Left Outer Join不返回主表中的所有记录

Question

When I do a left outer join, I expect to get all the records that the query would return prior to adding the joined table, but it is only returning records that match the joined table (ie: no record for '092387' exists in table 'documentation', so I just want null returned for 'filename' field for that record.) What am I doing wrong?

mysql> select documentation_reference.ref_docnumber
            , documentation.filename 
      from documentation_reference 
      left outer join documentation on ref_docnumber=documentation.docnumber      
      where documentation_reference.docnumber='TP-036' 
      and documentation.status!=3;
+---------------+-----------------+
| ref_docnumber | filename        |
+---------------+-----------------+
| SOP-0042      | SOP-0042r39.pdf |
+---------------+-----------------+
1 row in set (0.00 sec)

mysql> select ref_docnumber 
       from documentation_reference 
       where documentation_reference.docnumber='TP-036';
+----------------------+
| ref_docnumber        |
+----------------------+
| 092387               |
| 1100218B             |
| Applicable Item Spec |
| SOP-0042             |
+----------------------+
4 rows in set (0.00 sec)

Answer 1

Your where clause is converting the outer join back into an inner one.

The non matching rows preserved by the outer join will all have NULL values for documentation.status so your documentation.status != 3 condition will filter these back out (The result of the expression NULL !=3 is unknown not true ).

To avoid this issue use

select documentation_reference.ref_docnumber,
       documentation.filename
from   documentation_reference
       left outer join documentation
         on ref_docnumber = documentation.docnumber
            and documentation.status != 3
where  documentation_reference.docnumber = 'TP-036'  

Note that the documentation.status != 3 predicate is moved into the JOIN condition.

Answer 2

Check your documentation.status!=3 condition...it might be the culprit..it is I think eliminating your expected records.

Usually to debug this issue run your query in parts, firstly run-

select documentation_reference.ref_docnumber, documentation.filename from documentation_reference left outer join documentation on ref_docnumber=documentation.docnumber

check the results and then run query with the where -

select documentation_reference.ref_docnumber, documentation.filename from documentation_reference left outer join documentation on ref_docnumber=documentation.docnumber
where  documentation_reference.docnumber = 'TP-036'

Check the results they should change once you add the final where condition - documentation.status!=3

You should probably follow Martin's advice and run his code to get expected results.