I have to re-write a SQL statement with a subquery so that it has a join for my job. So far, this is what I have.
SELECT * FROM Table_A
WHERE TABLE_A.A_ID NOT IN
(SELECT LK.A_ID FROM Link_Table LK
LEFT JOIN Table_B B
ON B.B_ID = LK.B_ID)
I am really having a hard time with this. I feel like this is because of the link tables though. Can anyone give me advice on altering this query?
Seems like you want a LEFT JOin with a IS NULL in the where:
SELECT {Column list} --Don't use *
FROM dbo.Table_A A
LEFT JOIN dbo.Link_Table LK ON A.A_ID = LK.A_ID
WHERE LK.A_ID IS NULL;
You don't need the reference to Table_B at all here.
Personally, however, I would prefer an EXISTS, but that is a subquery again:
SELECT {Column List}
FROM dbo.Table_A A
WHERE NOT EXISTS (SELECT 1
FROM dbo.Link_Table LK
WHERE A.A_ID = LK.A_ID);
Related
I'd like to implement an antijoin on two table but using two keys so that the result is all rows in Table A that do not contain the combinations of [key_1, key_2] found in Table B. How can I write this query in SQL?
If you want an anti-left join, the logic is:
select a.*
from tablea a
left join tableb b on b.key_1 = a.key_1 and b.key_2 = a.key_2
where b.key_1 is null
As for me, I like to implement such logic with not exists, because I find that it is more expressive about the intent:
select a.*
from tablea a
where not exists (
select 1 from tableb b where b.key_1 = a.key_1 and b.key_2 = a.key_2
)
The not exists query would take advantage of an index on tableb(key_1, key_2).
select a.*
from table_a a
left anti join table_b b on a.key_1 = b.key_1 and a.key_2 = b.key_2;
I am trying to left join to tables using a query like this
SELECT * FROM table1 a, table2 b WHERE (a.ID = b.ID OR b.ID IS NULL)
In Oracle, this is equivalent to a LEFT JOIN (and in other databases as well, afaik).
Doing the same thing in DB2 (z/OS) produces an inner join - the b.ID IS NULL clause has no effect on the result, removing it does not change anything.
Is there a way to make this work in DB2? Is this something that should work according to ANSI SQL?
PS: I am aware that I can use the JOIN syntax, I'm just interested in why this doesn't work and if there is a way around this.
You can use
SELECT a.*, b.*
FROM tbl1 a LEFT JOIN tbl2 b ON a.id=b.id;
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 );
Why won't this work in Oracle?
Is there a way to make this work?
FROM table1 a,
table2 b,
table3 c
WHERE a.some_id = '10'
AND a.other_id (+)= b.other_id
AND a.other_id (+)= c.other_id
I want table1 to be left outer joined on multiple tables...
If I try to change it to use an ANSI join, I get compilation errors. I did the following:
FROM table2 b, table3 c
LEFT JOIN table1 a ON a.other_id = b.other_id and a.other_id = c.other_id
OK, looking at the examples from the Oracle docs, my recollection of the syntax was correct, so I'm turning my comment into an answer. Assuming that your goal is a left outer join where A is the base table, and you join matching rows from B and C, rewrite your query as follows (note that I'm just changing the prefixes; I like to have the source rowset on the right).
FROM table1 a,
table2 b,
table3 c
WHERE a.some_id = '10'
AND b.other_id (+)= a.other_id
AND c.other_id (+)= a.other_id
If that's not what you're trying to do, then the query is borked: you're doing a cartesian join of B and C, and then attempting an outer join from that partial result to A, with an additional predicate on A. Which doesn't make a lot of sense.
use ansi joins. They are way clearer IMO. BUt for some reason they don't work with materialized views...
You can do something like this.
FROM table1 a, table2 b, table3 c
WHERE a.some_id = '10'
AND a.other_id = b.other_id(+)
AND a.other_id = c.other_id(+)
I wanted to address separately this part of your question:
If I try to change it to ANSI join I get compilation errors. I did the following:
FROM table2 b, table3 c
LEFT JOIN table1 a ON a.other_id = b.other_id and a.other_id = c.other_id
In an ANSI join, at least in Oracle, you are operating on exactly two row sources. The LEFT JOIN operator in your example has table3 and table1 as its operands; so you cannot reference "b.otherid" in the ON clause. You need a new join operator for each additional table.
I believe what you are trying to do is outer join table 2 and table 3 to table 1. So what you should be doing is this:
FROM table1 a LEFT JOIN table2 b ON b.other_id = a.other_id
LEFT JOIN table3 c ON c.other_id = a.other_id
or Henry Gao's query if you want to use Oracle-specific syntax.
In oracle you cannot outer join the same table to more than one other table. You can create views that have joins in them, then join to that view. As side note, you also cannot outer join to an sub select, so that is not an option here either.
You could off course try the following (Table b and c being the BASE)
FROM (SELECT other_id FROM table2
UNION
SELECT other_id FROM table3) b
LEFT JOIN table1 a b.other_id = a.other_id
But then again I am an Oracle Nono
I have two tables that are joined together.
A has many B
Normally you would do:
select * from a,b where b.a_id = a.id
To get all of the records from a that has a record in b.
How do I get just the records in a that does not have anything in b?
select * from a where id not in (select a_id from b)
Or like some other people on this thread says:
select a.* from a
left outer join b on a.id = b.a_id
where b.a_id is null
select * from a
left outer join b on a.id = b.a_id
where b.a_id is null
The following image will help to understand SQL LET JOIN :
Another approach:
select * from a where not exists (select * from b where b.a_id = a.id)
The "exists" approach is useful if there is some other "where" clause you need to attach to the inner query.
SELECT id FROM a
EXCEPT
SELECT a_id FROM b;
You will probably get a lot better performance (than using 'not in') if you use an outer join:
select * from a left outer join b on a.id = b.a_id where b.a_id is null;
SELECT <columnns>
FROM a WHERE id NOT IN (SELECT a_id FROM b)
In case of one join it is pretty fast, but when we are removing records from database which has about 50 milions records and 4 and more joins due to foreign keys, it takes a few minutes to do it.
Much faster to use WHERE NOT IN condition like this:
select a.* from a
where a.id NOT IN(SELECT DISTINCT a_id FROM b where a_id IS NOT NULL)
//And for more joins
AND a.id NOT IN(SELECT DISTINCT a_id FROM c where a_id IS NOT NULL)
I can also recommended this approach for deleting in case we don't have configured cascade delete.
This query takes only a few seconds.
The first approach is
select a.* from a where a.id not in (select b.ida from b)
the second approach is
select a.*
from a left outer join b on a.id = b.ida
where b.ida is null
The first approach is very expensive. The second approach is better.
With PostgreSql 9.4, I did the "explain query" function and the first query as a cost of cost=0.00..1982043603.32.
Instead the join query as a cost of cost=45946.77..45946.78
For example, I search for all products that are not compatible with no vehicles. I've 100k products and more than 1m compatibilities.
select count(*) from product a left outer join compatible c on a.id=c.idprod where c.idprod is null
The join query spent about 5 seconds, instead the subquery version has never ended after 3 minutes.
Another way of writing it
select a.*
from a
left outer join b
on a.id = b.id
where b.id is null
Ouch, beaten by Nathan :)
This will protect you from nulls in the IN clause, which can cause unexpected behavior.
select * from a where id not in (select [a id] from b where [a id] is not null)