Declare a variable in RedShift
Amazon RedshiftAmazon Redshift Problem Overview
SQL Server
has the ability to declare a variable, then call that variable in a query like so:
DECLARE @StartDate date;
SET @StartDate = '2015-01-01';
SELECT *
FROM Orders
WHERE OrderDate >= @StartDate;
Does this functionality work in Amazon's RedShift? From the documentation, it looks that DECLARE
is used solely for cursors. SET
looks to be the function I am looking for, but when I attempt to use that, I get an error.
set session StartDate = '2015-01-01';
[Error Code: 500310, SQL State: 42704] [Amazon](500310) Invalid operation: unrecognized configuration parameter "startdate";
Is it possible to do this in RedShift
?
Amazon Redshift Solutions
Solution 1 - Amazon Redshift
Slavik Meltser's answer is great. As a variation on this theme, you can also use a WITH construct:
WITH tmp_variables AS (
SELECT
'2015-01-01'::DATE AS StartDate,
'some string' AS some_value,
5556::BIGINT AS some_id
)
SELECT *
FROM Orders
WHERE OrderDate >= (SELECT StartDate FROM tmp_variables);
Solution 2 - Amazon Redshift
Actually, you can simulate a variable using a temporarily table, create one, set data and you are good to go.
Something like this:
CREATE TEMP TABLE tmp_variables AS SELECT
'2015-01-01'::DATE AS StartDate,
'some string' AS some_value,
5556::BIGINT AS some_id;
SELECT *
FROM Orders
WHERE OrderDate >= (SELECT StartDate FROM tmp_variables);
The temp table will be deleted after the transaction execution.
Temp tables are bound per session (connect), therefor cannot be shared across sessions.
Solution 3 - Amazon Redshift
No, Amazon Redshift does not have the concept of variables. Redshift presents itself as PostgreSQL, but is highly modified.
There was mention of User Defined Functions at the 2014 AWS re:Invent conference, which might meet some of your needs.
Update in 2016: Scalar User Defined Functions can perform computations but cannot act as stored variables.
Solution 4 - Amazon Redshift
Note that if you are using the psql client to query, psql variables can still be used as always with Redshift:
$ psql --host=my_cluster_name.clusterid.us-east-1.redshift.amazonaws.com \
--dbname=your_db --port=5432 --username=your_login -v dt_format=DD-MM-YYYY
# select current_date;
date
------------
2015-06-15
(1 row)
# select to_char(current_date,:'dt_format');
to_char
------------
15-06-2015
(1 row)
# \set
AUTOCOMMIT = 'on'
...
dt_format = 'DD-MM-YYYY'
...
# \set dt_format 'MM/DD/YYYY'
# select to_char(current_date,:'dt_format');
to_char
------------
06/15/2015
(1 row)
Solution 5 - Amazon Redshift
You can now use user defined functions (UDF's) to do what you want:
CREATE FUNCTION my_const()
RETURNS CSTRING IMMUTABLE AS
$$ return 'my_string_constant' $$ language plpythonu;
Unfortunately, this does require certain access permissions on your redshift database.