Create a new table with columns with case statements and max function - sql

Question

I have some problems in creating a new table from an old one with new columns defined by case statements.
I need to add to a new table three columns, where I compute the maximum based on different conditions. Specifically,
if time is between 1 and 3, I define a variable max_var_1_3 as max((-1)*var),
if time is between 1 and 6, I define a variable max_var_1_6 as max((-1)*var),
if time is between 1 and 12, I define a variable max_var_1_12 as max((-1)*var),
The max function needs to take the maximum value of the variable var in the window between 1 and 3, 1 and 6, 1 and 12 respectively.
I wrote this
create table new as(
select t1.*,
(case when time between 1 and 3 then MAX((-1)*var)
else var
end) as max_var_1_3,
(case when time between 1 and 6 then MAX((-1)*var)
else var
end) as max_var_1_6,
(case when time between 1 and 12 then MAX((-1)*var)
else var
end) as max_var_1_12
from old_table t1
group by time
) with data primary index time
but unfortunately it is not working. The old_table has already some columns, and I would like to import all of them and then compare the old table with the new one. I got an error that says that should be something between ) and ',', but I cannot understand what. I am using Teradata SQL.
Could you please help me?
Many thanks

Answer 1

The problem is that you have GROUP BY time in your query while trying to return all the other values with your SELECT t1.*. To make your query work as-is, you'd need to add each column from t1.* to your GROUP BY clause.
If you want to find the MAX value within the different time ranges AND also return all the rows, then you can use a window function. Something like this:
CREATE TABLE new AS (
SELECT
t1.*,
CASE
WHEN t1.time BETWEEN 1 AND 3 THEN (
MAX(CASE WHEN t1.time BETWEEN 1 AND 3 THEN (-1 * t1.var) ELSE NULL END) OVER()
)
ELSE t1.var
END AS max_var_1_3,
CASE
WHEN t1.time BETWEEN 1 AND 6 THEN (
MAX(CASE WHEN t1.time BETWEEN 1 AND 6 THEN (-1 * t1.var) ELSE NULL END) OVER()
)
ELSE t1.var
END AS max_var_1_6,
CASE
WHEN t1.time BETWEEN 1 AND 12 THEN (
MAX(CASE WHEN t1.time BETWEEN 1 AND 12 THEN (-1 * t1.var) ELSE NULL END) OVER()
)
ELSE t1.var
END AS max_var_1_12,
FROM old_table t1
) WITH DATA PRIMARY INDEX (time)
;
Here's the logic:
check if a row falls in the range
if it does, return the desired MAX value for rows in that range
otherwise, just return that given row's default value (var)
return all rows along with the three new columns
If you have performance issues, you could also move the max_var calculations to a CTE, since they only need to be calculated once. Also to avoid confusion, you may want to explicitly specify the values in your SELECT instead of using t1.*.
I don't have a TD system to test, but try it out and see if that works.

Answer 2

I cannot help with the CREATE TABLE AS, but the query you want is this:
SELECT
t.*,
(SELECT MAX(-1 * var) FROM old_table WHERE time BETWEEN 1 AND 3) AS max_var_1_3,
(SELECT MAX(-1 * var) FROM old_table WHERE time BETWEEN 1 AND 6) AS max_var_1_6,
(SELECT MAX(-1 * var) FROM old_table WHERE time BETWEEN 1 AND 12) AS max_var_1_12
FROM old_table t;