"Martin" <co*****@hotmail.com> wrote in message
news:ac**************************@posting.google.c om...
I am now working on SQL Server 2000 having had previous experience on
a different database. Both of the OUTER JOIN syntaxes is different
from what I am used to and I am finding it slightly confusing.
For example, given two tables :
wipm_tbl_mi
wipm_tbl_wi (which may not have data in it for a specific record that
exists in the first table.)
If I use the old style syntax :
SELECT mi.workitemid, wi.datavalue
FROM wipm_tbl_mi mi , wipm_tbl_wi wi
WHERE mi.workitemid *= wi.workitemid
AND mi.workitemid = 1
AND wi.dataname = 'XXX'
I get back
1,NULL
when there is no matching record in wipm_tbl_wi, which is what I
expect.
However, if I try to use the SQL-92 syntax
SELECT mi.workitemid, wi.datavalue
FROM wipm_tbl_mi mi
LEFT OUTER JOIN wipm_tbl_wi wi
ON mi.workitemid = wi.workitemid
WHERE mi.workitemid = 1
AND wi.dataname = 'XXX'
I don't get anything back. Please can someone help me understand what
is wrong with the bottom query.
Thank you,
Martin
*= is the obsolete proprietary syntax used for outer joins by SQL Server
prior to version 6.5. OUTER JOIN is the Standard SQL way of doing
outer joins and should always be used. However, it's not simply syntactically
different but semantically different than *=. With outer joins, the join condition
must be evaluated separate from the condition applied to the rows resulting
from the join. The Standard SQL solution allows for this by being able to
specifiy the join condition for the outer join in the FROM clause and
the condition to be applied to the resulting rows of the outer join in the
WHERE clause. The *= syntax at best doesn't make this clear and at
worst won't allow you to specify the result you're looking for since both
kinds of conditions, for the join and the restriction of subsequent rows,
are specified in the WHERE clause.
It's always helpful to provide sample data in the form of INSERT
statements so a proposed solution can be tested, but try
SELECT mi.workitemid, wi.datavalue
FROM wipm_tbl_mi mi
LEFT OUTER JOIN
wipm_tbl_wi wi
ON mi.workitemid = wi.workitemid AND
wi.dataname = 'XXX'
WHERE mi.workitemid = 1
From the LEFT OUTER JOIN condition, every row of mi will be joined to
all rows in wi where mi.workitemid = wi.workitemid and wi.dataname = 'XXX'.
All rows of mi that don't satisfy this condition are joined to a single row
of all NULL values to indicate that there's no matching row from wi.
From the resulting outer join, the WHERE clause only keeps those rows
where mi.workitemid = 1. This will keep rows where wi.dataname is 'XXX'
or NULL, which is your desired result.
Regards,
jag
