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
Related
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.
I have read a number of posts from SO and I understand the differences between filtering in the where clause and on clause. But most of those examples are filtering on the RIGHT table (when using left join). If I have a query such as below:
select * from tableA A left join tableB B on A.ID = B.ID and A.ID = 20
The return values are not what I expected. I would have thought it first filters the left table and fetches only rows with ID = 20 and then do a left join with tableB.
Of course, this should be technically the same as doing:
select * from tableA A left join table B on A.ID = B.ID where A.ID = 20
But I thought the performance would be better if you could filter the table before doing a join. Can someone enlighten me on how this SQL is processed and help me understand this thoroughly.
A left join follows a simple rule. It keeps all the rows in the first table. The values of columns depend on the on clause. If there is no match, then the corresponding table's columns are NULL -- whether the first or second table.
So, for this query:
select *
from tableA A left join
tableB B
on A.ID = B.ID and A.ID = 20;
All the rows in A are in the result set, regardless of whether or not there is a match. When the id is not 20, then the rows and columns are still taken from A. However, the condition is false so the columns in B are NULL. This is a simple rule. It does not depend on whether the conditions are on the first table or the second table.
For this query:
select *
from tableA A left join
tableB B
on A.ID = B.ID
where A.ID = 20;
The from clause keeps all the rows in A. But then the where clause has its effect. And it filters the rows so on only id 20s are in the result set.
When using a left join:
Filter conditions on the first table go in the where clause.
Filter conditions on subsequent tables go in the on clause.
Where you have from tablea, you could put a subquery like from (select x.* from tablea X where x.value=20) TA
Then refer to TA like you did tablea previously.
Likely the query optimizer would do this for you.
Oracle should have a way to show the query plan. Put "Explain plan" before the sql statement. Look at the plan both ways and see what it does.
In your first SQL statement, A.ID=20 is not being joined to anything technically. Joins are used to connect two separate tables together, with the ON statement joining columns by associating them as keys.
WHERE statements allow the filtering of data by reducing the number of rows returned only where that value can be found under that particular column.
I'm a complete novice teaching myself SQL by writing and modifying a few queries and reports at work.
I've got something of a handle on the various types of JOINs and I've used INNER JOIN a few times with decent success.
What I'm stuck on should be a simple task, but my Google-Fu must be weak. Here's what I'm trying to do.
Say I have 3 tables, Table_A, Table_B, and Table_C, and each table has a column called [Serial_Number].
What I'm wanting to select is 3 of the other columns if A.Serial_Number = B.Serial_Number OR C.Serial_Number.
I've tried doing:
SELECT
*
FROM
Table_A AS A
INNER JOIN Table_B AS B ON A.Serial_Number = B.Serial_Number
INNER JOIN Table_C AS C ON A.Serial_Number = C.Serial_Number
But this always yields 0 results as the nature of the data dictates that if A matches B, it will never match C and vice versa. I also tried a LEFT OUTER JOIN as the second clause, but this just includes NULLs from Table_C that have already matched on Table_B.
All the searches I have done relating to JOINs on multiple tables seem to be about using JOINS to further exclude records, where I'm actually wanting to INCLUDE more records.
Like I said, I'm sure this is really simple, just needing a nudge in right direction.
Thanks!
The use of two inner joins here is akin to saying
If A.Serial_Number = B.Serial_Number AND
A.Serial_Number = C.Serial_Number
Using left outer join on the second clause - by which i presume you mean second join - would perform a left join on a result set already filtered by A.Serial_Number = B.Serial_Number by the first inner join. Given that B.Serial_Number doesn't relate to C.Serial_Number you wouldn't expect the an equijoin to return any result from tablec.
What you want is a left outer join like you tried but for both tableb and tablec.
Select *
From tablea
Left join tableb on tableb.Serial_Number = tablea.Serial_Number
Left join tablec on tablec.Serial_Number = tablea.Serial_Number
This way regardless of whether tablea.Serial_Number is in tableb it will still be returned and thus available to be joined to tablec
Agreed. Your output for your inner joins is producing NULLs which is why it is resulting in 0. I would suggest modifying your INNER JOIN.
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.
I was wondering, is there a way to make a kind of one to one left outer join:
I need a join that matches say table A with table B, for each record on table A it must search for its pair on table B, but there exists only 1 record that matches that condition, so when it has found its pair on B, it must stop and continue with the next row at table A.
What I have is a simple LEFT OUTER JOIN.
select * from A left outer join B on A.ID = B.ID order by (NAME) asc
Thanks in advance!
SQL doesn't work this way. In the first place it does not look at things row-by-row. In the second place what defines the record you want to match on?
Assuming you don't really care which row is selcted, something like this might work:
SELECT *
From tableA
left outer join
(select b.* from tableb b1
join (Select min(Id) from tableb group by id) b2 on b1.id - b2.id) b
on a.id = b.id
BUt it still is pretty iffy that you wil get the records you want when there are multiple records with the id in table b.
The syntax you present in your question is correct. There is no difference in the query for joining on a one-to-one relationship than on a one-to-many.