This came up at work and this really just seems like a puzzle...
Query 1:
SELECT * FROM
SYSIBM.SYSTABLES A LEFT JOIN
SYSIBM.SYSTABLESPACE B
ON A.DBNAME = B.DBNAME
AND A.TSNAME = B.NAME
AND A.TSNAME LIKE 'HIB%'
;
This query does NOT filter on the HIB% criteria, contrary to what it LOOKS like the query should be doing. I would THINK that putting that criteria in the JOIN should deliver identical results.
Query 2 works as expected:
SELECT * FROM
(SELECT * FROM SYSIBM.SYSTABLES
WHERE TSNAME LIKE 'HIB%') A
LEFT JOIN
SYSIBM.SYSTABLESPACE B
ON A.DBNAME = B.DBNAME
AND A.TSNAME = B.NAME
;
What is wrong with the first query?
TRY
SELECT * FROM SYSIBM.SYSTABLES A
LEFT JOIN SYSIBM.SYSTABLESPACE B
ON A.DBNAME = B.DBNAME AND A.TSNAME = B.NAME
WHERE A.TSNAME LIKE 'HIB%' ;
When you put the condition in the left join that is used to filter the records for the join, but since is a left join it is returning all the records from table A becasue you put no conditions on that.
Now it would have been different if the condition was on table B. The it belongs in the left join (unless it is something like B.id is null) or you will turn the query back into an inner join.
Have you tried to move the "Like"-part to a where clause?
SELECT *
FROM SYSIBM.SYSTABLES A
LEFT JOIN SYSIBM.SYSTABLESPACE B ON A.DBNAME = B.DBNAME AND A.TSNAME = B.NAME
WHERE A.TSNAME LIKE 'HIB%'
;
Related
I'm trying to run this SQL Expression in Access:
Select *
From ((TableA
Left Join TableB
On TableB.FK = TableA.PK)
Left Join TableC
On TableC.FK = TableB.PK)
Left Join (SELECT a,b,c FROM TableD WHERE b > 1) AS TableD
On (TableD.FK = TableC.PK AND TableA.a = TableD.a)
but it keeps getting error: Join-Expression not supported.
Whats the problem?
Sorry, im just starting with Jet-SQL and in T-SQL its all fine.
Thanks
The issue is that the final outer join condition TableA.a = TableD.a will cause the query to contain ambiguous outer joins, since the records to which TableA is joined to TableD will depend upon the results of the joins between TableA->TableB, TableB->TableC and TableC->TableD.
To avoid this, you'll likely need to structure your query with the joins between tables TableA, TableB & TableC existing within a subquery, the result of which is then outer joined to TableD. This unambiguously defines the order in which the joins are evaluated.
For example:
select * from
(
select TableA.a, TableC.PK from
(
TableA left join TableB on TableA.PK = TableB.FK
)
left join TableC on TableB.PK = TableC.FK
) q1
left join
(
select TableD.a, TableD.b, TableD.c, TableD.FK from TableD
where TableD.b > 1
) q2
on q1.a = q2.a and q1.PK = q2.FK
Consider relating every join to the FROM table to avoid having to nest relations.
SELECT *
FROM ((TableA
LEFT JOIN TableB
ON TableB.FK = TableA.PK)
LEFT JOIN TableC
ON TableC.FK = TableA.PK)
LEFT JOIN
(SELECT FK,a,b,c
FROM TableD WHERE b > 1
) AS TableD
ON (TableD.FK = TableA.PK)
AND (TableD.a = TableA.a)
Here is pseudocode of what I am trying to achieve
Select * from TableA
if(TableA.criteria != 1)
inner join TableB.criteria = TableA.criteria
I must do it in view can use sp, functions, etc.
Thanks for any help you will be able to provide
Is this what you want?
Select a.*, b.* from TableA a join TableB b
on b.criteria = case when a.Criteria = 1
then 1 else null end
This may be what you want:
select a.*, b.*
from a left join
b
on b.criteria = a.criteria and b.criteria = 1;
The left join will keep all rows in a, even when the on condition is not true. The extra columns from b will be NULL, unless the criterias are both "1".
Can anyone help me figure out the correct WHERE clause for the following scenario:
select A.name
from tableA A, tableB B
where A.id = B.id
and
(
B.field = 5
OR
B.hasNoRowForJoinedID
)
I feel like I'm missing something really obvious here in how to accomplish this, but I can't for the life of me put my finger on it.
