I have a table A joined with a table B which give me a result set.
I want to join a table C to the previous ones in order to restrict the result set. But in case there is no result with this join, I would like to have the same result set than before (without taking care of C).
Can you think of way to do that in SQL ?
SELECT *
FROM TableA
INNER JOIN TableB
ON TableA.ID = TableB.TableAID
LEFT JOIN TableC
ON TableC.ID = TableB.TableCID
This will return all rows from Tables A & B but only the rows from TableC where the ON criteria match.
Otherwise conditional joins don't really apply in standard SQL. If you are using SQL Server you can perform some stored procedure logic to check the results from TableC and if there are none then only get data from Table A & B. But this approach with be provider specific
Not possible with regular SQL since it involves logic.
Your best bet is to make a small script, e.g. (in pseudo code)
select * into #tmp from x inner join y inner join z where blabla;
if (exists (select * from #tmp))
BEGIN
select * from #tmp
END
else
BEGIN
select * from x inner join y where blabla;
END
Edit:
But if I were you, I would just always join with C using a LEFT JOIN, so you can see if the result was in one or the other result set...
e.g.
select x.*, y.*, case when z.id is null then 0 else 1 end from x inner join y left join z on blabla where blabla;
But that of course assumes you are able to alter the code path that reads the result.
I see a problem in the LEFT/OUTER JOIN methods. If you do it you could get some results that are in A and B but not in C. If I understand well the porpouse is join AB with C, I mean the result when crossing with C must include the three restrictions. So the #Cine solution is the apropiate to this case.
Related
Lets say I have a query where one field can appear in either Table A or Table B but not both. So to retrieve it I use Coalesce.
Something like
Select
...
Coalesce(A.Number,B.Number) Number
...
From Table A
Left Join Table B on A.C= B.C
Now lets say I want to join another table to that Number field
should I just do
Join Table Z on Z.Z = Coalesce(A.Number,B.Number)
Or is it better to wrap my original table in a query and join on the definite result. So something like
Select * from (
Select
...
Coalesce(A.Number,B.Number) Number
...
From Table A
Left Join Table B on A.C= B.C
) T
left join Table Z on Z.Number= T.Number
Does this make a difference?
if i were joining another table to the result of the first query instead of a sub query i would place the first part in a CTE whenever possible, i believe the performance would be the same as a subquery but CTEs are more readable in my opinion.
with cte1 as
(
Select
...
Coalesce(A.Number,B.Number) Number
...
From Table A
Left Join Table B
on A.C= B.C
)
select *
from cte1 a
Join Table Z
on Z.Z = a.number
I have the following scenario on a SQL Server 2008 R2:
The following queries returns :
select * from TableA where ID = '123'; -- 1 rows
select * from TableB where ID = '123'; -- 5 rows
select * from TableC where ID = '123'; -- 0 rows
When joining these tables the following way, it returns 1 row
SELECT A.ID
FROM TableA A
INNER JOIN ( SELECT DISTINCT ID
FROM TableB ) AS D
ON D.ID = A.ID
INNER JOIN TableC C
ON A.ID = C.ID
ORDER BY A.ID
But, when switching the inner joins order it does not returns any row
SELECT A.ID
FROM TableA A
INNER JOIN TableC C
ON A.ID = C.ID
INNER JOIN ( SELECT DISTINCT ID
FROM TableB ) AS D
ON D.ID = A.ID
ORDER BY A.ID
Can this be possible?
Print Screen:
For inner joins, the order of the join operations does not affect the query (it can affect the ordering of the rows and columns, but the same data is returned).
In this case, the result set is a subset of the Cartesian product of all the tables. The ordering doesn't matter.
The order can and does matter for outer joins.
In your case, one of the tables is empty. So, the Cartesian product is empty and the result set is empty. It is that simple.
As Gordon mentioned, for inner joins the order of joins doesn't matter, whereas it does matter when there's at least one outer join involved; however, in your case, none of this is pertinent as you are inner joining 3 tables, one of which will return zero rows - hence all combinations will result in zero rows.
