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.
Related
I am new to Hive and have some problems. I try to find a answer here and other sites but with no luck... I also tried many different querys that come to my mind, also without success.
I have my source table and i want to create new table like this.
Were:
id would be number of distinct counties as auto increment numbers and primary key
counties as distinct names of counties (from source table)
You could follow this approach.
A CTAS(Create Table As Select)
with your example this CTAS could work
CREATE TABLE t_county
ROW FORMAT DELIMITED FIELDS TERMINATED BY ','
STORED AS TEXTFILE AS
WITH t AS(
SELECT DISTINCT county, ROW_NUMBER() OVER() AS id
FROM counties)
SELECT id, county
FROM t;
You cannot have primary key or foreign keys on Hive as you have primary key on RBDMSs like Oracle or MySql because Hive is schema on read instead of schema on write like Oracle so you cannot implement constraints of any kind on Hive.
I can not give you the exact answer because of it suppose to you must try to do it by yourself and then if you have a problem or a doubt come here and tell us. But, what i can tell you is that you can use the insertstatement to create a new table using data from another table, I.E:
create table CARS (name string);
insert table CARS select x, y from TABLE_2;
You can also use the overwrite statement if you desire to delete all the existing data that you have inside that table (CARS).
So, the operation will be
CREATE TABLE ==> INSERT OPERATION (OVERWRITE?) + QUERY OPERATION
Hive is not an RDBMS database, so there is no concept of primary key or foreign key.
But you can add auto increment column in Hive. Please try as:
Create table new_table as
select reflect("java.util.UUID", "randomUUID") id, countries from my_source_table;
As per my design I want to fetch rowid as in
select rowid r from table_name;
into a C variable. I was wondering what is the max size / length in characters of the rowid.
Currently in one of the biggest tables in my DB we have the max length as 18 and its 18 throughout the table for rowid.
Thanks in advance.
Edit:
Currently the below block of code is iterated and used for multiple tables hence in-order to make the code flexible without introducing the need of defining every table's PK in the query we use ROWID.
select rowid from table_name ... where ....;
delete from table_name where rowid = selectedrowid;
I think as the rowid is picked and used then and there without storing it for future, it is safe to use in this particular scenario.
Please refer to below answer:
Is it safe to use ROWID to locate a Row/Record in Oracle?
I'd say no. This could be safe if for instance the application stores ROWID temporarily(say generating a list of select-able items, each identified with ROWID, but the list is routinely regenerated and not stored). But if ROWID is used in any persistent way it's not safe.
A physical ROWID has a fixed size in a given Oracle version, it does not depend on the number of rows in a table. It consists of the number of the datafile, the number of the block within this file, and the number of the row within this block. Therefore it is unique in the whole database and allows direct access to the block and row without any further lookup.
As things in the IT world continue to grow, it is safe to assume that the format will change in future.
Besides volume there are also structural changes, like the advent of transportable tablespaces, which made it necessary to store the object number (= internal number of the table/partition/subpartion) inside the ROWID.
Or the advent of Index organized tables (mentioned by #ibre5041), which look like a table, but are in reality just an index without such a physical address (because things are moving constantly in an index). This made it necessary to introduce UROWIDs which can store physical and index-based ROWIDs.
Please be aware that a ROWID can change, for instance if the row moves from one table partition to another one, or if the table is defragmented to fill the holes left by many DELETEs.
According documentation ROWID has a length of 10 Byte:
Rowids of Row Pieces
A rowid is effectively a 10-byte physical address of a row.
Every row in a heap-organized table has a rowid unique to this table
that corresponds to the physical address of a row piece. For table
clusters, rows in different tables that are in the same data block can
have the same rowid.
Oracle also documents the (current) format see, Rowid Format
In general you could use the ROWID in your application, provided the affected rows are locked!
Thus your statement may look like this:
CURSOR ... IS
select rowid from table_name ... where .... FOR UPDATE;
delete from table_name where rowid = selectedrowid;
see SELECT FOR UPDATE and FOR UPDATE Cursors
Oracle even provides a shortcut. Instead of where rowid = selectedrowid you can use WHERE CURRENT OF ...
Just want to know is this possible.
Say that if i have value 'X' and iam sure that this is referenced in some other tables as PK value but not sure about exactly which table is that, so i would like to know the list of those tables.
Pseudo query of above what i mentioned
SELECT TABLE_NAME FROM DBA_TABLES WHERE <<ATLEAST ONE OF THE TABLE ROW PK VALUE IS MATCHING EQUAL TO 'X'>>;
I wonder is it safe to use rowid for row matching?
I have following query:
select * from a,
(select a.rowid rid, <some_columns_omitted> from a, b, c where a.some_column = b.some_column ... <joining_omitted>
union all
select a.rowid rid, <some_columns_omitted> from a, d, e where a.some_column = d.some_column ... <joining_omitted>
union all ....) sub_query
where a.rowid = sub_query.rid
Will using rowid for row matching be as safe as using primary key?
See this related question:
Oracle ROWID as function/procedure parameter
Oracle guarantees that, as long as the row exists, its rowid does not change. Rowid will change only in very special occasions (table rebuild, partition table with row movement enabled, index-organized table with update to the pk). On heap tables, an update will not cause the rowid to change, even if the row is migrated (because it doesn't fit in the block anymore).
In any cases the rowid is part of the metadata of a row and will be kept consistent for the duration of a query, with the same consistency mechanism that keeps column data consistent (multiversion read consistency...).
Furthermore, it is safe to use rowid accross queries if you lock the row for update (same as primary key). Accessing rows by rowid is also faster than a primary key lookup (since a primary key lookup is an index scan + a rowid access).
I believe it is OK to use the rowid, but I do not like that. You have a primary key for that purpose, please use that. I believe Oracle currently guarantees that rowid will not change during query run, but this is a bad practice. For instance if it works perfectly who does guarantee that this will work perfectly on a newer Oracle version when you migrate the database?
If you consider that under the hood Oracle itself uses ROWIDs to process a query (think "TABLE ACCESS BY ROWID" in an execution plan) you better believe that ROWIDs will be reliable for the duration of a query. (I'm also going by the premise that readers don't block writers so Oracle wouldn't be doing any special locking as it's processing records.)
If it was a case of recording ROWIDs for use in a subsequent SQL statement then I'd be a little wary, but for a self-contained query, I'd say you'll be ok.
I have two tables like bellow shows figures
I need to select records as bellow shown figure. with AH_ID need to join in second table and ATT_ID will be the column header and ATT_DTL_STR_VALUE need to get as that column relevant value
Required output
Sounds like you have an Entity-Attribute-Value data model which relational DBs aren't the best at modeling. You may want to look into a key-value store.
However, as Justin suggested, if you're using 11g you can use th pivot clause as follows:
SELECT *
FROM (
SELECT T1.AH_ID, T1.AH_DESCRIPTION, T2.ATT_ID, T2.ATT_DTL_STR_VALUE
FROM T1
LEFT OUTER JOIN T2 ON T1.AH_ID = T2.AH_ID
)
PIVOT (MAX(ATT_DTL_STR_VALUE) FOR (ATT_ID) IN (1));
This statement requires you to hard-code in ATT_ID however there are ways to do it dynamically. More info can be found here.