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 ...
Related
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
I have created a new table in hive (T1) with columns c1,c2,c3,c4. I want to populate data into this table by querying from other existing tables(T2,T3).
E.g c1 and c2 come from a query run on T2 & the other columns c3 and c4 come from a query run on T3.
Is this possible in hive ? I have done immense research but still am unable to find a solution to this
Didn't something like this work?
create table T1 as
select t2.c1, t2.c2, t3.c3, t3.c4 from (some query against T2) t2 JOIN (some query against T3) t3
Obviously replace JOIN with whatever is needed. I assume some join between T2 and T3 is possible or else you wouldn't be putting their columns alongside each other in T1.
According to the hive documentation, you can use the following syntax to insert data:
INSERT INTO TABLE tablename1 [PARTITION (partcol1=val1, partcol2=val2 ...)] select_statement1 FROM from_statement;
Be careful that:
Values must be provided for every column in the table. The standard SQL syntax that allows the user to insert values into only some columns is not yet supported. To mimic the standard SQL, nulls can be provided for columns the user does not wish to assign a value to.
So, I would make a JOIN between the two existing table, and then insert only the needed values in the target table playing around with SELECT. Or maybe creating a temporary table would allow you to have more control over the data. Just remember to handle the problem with NULL, as stated in the official documentation. This is just an idea, I guess there are other ways to achieve what you need, but could be a good place to start from.
I wonder which way is faster
SELECT Id FROM T1
INTERSECT
SELECT Id FROM T2
or
SELECT T1.Id
FROM T1
INNER JOIN T2 ON T1.Id=T2.Id
At the moment, SQLite implements INTERSECT by copying the results of the two queries into two temporary sorted tables, and then looking up each Id value of the first table in the second table.
An INNER JOIN is implemented as a nested loop join, i.e., each Id value of one table is looked up in the other table. (SQLite chooses the other table as the one with an index on Id; if neither table has such an index, it creates a temporary index.)
So the pratical difference is that INTERSECT always creates temporary tables, while JOIN can work directly on the actual tables.
(If T1 and T2 were complicated subqueries, JOIN would also need temporary tables, and there would be no difference.)
I have table A and table B with same columns, say we have 100 columns. I want to insert the data in table A to table B, I have below insert statement: INSERT INTO B (column1, column2,....) select column1, column2..., column100 from A.
Is there a way I can insert it without list all columns? Thanks!
Unless table B is pretty much identical to table A, listing all of the fields is the only option.
e.g. if they were exact duplicates, with same fields and types, in the same order, then
INSERT INTO `B` SELECT * FROM `A`
is all you need. But as soon as there's a mismatch between the fields, then you've got to get dirty and list the fields. This is especially true if B's fields are different names than what's in A.
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.