Unable to use nested window functions in Looker - sql

Question

which is similar to MYSQL, and having trouble understanding why I'm unable to use SUM & POWER together in a window function. Specifically the SUM(POWER("DELTA"... line throws the following error:
SQL compilation error: Window function [AVG(CAST(VALUE AS NUMBER(38,3))) OVER (PARTITION BY ID)] may not be nested inside another window function.
Removing this line or moving it to the second select statement fixes the error, and all the other . I think this is a more fundamental SQL misunderstanding I have. Any thoughts would be much appreciated!
WITH UTILS AS (
SELECT
ID,
VALUE AS "TEMP_CELSIUS,
AVG(VALUE) OVER(PARTITION BY ID) AS "TEMP_AVG",
VAR_POP(VALUE) OVER(PARTITION BY ID) As "TEMP_VAR",
STDDEV_POP(VALUE) OVER(PARTITION BY ID) As "TEMP_STD",
COUNT(VALUE) OVER(PARTITION BY ID) As "DEVICE_N",
(VALUE-"TEMP_AVG") AS "DELTA",
SUM(POWER("DELTA", 3)) OVER(PARTITION BY ID) AS "SKEW2"
FROM
TABLE1
)
SELECT
"SKEW2"
FROM
UTILS

Answer 1

Just to be clear, your problem wasn't POWER exactly, but that POWER referred to TEMP_AVG, a derived column involving a window function.
In standard SQL, we also can't refer to one derived column in a subsequent item in the same select list.
This is a problem:
SELECT a + b AS c
, c + d AS e
FROM t1
;
Here are some fragments of the standard SQL specification that refer specifically to window functions and some supporting items.
You asked about "fundamental SQL". While your database might support non-standard behavior, it might be helpful to understand the behavior provided by standard SQL, at least from a foundational perspective, as a starting point.
This is just a small part of that detail from a slightly older version of the specification (the foundation document dated 2011-12). Much of this hasn't changed.
In Section: 7.12 <query specification> we have:
<query specification> ::= SELECT [ <set quantifier> ] <select list> <table expression>
You can see that your select list is followed by a table expression.
Your question is basically about what is allowed in the <select list>.
But to understand the select list behavior, we need to peek for a moment at the term called <table expression>:
<table expression> ::=
<from clause>
[ <where clause> ]
[ <group by clause> ]
[ <having clause> ]
[ <window clause> ]
In the corresponding Syntax Rules for a <query expression>, we find the following:
2) Let T be the result of the <table expression> simply contained in QSPEC.
Basically, T is the result of your FROM clause, which optionally includes WHERE, GROUP BY, etc.
Now let's jump to the expression you asked about, the window function.
The Syntax Rule which applies:
11) Each column reference contained in a <window function> shall unambiguously reference a column of T.
In your case, you were referring to a derived column in the current select list, not to a column of T.
That's the problem. The standard limits you to just columns of T.
Be aware, I'm picking out very tiny bits from the huge specification. But I think this points out the basic detail.
One common approach is to simply move the window function to a subsequent query expression, so that it refers to a T which contains the first derived column.
WITH cte1 AS (
SELECT ...
, AVG(...) OVER ... AS temp_avg
FROM t1
)
SELECT ...
, SUM(temp_avg) OVER ...
FROM cte1
;
In some cases, if you wish, you can just repeat an expression used in the derived column, without using separate CTE terms or derived tables. At least in standard SQL, we can't do that with two window functions.