[Solved] Given the following query, what will be the output? WITH cte1 AS (SELECT 1 AS num), cte2 AS (SELECT num * 2 AS double_num FROM cte1) SELECT * FROM cte2; | PostgreSQL

PostgreSQL - Common Table Expressions

Given the following query, what will be the output?

WITH cte1 AS (SELECT 1 AS num), cte2 AS (SELECT num * 2 AS double_num FROM cte1) SELECT * FROM cte2;

A[{num: 2}]

B[{num: 1, double_num: 2}]

C[{double_num: 2}]

DSyntax error

Step-by-Step Solution

Solution:

Step 1: Analyze cte1
cte1 returns one row with column num = 1.
Step 2: Analyze cte2 using cte1
cte2 selects num * 2 as double_num, so double_num = 1 * 2 = 2.
Step 3: Final SELECT from cte2
Only double_num column is selected, so output is [{double_num: 2}].
Final Answer:
[{double_num: 2}] -> Option C
Quick Check:
cte2 output columns = double_num [OK]

Quick Trick: Check which columns each CTE outputs carefully [OK]

Common Mistakes:

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Given the following query, what will be the output?