[Solved] Why does this query cause an error?SELECT id, name FROM Table1 UNION SELECT id FROM Table2; — Ans: Different number of columns in SELECT statements | PostgreSQL

PostgreSQL - Set Operations and Advanced Queries

Why does this query cause an error?
SELECT id, name FROM Table1 UNION SELECT id FROM Table2;

ADifferent number of columns in SELECT statements

BUNION requires ORDER BY clause

CTable1 and Table2 must have same number of rows

DUNION cannot combine different tables

Step-by-Step Solution

Solution:

Step 1: Check column counts in SELECTs
The first SELECT returns 2 columns (id, name), second returns 1 column (id). UNION requires same number of columns.
Step 2: Verify other options
ORDER BY is optional, tables can differ in rows, UNION can combine different tables.
Final Answer:
Different number of columns in SELECT statements -> Option A
Quick Check:
UNION requires same column count [OK]

Quick Trick: UNION needs same number of columns in both SELECTs [OK]

Common Mistakes:

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Why does this query cause an error?