LEFT JOIN not exactly working

Question

Hi all,
Hoping you can help me with my SQL query. I want to do a
simple LEFT JOIN between two tables...fair enough right? Yeah,
well, it works and all rows are returned from the LEFT table even
if there are nmo matching rows in the RIGHT table.
BUT, when I apply a few conditions using WHERE after this
LEFT JOIN the whole point of the LEFT JOIN seems to not work. I
only get rows from the LEFT where they match in the RIGHT.
For example...this following code works and returns all LEFT
rows despite there not being a match in the RIGHT.
<cfquery name="getSchemaFields" datasource="#request.dsn#"
username="#request.username#" password="#request.password#">
SELECT schema_#getSchemas.schema_token#_fields.*,
fields_content.*
FROM schema_#getSchemas.schema_token#_fields
LEFT JOIN fields_content
ON schema_#getSchemas.schema_token#_fields.field_type_uuid =
fields_content.field_content_field_uuid
</cfquery>
But, what I really need is to apply some conditions on the
rows I want from the right table based on an ID like as follows...
<cfquery name="getSchemaFields" datasource="#request.dsn#"
username="#request.username#" password="#request.password#">
SELECT schema_#getSchemas.schema_token#_fields.*,
fields_content.*
FROM schema_#getSchemas.schema_token#_fields
LEFT JOIN fields_content
ON schema_#getSchemas.schema_token#_fields.field_type_uuid =
fields_content.field_content_field_uuid
WHERE fields_content.field_content_item_id = <cfqueryparam
cfsqltype="cf_sql_integer" value="#getSchemas.item_id#" />
OR fields_content.field_content_item_id = NULL
AND fields_content.field_content_item_uuid = <cfqueryparam
cfsqltype="cf_sql_varchar" value="#getSchemas.item_uuid#" />
OR fields_content.field_content_item_uuid = NULL
</cfquery>
Now I don't get any errors but what I do get is rows from the
left ONLY when they match rows in the RIGHT. I still need all the
rows from the LEFT despite this. The WHERE conditions seems to stop
this happening.
I know it's probably blindingly simple but I just can't get
my head around it and I'm pulling my hair out about it!
Hope somebody can assist me in this.
Many thanks in advance!!
Mikey.

Answer 1

> Now I don't get any errors but what I do get is rows
from the left ONLY when
> they match rows in the RIGHT. I still need all the rows
from the LEFT despite
> this. The WHERE conditions seems to stop this happening.
You need to stop to think about how the query is being
executed.
FIRST the FROM recordset is created which is the result of
the join
statement. This will have all your "left" rows, and null-data
for
unmatched "right" rows.
THEN the WHERE clause is executed, which will filter out any
rows from the
recordset create in the FROM recordset.
So if yuor data is this:
TBL_FRUIT
FRT_ID, FRT_NAME
1, Apple
2, Banana
3, Cherry
TBL_STOCK (<sung>we have no bananas, we have no bananas
today</sung>)
STK_ID, FRT_ID, STK_LEVEL
1, 1, 12
2, 3, 100
And if you FROM clause is this:
from TBL_FRUIT F left outer join TBL_STOCK S on F.FRT_ID =
S.FRT_ID
You get a record set thus:
1, Apple, 12
2, Banana, null
3, Cherry, 100
Now if you have a WHERE filter, thus:
WHERE S.STK_LVEL > 0
You're going tobe filtering out the banana row, because it
DOESN'T have a
STK_LEVEL > 0
1, Apple, 12
3, Cherry, 100
You need to get all your filtering done on the STOCK table
*before*
left-joining it to the FRUIT table, if you want to have all
the fruit rows
in the final result.
Dan's detailed how to effect this.
Make sense?
Adam