state isn't 4 for those rows). I also tried WHERE state = 4 OR state IS NULL , doesn't work either (since state technically isn't NULL when there is no state).

So what I need is a WHERE statement which is only evaluated when there actually is a row, does such a thing exist?

If not I see two options: join (SELECT * FROM tableB WHERE state = 4) instead of table B, or create a view with the same WHERE statement and join that instead. What's the best option performance wise?

This is SQL Server 2008 R2 by the way.

Answer 1

You put the conditions in the on clause. Example:

select a.this, b.that
from TableA a
left join TableB b on b.id = a.id and b.State = 4

Answer 2

You can add state = 4 to the join condition.

select * 
from T1
  left outer join T2
    on T1.T1ID = T2.T1ID and
       T2.state = 4

Answer 3

Even easier than a subquery is expanding the on clause, like;

select  *
from    TableA a
left    join
        TableB b
on      a.b_id = b.id
        and b.state = 4

All rows from TableA will appear, and only those from TableB with state 4.

SQL Server will probably execute the view, expanded on , and subquery in exactly the same way. So performance wise, there should be little difference.

Answer 4

Alternative approach: (1) inner join to table B where state equals 4, (2) antijoin to table B to find rows that don't exist, (3) union the results:

SELECT A1.ID, A1.colA, B1.ColB
  FROM tableA AS A1
       INNER JOIN tableB AS B1
          ON A1.ID = B1.ID
             AND B1.state = 4
UNION
SELECT A1.ID, A1.colA, '{{MISSING}}' AS ColB
  FROM tableA AS A1
 WHERE NOT EXISTS (
                   SELECT * 
                     FROM tableB AS B1
                     WHERE A1.ID = B1.ID
                  );

Alternatively:

SELECT A1.ID, A1.colA, B1.ColB
  FROM tableA AS A1
       JOIN tableB AS B1
          ON A1.ID = B1.ID
             AND B1.state = 4
UNION
SELECT ID, colA, '{{NA}}' AS ColB
  FROM tableA 
 WHERE ID IN (
              SELECT ID
                FROM tableA
              EXCEPT 
              SELECT ID
                FROM tableB
             );