I have a list of tables in Powerquery after partitioning from an original big table. I am trying to perform a nested join on each of the tables. Each of the 3 tables will have its own nested join, although the tables joined to and the key-containing columns are different for each table. Eg. List contains table1, table2,table3, table1 to be joined to Table A with table1[Host] as the key, table2 to be joined to Table B with table2[Location] as the key, table3 to be joined to Table C with table3[Name] as the key.
I can do the joins on each table separately (as opposed to having a list of tables), and then combine the output, but it gets unwieldy if there are many tables to be joined. Was wondering if there is a "neater" way to do so.
On a related note, is it possible to invoke different functions (1 function for each table in a list of tables), and how?
Cheers
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I know that ROWID is distinct for each row in different tables.But,I am seeing somewhere that two tables are being merged using rowid.So,I also tried to see it,but I am getting the blank output.
I have person table which looks as:
scrowid is the column which contains rowid as:
alter table ot.person
add scrowid VARCHAR2(200) PRIMARY KEY;
I populated this person table as:
insert into ot.person(id,name,age,scrowid)
select id,name, age,a.rowid from ot.per a;
After this I also created another table ot.temp_person by same steps.Both table has same table structure and datatypes.So, i wanted to see them using inner join and I tried them as:
select * from ot.person p inner join ot.temp_person tp ON p.scrowid=tp.scrowid
I got my output as empty table:
Is there is any possible way I can merge two tables using rowid? Or I have forgotten some steps?If there is any way to join these two tables using rowid then suggest me.
Define scrowid as datatype ROWID or UROWID then it may work.
However, in general the ROWID may change at any time unless you lock the record, so it would be a poor key to join your tables.
I think perhaps you misunderstood the merging of two tables via rowid, unless what you actually saw was a Union, Cross Join, or Full Outer Join. Any attempt to match rowid, requardless of you define it, doomed to fail. This results from it being an internal definition. Rowid in not just a data type it is an internal structure (That is an older version of description but Oracle doesn't link documentation versions.) Those fields are basically:
- The data object number of the object
- The data block in the datafile in which the row resides
- The position of the row in the data block (first row is 0)
- The datafile in which the row resides (first file is 1). The file
number is relative to the tablespace.
So while it's possible for different tables to have the same rowid, it would be exteremly unlikely. Thus making an inner join on them always return null.
I want to insert data from two different tables (say table A and table B ) into a third table (table C) in oracle.
I have written two different cursors for fetching data from table A and B separately, and populated two collections based on these two tables.
Now, i want to insert the data in those two collections into the third table (table C), how can i get this done.
Now there are two common columns that are present in both the columns, say for example ID and YEARMONTH, these two columns are there in all tables (A, B and C).
I have tried doing a merge based on these two fields.
but i am looking for an efficient and more convenient way to do this.
You didn't provide code you wrote, so I'll guess: cursors mean PL/SQL. If you're doing it in a loop, row-by-row, it'll be slow-by-slow.
As there are common columns in both tables (A and B), I'd suggest doing it in pure SQL: join those two tables and insert the result into C. Something like
insert into c (id, yearmonth, ...)
select a.id, a.yearmonth, ...
from a join b on a.id = b.id;
Make sure that indexes exist on columns you use to join tables. Or, even better, compare explain plans in both cases (with and without indexes) and choose an option which seems to be the best.
insert into tableC
select * from tableA where ...
union
select * from tableB where ...
I have two tables tbl_customer and tbl_policy and i need to join these two tables. But there is no direct relationship of Pkey and Fkey between these two tables. In this case i know that tbl_customer has a field int_policy and in tbl_policy and its primary key int_policy_pk and these two column can be used to join the table.
But in a generalized way i need to write a function to find this relationship (If i don't know about those fields). Is there any algorithms or other ways to do this?
Let's assume I have two tables of the same row count. Both tables contain a column that allows for 1-1 join between them.
If those tables were turned into one table instead and thus JOIN statement eliminated from the query, would there be any performance benefit of that?
Another example... Let's assume I have table with 10 columns. From that table I created new table but only taking one column. If I issue statement selecting that one column with WHERE predicate on the same column would there be any performance difference in executing this query on both tables?
What I'm trying to get to is if performance is the same in above cases is it safe to say tables are only containers wrapping number of columns together?
I did run couple tests but with non conclusive results.
Let's assume I have two tables of the same row count. Both tables
contain a column that allows for 1-1 join between them. If those
tables were turned into one table instead and thus JOIN statement
eliminated from the query, would there be any performance benefit of
that?
Performing that join for every query is of course more expensive than materializing the table once and then reading it. So yes, there would be a performance benefit.
Another example... Let's assume I have table with 10 columns. From
that table I created new table but only taking one column. If I issue
statement selecting that one column with WHERE predicate on the same
column would there be any performance difference in executing this
query on both tables?
No, there would be no difference, since tables are represented as collections of columns, which are each stored in their own file.
What I'm trying to get to is if performance is the same in above cases
is it safe to say tables are only containers wrapping number of
columns together?
That is indeed safe to say.
I am on an oracle DB. Lets say I have one view that joins to three tables. The view has two fields each. Each field only needs data from two of the three tables.
If I query the view and return only one field, does the view still join to three tables or just to the two tables that it needs to calculate the field?
Generally it will have to hit the three tables.
Consider
SELECT A.VAL, B.VAL, C.VAL FROM A JOIN B ON A.ID = B.ID JOIN C ON A.ID = C.ID
It is possible that a single ID in "A" to have zero, 1 or multiple matches in either B or C. If table "C" were empty, the view would never return a row, so even just querying A.VAL or B.VAL, it would still need to see if there was a corresponding row in "C".
The exception is when, because of an enforced referential integrity constraint, the optimizer knows that a row in 'B' will always have a parent row in 'A'. In that case, a select of B.VAL would not need to actually check the existence of the parent row in 'A'. This is demonstrated by this article
That likely depends on the type of join being used. If they are all inner joins, it will definitely need to examine all three tables.
In general, the database engine would join all three tables to ensure it got the right answer.
Oracle will sometimes eleminate one of the tables where this does not change the result.
This can be done if:-
There is a foreign key constraint to the table to be eleminated (i.e. a row in the table
can be guaranteed to be found)
The table is otherwise unused. i.e. not selected from, in the where clause, etc.