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
);
CREATE VIEW materialized_view WITH SCHEMABINDING AS
SELECT ...
FROM ext.external_table
Fails with
The option 'SCHEMABINDING' is not supported with external tables.
If I understand correctly SCHEMABINDING is necessary to make a materialized view.
How can I correct this query?
You cannot create an indexed view based on tables that are in a different database.
I think your options are:
a) create the indexed view in the other database and create a regular view in this database to query that indexed view
b) create a copy of the table in this database and a mechanism to update this table whenever the data is changed in the table which is in the other database; this could be done with triggers, replication, a stored procedure called on a schedule, etc.
I use PostgreSQL for time series data. There is an Event table and partitioned tables like Event_2016, Event_2017 which inherit from Event with a CONSTRAINT CHECK for the date range. So when querying the Event table, PostgreSQL uses only the child tables that are needed.
To roll up events I use an EventByDay materialized view. Refreshing this requires reading from all Event_* tables.
Can I use Materialized Views the same way as tables above to limit the amount of data in each Materialized View? (EventByDay_2016 inherits from EventByDay).
No, a MVIEW can not participate in table inheritance.
But you can create a (regular) child table and then use insert into .. select ... using the query from the MVIEW. If you want to store the MVIEW's query in the database then create a view that you use for populating the child table.
Something like this:
Initial setup:
create view v_event_by_day_2016
as
-- this is the complete query from your MVIEW
select ...;
create table event_by_day_2016
as
select *
from v_view_one;
alter table event_by_day_2016 inherit event_by_day;
alter table event_by_day_2016 add constraint check (...);
Refresh the table:
truncate table event_by_day_2016;
insert into event_by_day_2016
select *
from v_event_by_day_2016;
Alternatively you can use delete to so that the data in the child table can be refreshed in a transactional manner.
im using a platform called CKAN which saves datasets. When a dataset is added it creates a table with a (seemingly) random name. There are certain datasets that I want to use the data from. Therefore I want to map the relation between the table in another table and the data that is inside.
I would like to use this mapped variable (table name) in a select query as FROM statement.
SELECT * FROM (SELECT tablename FROM mappingtable WHERE id=1)
How do I do this?
Edit: As what kind of data type do I store the table name?
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