Identify the error in this query intended to calculate running totals:
medium📝 Debug Q6 of 15
SQL - Advanced Query Patterns
Identify the error in this query intended to calculate running totals: SELECT id, amount, (SELECT SUM(amount) FROM sales WHERE id < id) AS running_total FROM sales ORDER BY id;
AThe ORDER BY clause is missing.
BThe subquery condition 'id < id' compares the same column without alias, causing always false condition.
CThe query uses window functions which are not allowed.
DThe SUM() function cannot be used in subqueries.
Step-by-Step Solution
Solution:
Step 1: Analyze the subquery condition
The condition 'id < id' compares the same column from the same table without alias, so it is always false.
Step 2: Understand the effect on running total
Because the condition is always false, the subquery returns NULL or zero, so running_total is incorrect.
Final Answer:
The subquery condition 'id < id' compares the same column without alias, causing always false condition. -> Option B
Quick Check:
Incorrect subquery condition = Always false [OK]
Quick Trick:Use table aliases to distinguish columns in subqueries [OK]
Common Mistakes:
Not using aliases causing ambiguous column references
Misunderstanding subquery logic
Assuming SUM() can't be in subqueries
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