SQL - Aggregate Functions
Identify the error in the following SQL query:
SELECT SUM(NULLIF(sales, NULL)) FROM transactions;Identify the error in the following SQL query: SELECT SUM(NULLIF(sales, NULL)) FROM transactions;
medium📝 Debug Q6 of 15
Step-by-Step Solution
Solution:
Step 1: Understand NULLIF usage
NULLIF(sales, NULL) is valid SQL syntax. Since any value = NULL is UNKNOWN (not true), NULLIF always returns sales, preserving NULLs.Step 2: Check for errors
No syntax error; no GROUP BY needed for table-wide aggregate; SUM works fine with it (equivalent to SUM(sales)).Final Answer:
No error, query is correct -> Option AQuick Check:
NULLIF(col, NULL) valid, no error = D [OK]
Quick Trick: NULLIF(column, NULL) is valid and equivalent to the column itself [OK]
Common Mistakes:
MISTAKES- Thinking NULLIF cannot take NULL as second argument
- Assuming SUM requires GROUP BY for single aggregate
- Believing SUM cannot aggregate NULLIF results
Master "Aggregate Functions" in SQL
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