You cannot reproduce the erratic behavior with the queries as they are shown in this question since they will always return zero records. You can try it again on your end to see what you come up with, and if you do find a difference, please share it with us then.
For the future, whenever you have something like this, creating some dummy data either in the form of insert statements or in rextester or the like, you make it that much easier for someone to help you.
Best of luck.
please help on below query
select * from processed_h where c_type not in (select convert(int,n_index) from index_m where n_index <>'0') **-- 902 rows**
select * from processed_h where c_type not in (2001,2002,2003) **-- 902 rows**
select convert(int,n_index) from index_m where n_index <>'0' **--- 2001,2002,2003**
I tried to convert the not in to LEFT JOIN as below but it is giving me 40,000 rows returned what I am doing wrong
select A.* from processed_h A LEFT JOIN index_m B on A.c_type <> convert(int,B.n_index) and B.n_index <>'0' --40,000 + rows
A LEFT JOIN returns ALL rows from the "left-hand" table regardless of whether the condition matches or not, which is why you are getting the "extra" rows.
An INNER JOIN might give you the same number of rows, but if there are multiple matches in the "right-hand" table then you'll still get more rows than you expect.
If NOT IN gives you the expected results then I'd stick with that. You probably aren;t going to see significant improvements with a join. The only reason I would change to an INNER JOIN is if I needed columns from the joined table in my output.
For the equivalent of a NOT IN using a left join, you need to link the tables as though the results in the linked table should be IN the resultset, then select only those records where the outer joined table did not return a record - like so:
select A.* from processed_h A
LEFT JOIN index_m B on A.c_type = convert(int,B.n_index) and B.n_index <>'0'
WHERE B.n_index IS NULL
However, you might get better performance using a NOT EXISTS query instead:
select A.* from processed_h A
where not exists
(select 1 from index_m B where B.n_index <>'0' and A.c_type = convert(int,B.n_index) )
Table A
Table B
I tried to use LEFT OUTER JOIN but it seems not working..
I want the query to extract all data from Table A with 0 as average score if there is no data yet for the specified parameter. Meaning, in Figure 3, it should have shown ID 2 with 0 on s. Can anyone help me figure out the solution?
You have the table names switched in the join. To keep all of Table A then it needs to be the table listed on the left side of the left join. Also anything that you want to only affect the output of table B, and not filter the entire results, should be moved to the left join on clause. Should be:
SELECT a.id,
Avg(Isnull(b.score, 0)) AS s
FROM a
LEFT OUTER JOIN b
ON a.id = b.id
AND b.kind = 'X'
GROUP BY a.id
I'm trying to do
SELECT * FROM a, b
However, it doesn't return anything if one of the tables is empty. How do I make it so it returns 'a' even if the other one is empty?
Using two tables in the from clause is functionally equivalent to a cross join:
select *
from A
cross join
B
This returns a row of A for every row in B. When B is empty, the result is empty too. You can fix that by using a left join. With a left join, you can return rows even if one of the tables is empty. For example:
select *
from A
left join
B
on 1=1
As the condition 1=1 is always true, this is just like a cross join except it also works for empty tables.
SELECT * FROM a LEFT JOIN b ON a.ID = b.ID
Will return everything from a even if b is empty.
You should do a left join.
Like this
SELECT *
FROM A
LEFT JOIN B ON A.ID = B.ID
Then you receive the rows in A and the respective row in B if exists.
SELECT a.*, b.* FROM a LEFT JOIN b ON a.id = b.id
in this example id is just example name for join key
The query mentioned above display join of both tables if a contain 2 record and b contain 7 records it displays 7*2 = 14 records. In your case one of the table is empty( with 0 records), it will not display any data. If still you want to display data and tables are not having any relationship, you need to check if count of both tables greater that 0. Otherwise display records from only one table which is not empty.