You are using an outdated SQL Syntax. To perform the LEFT OUTER JOIN based your your request above, you can do the following:
SELECT A.name
FROM A
LEFT OUTER JOIN B ON A.id = B.id
WHERE (B.field = 5 OR B.field IS NULL)
Use proper join syntax and not the outdated ones:
select A.name
from tableA A
left join tableB B on A.id = B.id and B.field = 5
If i write a sql:
select *
from a,b
where a.id=b.id(+)
and b.val="test"
and i want all records from a where corresponding record in b does not exist or it exists with val="test", is this the correct query?
You're much better off using the ANSI syntax
SELECT *
FROM a
LEFT OUTER JOIN b ON( a.id = b.id and
b.val = 'test' )
You can do the same thing using Oracle's syntax as well but it gets a bit hinkey
SELECT *
FROM a,
b
WHERE a.id = b.id(+)
AND b.val(+) = 'test'
Note that in both cases, I'm ignoring the c table since you don't specify a join condition. And I'm assuming that you don't really want to join A to B and then generate a Cartesian product with C.
Move the condition into the JOIN clause and use the ANSI standard join pattern.
SELECT NameYourFields,...
FROM A
LEFT OUTER JOIN B
ON A.ID = B.ID
AND B.VAL = 'test'
INNER JOIN C
ON ...
A LEFT OUTER JOIN is one of the JOIN operations that allow you to specify a join clause. It preserves the unmatched rows from the first (left) table, joining them with a NULL row in the shape of the second (right) table.
So you can do as follows :
SELECT
FROM a LEFT OUTER JOIN b
ON a.id = b.id
--Note that you have used double quote "test" which is not used for varchar in SQL you should use single quote 'test'
AND b.val = 'test';
SELECT * FROM abc a, xyz b
WHERE a.id = b.id
AND b.val = 'test'
I have a long stored procedure and I would like to make a slight modification to the procedure without having to create a new one(for maintenance purposes).
Is it possible to use a IF or CASE in the FROM statement of the select statement to join other tables?
Like this:
from tableA a
join tableB b a.indexed = c.indexed
IF #Param='Y'
BEGIN
join tableC c a.indexed = c.indexed
END
It didn't seem to work for me. But I am wondering if this is even possible and/or if this even makes sense to do.
Thanks.
No, it is not possible. You can only accomplish this through the use of dynamic SQL.
The Curse and Blessings of Dynamic SQL
An Intro to Dynamic SQL
I would not advise using Dynamic SQL, there are most likely better ways to perform this operation but you would have to provide more info.
You can achieve something like it if you have a left outer join
Consider
declare #param bit = 1
select a.*, b.*, c.* from a
inner join b on a.id = b.a_id
left outer join c on b.id = c.b_id and #param = 1
This will return all columns from a, b, c.
Now try with
declare #param bit = 0
This will return all columns from a and b, and nulls for columns of c.
It won't work if both joins are inner.
No this is not possible. Your best bet would probably be to select from both tables and only include the data your care about. If you provide an example of what you are trying to do I can provide a better answer.
Attempt at an example:
SELECT t1.id, COALESCE(t2.name, t3.name)
FROM Table1 as t1
LEFT JOIN Table2 as t2
ON t1.id = t2.id
LEFT JOIN Table2 as t3
ON t1.id = t3.id
While what you proposed is not possible, you can play with your where conditions:
from tableA a
inner join tableB b ON a.indexed = c.indexed
left join tableC c ON a.indexed = c.indexed AND 1 = CASE #Param WHEN 'Y' THEN 1 ELSE 0 END
More performant would be to just doing a big
IF #Param='Y' THEN
from tableA a
inner join tableB b ON a.indexed = c.indexed
ELSE
from tableA a
inner join tableB b ON a.indexed = c.indexed
left join tableC c ON a.indexed = c.indexed
You haven't revealed you SELECT clause. The essence of what you want is as follows:
SELECT indexed
FROM tableA
INTERSECT
SELECT indexed
FROM tableB
INTERSECT
SELECT indexed
FROM tableC
WHERE #Param = 'Y'
Then use this table expression as dictated by your SELECT clause e.g. say you only want to project tableA:
WITH T
AS
(
SELECT indexed
FROM tableA
INTERSECT
SELECT indexed
FROM tableB
INTERSECT
SELECT indexed
FROM tableC
WHERE #Param = 'Y'
)
SELECT *
FROM tableA
WHERE indexed IN ( SELECT indexed FROM T );