SQL - Set Operations
What is the issue with this SQL query?
SELECT employee_id, employee_name FROM Employees
UNION
SELECT id FROM Contractors;
What is the issue with this SQL query?
medium📝 Debug Q6 of 15
Step-by-Step Solution
Solution:
Step 1: Count columns in each SELECT
First SELECT returns 2 columns (employee_id, employee_name), second SELECT returns 1 column (id).Step 2: Check set operation rules
UNION requires both SELECT statements to have the same number of columns.Final Answer:
The number of columns in both SELECT statements do not match. is correct because the column counts differ.Quick Check:
Column count mismatch causes error [OK]
Quick Trick: Both SELECTs must have same column count [OK]
Common Mistakes:
MISTAKES- Ignoring column count mismatch
- Assuming UNION works with different column numbers
- Confusing column names with counts
Master "Set Operations" in SQL
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