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
Related
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
);
I am trying to modify a column data type and name in one command.
Is it possible to apply multiple alter statements on one column in one code line? Is that the correct syntax?
ALTER TABLE tb_table
ALTER COLUMN colum_date
SET DATA TYPE TIMESTAMP NOT NULL
RENAME COLUMN colum_date TO colum_timestamp;
I have been looking for related bibliography but could not find much information on the web.
According to the documentation renaming a column is an action that cannot be combined with others but requires its own statement. So no, you cannot do what you want in one statement.
But you can do it in one transaction. Setting the datatype and setting the NOT NULL constraint requires two actions though. But both of that actions can be in one statement.
BEGIN TRANSACTION;
ALTER TABLE tb_table
ALTER COLUMN colum_date
SET DATA TYPE TIMESTAMP,
ALTER COLUMN colum_date
SET NOT NULL;
ALTER TABLE tb_table
RENAME COLUMN colum_date
TO colum_timestamp;
COMMIT TRANSACTION;
I'm trying to change all the USER-DEFINED columns to TEXT in a specific view using pgsql.
Is it possible to do that in a single alter table query ou do I need to check first what columns contain that dataype and then perform the dataype change one by one?
This is what I'm trying:
ALTER TABLE if exists "schemaName"."Table_A"
ALTER COLUMN (
select column_name
from information_schema.columns inf
where table_name = 'Table_A' and inf.data_type = 'USER-DEFINED')
TYPE TEXT;
I'm getting and error in the subquery start "("
You need to do this one by one. Generally speaking such DDL statements cannot work on several objects in one statement.
For ALTER TABLE, see: https://www.postgresql.org/docs/12/sql-altertable.html.
FOR ALTER VIEW, see: https://www.postgresql.org/docs/current/sql-alterview.html
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 do I add multiple columns in one query statement in PostgreSQL using pgadmin3?
Try this :
ALTER TABLE table ADD COLUMN col1 int, ADD COLUMN col2 int;
Use ALTER TABLE with ADD COLUMN subcommand.
ALTER TABLE:
This changes the definition of an existing table using a subcommand, for example: ADD COLUMN.
ADD COLUMN [ IF NOT EXISTS ]:
This form adds a new column to the table, using the same syntax as CREATE TABLE. If IF NOT EXISTS is specified and a column already exists with this name, no error is thrown.
Adding a Column
The most basic syntax is:
ALTER TABLE table_name
ADD COLUMN new_column_name data_type constraint;
In this syntax:
First, specify the name of the table that you want to add a new column to after the ALTER TABLE keyword.
Second, specify the name of the new column as well as its data type and constraint after the ADD COLUMN keywords.
Adding a Column using IF NOT EXIST clause
ALTER TABLE table_name
ADD COLUMN IF NOT EXISTS new_column_name data_type constraint;
This option gives PostgreSQL instructions to add the new column only in case the column name does not already exist in the table. If it does, you will receive a corresponding response; otherwise, it will create one.
ALTER TABLE IF EXISTS TABLEname
add ADD COLUMN IF NOT EXISTS column_name data_type [column_constraint];
detailed query where column_constraints are optional