I am doing a select with right join. And I have some rows with IDS from table A but not all rows have corresponding IDs in Table B.
I wish to replace null IDS from table B with IDS of table A based on other row.
That alternative row has no null values. How can it be done?
SQL has a command for that...
ISNULL(TableB.Id, TableA.Id) AS SomeId
In the event of TableB.Id being NULL, you'll get TableA.Id as SomeId.
THIS ADDRESSES THE ORIGINAL VERSION OF THE QUESTION.
First, I strongly recommend left join over right join. It is easier to read a query when the logic is "keep all the rows in the first table (and I know what that is)".
You can use coalesce(). In a select query, you would do:
select coalesce(b.id, a.id)
from a left join
b
on a.anothercol = b.anothercol;
However, I suspect you intend:
update b
set id = (select a.id from a where a.anothercol = b.anothercol)
where id is null;
Related
I face issue about duplicate data when join table, here my sample data table I have
-- Table A
I want to join with
-- Table B
this my query notation for join both table,
select a.trans_id, name
from tableA a
inner join tableB b
on a.ID_Trans = b.trans_id
and this the result, why I get the duplicating data which should show only two lines of data, please help me to solve this case.
Firstly, as you have been told multiple times in the comments, this is working exactly as you have written, and (more importantly) as intended. You have 2 rows in tableA and those 2 rows match 2 rows in your table tableB according to the ON clause. This means that each join operation, for the each of the rows in tableA, results in 2 rows as well; thus 4 rows (2 * 2 = 4).
Considering that your table, TableA only has one column then it seems that you should be cleaning up that data and deleting the duplicates. There are plenty of examples on how to do that already (example).
Perhaps the column you show us in TableA is one many, and thus instead you have a denormalisation issue, and instead there should be another table with the details of Id_trans and a PRIMARY KEY or UNIQUE CONSTRAINT/INDEX on it. Then you would join fron that table to TableB.
Finally, what you might be after is an EXISTS, which would look like this:
SELECT B.trans_id, B.[name]
FROM dbo.TableB B
WHERE EXISTS(SELECT 1
FROM dbo.TableA A
WHERE A.ID_Trans = B.trans_id); --Odd that it's called ID_Trans in one table, and Trans_ID in another
As the comments mentioned your query does exactly what you asked it to do but I think you wanted something like:
select a.trans_id, a.name, b.name
from tableA a
inner join tableB b on a.trans_id = b.trans_id
group by a.trans_id, a.name, b.name
Since there are two rows in both table with same ID join will make them four. You can use distinct to remove duplicates:
select distinct a.trans_id, name
from tableA a
inner join tableB b
on a.id_trans = b.trans_id
But I would suggest to use exists:
select trans_id, name
from tableB b
exists (select 1 from tableA a where a.trans_id=b.trans_id)
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 have table A, which contains a row for an int representing the table B object they relate to.
Multiple As can reference the same B. B does not reference A
I want to return As ordered by a row in the B object they relate to.
Is there a way to do this in one SQL statement? Or even 2?
Thank you.
You can put anything in your SELECT list and ORDER BY any column you'd like as long as it's in tablea or tableb
SELECT a.ID
FROM tablea
INNER JOIN tableb ON tablea.ID = tableb.ID
ORDER BY tableb.ID
Have you tried using
Select (columns that you want to display)
from TableA INNER JOIN TableB
ON TableA.col = TableB.col
Order By TableB.ColumnName
Here is my obstacle.
I have two tables. Table A contains more rows than Table B. I have to merge the results and if Table A does not contain a row from Table B then I insert it into the new set. If however, a row from Table A contains a row with the same primary key as Table B, the new set will take the row from Table B.
Would this best be done in a cursor or is there an easier way to do this? I ask because there are 20 million rows and while I am new to sql, i've heard cursors are expensive.
Your phrasing is a little vague. It seems that you want everything from TableB and then rows from TableA that have no matching primary key in B. The following query solves this problem:
select *
from tableB union all
select *
from tableA
where tableA.pk not in (select pk from tableB)
Yep, cursors are expensive.
There's a MERGE command in later versions of SQL that will do this in one shot, but it's sooo cumbersome. Better to do it in two pieces - first:
UPDATE A SET
field1 = B.field1
,field2 = B.field2
, etc
FROM A JOIN B on B.id = A.id
Then:
INSERT A SELECT * FROM B --enumerate fields if different
WHERE B.id not in (select id FROM A)
An OUTER JOIN should do what you need and be more efficient than a cursor.
Try this query
--first get the rows that match between TableA and TableB
INSERT INTO [new set]
SELECT TableB.* --or columns of your choice
FROM TableA LEFT JOIN TableB ON [matching key criteria]
WHERE TableB.[joining column/PK] IS NOT NULL
--then get the rows from TableA that don't have a match
INSERT INTO [new set]
SELECT TableA.* --you didn't say what was inserted if there was no matching row
FROM TableA LEFT JOIN TableB ON [matching key criteria]
WHERE TableB.[joining column/PK] IS NULL
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.