Is there a way to add a column alias to an existing table, which just references another existing column in the table? such that reads and writes to the new column name will go to the existing column name. Sort of how a view in postgres can act as a read / write alias:
create view temp_order_contacts as (select * from order_emails)
This will make read / write possible to order_emails table but by calling temp_order_contacts instead.
Is there something similar but for columns?
Assuming this is for backwards compatibility; you want to rename a column, but you also want to existing queries to still work.
You can rename the table and create a view with the original name.
-- Move the existing table out of the way.
alter table some_table rename to _some_table;
-- Create a view in its place.
create view some_table as (
select
*,
-- provide a column alias
some_column as some_other_column
from _some_table
);
Related
It often happens that the original name something is given is not the best name. Maybe requirements shifted slightly, or maybe as time went on, a better understanding of the concept being represented developed. Sometimes one name is used in developing a feature, but testing with real users reveals that a better name is needed, and it'd be nice to have the names for things in the DB match the names used in the UI.
PostgreSQL lets you rename tables and columns via alter table, but often this isn't feasible in a production without significant downtime. If existing clients are using the old name, you can't just yank it out from under them.
I was hoping there was some way to "add a name" to a table or column, so that old_name and new_name will both work, and then at a later time remove the old name. Then a name migration could work like this:
add new_name
modify all clients to use new_name instead of old_name
once all clients have been updated, remove old_name
Is there a way to do this? If not, is there a recommended procedure for renaming a column while minimizing downtime? How about a table?
Some recipes for renaming tables and/or columns in a production system that seem to work. However, I've only tested these on a small test database, not on a large production system. Renaming and view creation are both supposed to be very fast, though.
Renaming a table
Rename the table, and temporarily add an updatable view with the old name:
begin;
alter table old_table rename to new_table;
create view old_table as select * from new_table;
commit;
Migrate clients to use the new table name.
Once all clients are migrated, drop the view:
drop view old_table;
Renaming columns without renaming the table
Renaming a column without renaming the table is a bit more complicated,
because we can't have a view shadow a table (apparently).
Rename the column(s), temporarily rename the table, and add an updatable
view that adds the old name for the column with the table's correct name:
begin;
alter table my_table rename column old_name to new_name;
alter table my_table rename to my_table_tmp;
create view my_table as
select *, new_name as old_name from my_table_tmp;
commit;
Migrate clients to use the new column name.
Once all clients are migrated, drop the view, rename the table back:
begin;
drop view my_table;
alter table my_table_tmp rename to my_table;
commit;
Renaming a table and some of its columns simultaneously
Rename the table and columns, and temporarily add an updatable view with the old name and old columns:
begin;
alter table old_table rename to new_table;
alter table new_table rename column to old_name to new_name;
create view old_table as
select *, old_name as new_name from new_table;
commit;
Instead of select *, old_name as new_name, it might be better to only have
the original set of columns, so clients have to migrate to the new table
name to get the new column names:
create view old_table as
select unchanged_name, old_name as new_name from new_table;
Migrate clients to use the new table and column names.
Once all clients are migrated, drop the view:
drop view old_table;
I have a column where I want to change the data type. I currently am using Redshift. I know I can use the alter table statement to change the datatype, but this would change the order of the columns.
Is there a way to change the datatype without changing the order of the column?
I would recommend creating a new table with the schema you want and copying it over from the old table using a insert into new_table (select * from old_table) statement (here you can also do any casting to the new data type), after which you can drop the old table and rename the new one:
drop table old_table;
alter table new_table rename to old_table;
Using ALTER TABLE table_name ALTER COLUMN column_name TYPE new_data_type will not change the order of the columns in your table.
Please note that this clause can only changes the size of a column defined as a VARCHAR data type.
There are also other limitations described in AWS documentation of ALTER TABLE
Good evening
Hello everyone
How to rename view column in oracle.
i have two table
and create view of this table. can we rename of view column.
You can't modify the name of a column in an existing view; there is no alter view rename ... clause similar to alter table rename ....
You will need to drop and recreate the view. When you recreate it you can either specify the view column names explicitly, with whatever names you want, e.g.:
create view your_view (new_name) as select old_name from your_table;
or use column aliases in the view query, e.g.:
create view your_view as select old_name as new_name from your_table;
I am working with hive version 0.9 and I need delete columns of a hive table. I have searched in several manuals of hive commands but I only I have found commands to version 0.14. Is possible to delete a column of a hive table in hive version 0.9? What is the command?
Thanks.
We can’t simply drop a table column from a hive table using the below statement like sql.
ALTER TABLE tbl_name drop column column_name ---- it will not work.
So there is a shortcut to drop columns from a hive table.
Let’s say we have a hive table.
From this table I want to drop the column Dob. You can use the ALTER TABLE REPLACE statement to drop a column.
ALTER TABLE test_tbl REPLACE COLUMNS(ID STRING,NAME STRING,AGE STRING); you have to give the column names which you want to keep in the table
There isn't a drop column or delete column in Hive.
A SELECT statement can take regex-based column specification in Hive releases prior to 0.13.0, or in 0.13.0 and later releases if the configuration property hive.support.quoted.identifiers is set to none.
That being said you could create a new table or view using the following:
drop table if exists database.table_name;
create table if not exists database.table_name as
select `(column_to_remove_1|...|column_to_remove_N)?+.+`
from database.some_table
where
...
;
This will create a table that has all the columns from some_table except the columns named column_to_remove_1, ... , to column_to_remove_N. You can also choose to create a view instead.
ALTER TABLE table_name REPLACE COLUMNS ( c1 int, c2 String);
NOTE: eliminate column from column list. It will keep matched columns and removed unmentioned columns from table schema.
we can not delete column from hive table . But droping a table(if its external) in hive and the recreating table(with column excluded) ,wont delete ur data .
so what can u do is(if u dont have table structure) run this command :
show create table database_name.table_name;
Then you can copy it and edit it (with column eliminated).Afterwards you can do as per invoke the shell
table details are empid,name,dept,salary ,address. i want remove address column. Just write REPLACE COLUMNS like below query
jdbc:hive2://> alter table employee replace columns(empid int, name string,dept string,salary int);
As mentioned before, you can't drop table using an alter statement.
Alter - replace is not guaranteed to work in all the cases.
I found the best answer for this here:
https://stackoverflow.com/a/48921280/4385453
How to clone or copy records in same table in PostgreSQL by creating temporary table.
trying to create clones of records from one table to the same table with changed name(which is basically composite key in that table).
You can do it all in one INSERT combined with a SELECT.
i.e. say you have the following table definition and data populated in it:
create table original
(
id serial,
name text,
location text
);
INSERT INTO original (name, location)
VALUES ('joe', 'London'),
('james', 'Munich');
And then you can INSERT doing the kind of switch you're talking about without using a TEMP TABLE, like this:
INSERT INTO original (name, location)
SELECT 'john', location
FROM original
WHERE name = 'joe';
Here's an sqlfiddle.
This should also be faster (although for tiny data sets probably not hugely so in absolute time terms), since it's doing only one INSERT and SELECT as opposed to an extra SELECT and CREATE TABLE plus an UPDATE.
Did a bit of research, came up with a logic :
Create temp table
Copy records into it
Update the records in temp table
Copy it back to original table
CREATE TEMP TABLE temporary AS SELECT * FROM ORIGINAL WHERE NAME='joe';
UPDATE TEMP SET NAME='john' WHERE NAME='joe';
INSERT INTO ORIGINAL SELECT * FROM temporary WHERE NAME='john';
Was wondering if there was any shorter way to do it.