[Solved] Given the table edges(source INT, target INT) with data:(1, 2), (2, 3), (3, 4), (2, 5)What will the following query return?... | PostgreSQL

PostgreSQL - Common Table Expressions

Given the table edges(source INT, target INT) with data:
(1, 2), (2, 3), (3, 4), (2, 5)
What will the following query return?

WITH RECURSIVE path AS (SELECT source, target FROM edges WHERE source = 1 UNION ALL SELECT e.source, e.target FROM edges e JOIN path p ON e.source = p.target) SELECT * FROM path ORDER BY source, target;

ARows: (1,2), (2,3), (2,5), (3,4)

BRows: (1,2), (2,3), (3,4)

CRows: (1,2), (2,5)

DEmpty result

Step-by-Step Solution

Solution:

Step 1: Identify the anchor and recursive parts
The anchor selects edges where source=1: (1,2). The recursive part joins edges where source matches previous target.
Step 2: Trace recursive expansion
From (1,2), next edges with source=2 are (2,3) and (2,5). From (2,3), next edge with source=3 is (3,4). No further edges from 5 or 4.
Final Answer:
Rows: (1,2), (2,3), (2,5), (3,4) -> Option A
Quick Check:
Recursive join expands all reachable edges [OK]

Quick Trick: Follow edges from start node stepwise [OK]

Common Mistakes:

Ignoring multiple edges from same source
Stopping recursion too early
Misordering results

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Given the table edges(source INT, target INT) with data:

Step 1: Identify the anchor and recursive parts

Step 2: Trace recursive expansion

Final Answer:

Quick Check:

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