[Solved] Which of the following SQL queries correctly uses UNION with matching columns?-- Table A: (id INT, name VARCHAR)-- Table B:... | SQL

SQL - Set Operations

Which of the following SQL queries correctly uses UNION with matching columns?

-- Table A: (id INT, name VARCHAR)
-- Table B: (user_id INT, username VARCHAR)

A) SELECT id, name FROM A UNION SELECT user_id, username FROM B;
B) SELECT id FROM A UNION SELECT user_id, username FROM B;
C) SELECT id, name FROM A UNION SELECT user_id FROM B;
D) SELECT id, name FROM A UNION SELECT user_id, username, email FROM B;

ASELECT id, name FROM A UNION SELECT user_id FROM B;

BSELECT id FROM A UNION SELECT user_id, username FROM B;

CSELECT id, name FROM A UNION SELECT user_id, username FROM B;

DSELECT id, name FROM A UNION SELECT user_id, username, email FROM B;

Step-by-Step Solution

Solution:

Step 1: Check column counts in each query
SELECT id, name FROM A UNION SELECT user_id, username FROM B; selects 2 columns from both queries, matching counts. Other options have mismatched column counts (1 vs 2, 2 vs 1, or 2 vs 3).
Step 2: Verify column types compatibility
Columns in SELECT id, name FROM A UNION SELECT user_id, username FROM B; are INT and VARCHAR in both queries, which are compatible.
Final Answer:
SELECT id, name FROM A UNION SELECT user_id, username FROM B; -> Option C
Quick Check:
Equal columns and compatible types = A [OK]

Quick Trick: Count columns and check types match in both queries [OK]

Common Mistakes:

MISTAKES

Mismatched column counts cause errors
Ignoring column type compatibility
Assuming extra columns are allowed

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More SQL Quizzes

Which of the following SQL queries correctly uses UNION with matching columns?

Step 1: Check column counts in each query

Step 2: Verify column types compatibility

Final Answer:

Quick Check:

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