You wrote this query: SELECT COUNT(column_name) FROM table_name; but it returns 0. The column has some NULL and some non-NULL values. What is the most likely problem?
medium📝 Debug Q14 of 15
SQL - Aggregate Functions
You wrote this query: SELECT COUNT(column_name) FROM table_name; but it returns 0. The column has some NULL and some non-NULL values. What is the most likely problem?
ACOUNT(column_name) counts NULL values, so it should not return 0.
BThe column_name is misspelled or does not exist in the table.
CCOUNT(*) should be used instead of COUNT(column_name) to count non-NULL values.
DThe table is empty, so COUNT(column_name) returns 0.
Step-by-Step Solution
Solution:
Step 1: Check column existence
If COUNT(column_name) returns 0 but column has non-NULL values, likely the column name is wrong or missing.
Step 2: Understand COUNT behavior
COUNT(column_name) counts non-NULL values; if column exists and has non-NULLs, result won't be zero.
Final Answer:
Column name is misspelled or does not exist. -> Option B
Quick Check:
Wrong column name causes zero count [OK]
Quick Trick:Check column spelling if COUNT(column) returns zero unexpectedly [OK]
Common Mistakes:
MISTAKES
Assuming COUNT(column) counts NULLs
Using COUNT(*) when column is misspelled
Ignoring empty table possibility without checking
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