[Solved] CTE query associated with \"Where Clause\" not executing before evaluation condition in Where clause

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CTE query associated with \"Where Clause\" not executing before evaluation condition in Where clause

I am facing a strange behaviour while executing the below query

WITH X1 AS 
(
  SELECT 1 AS N1, 0 AS N2
  UNION ALL
  SELECT 1 AS N1, 1 AS N2
)
,X2 AS
(
 SELECT * FROM X1 WHERE N2>0
)
SELECT N1,N2,N1/N2 FROM X2

Output:

However when I put below where condition in above select statement

SELECT N1,N2,N1/N2 FROM X2
WHERE N1/N2>0

I get "Divide by zero error encountered." Please can anyone explain why I am getting this error?

How to optimize this SQL query?

The following recommendations will help you in your SQL tuning process.
You'll find 3 sections below:

Description of the steps you can take to speed up the query.
The optimal indexes for this query, which you can copy and create in your database.
An automatically re-written query you can copy and execute in your database.

The optimization process and recommendations:

Avoid Selecting Unnecessary Columns (query line: 8): Avoid selecting all columns with the '*' wildcard, unless you intend to use them all. Selecting redundant columns may result in unnecessary performance degradation.

The optimized query:

WITH X1 AS (SELECT
        1 AS N1,
        0 AS N2 
    UNION
    ALL SELECT
        1 AS N1,
        1 AS N2), X2 AS (SELECT
        * 
    FROM
        X1 
    WHERE
        X1.N2 > 0) SELECT
        X2.N1,
        X2.N2,
        X2.N1 / X2.N2 
    FROM
        X2

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* original question posted on StackOverflow here.