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What does COUNT(column_name) count in SQL?

easy📝 Conceptual Q2 of 15
SQL - Aggregate Functions
What does COUNT(column_name) count in SQL?
AOnly rows where <code>column_name</code> is NOT NULL
BOnly rows where <code>column_name</code> is NULL
CAll rows in the table
DRows where any column is NOT NULL
Step-by-Step Solution
Solution:

  1. Step 1: Understand COUNT(column_name) behavior

    COUNT(column_name) counts only rows where the specified column is NOT NULL.

  2. Step 2: Contrast with COUNT(*)

    Unlike COUNT(*), it ignores rows where the column has NULL values.

  3. Final Answer:

    Only rows where column_name is NOT NULL -> Option A

  4. Quick Check:

    COUNT(column) = counts non-NULL column values [OK]
Quick Trick: COUNT(column) skips NULLs in that column only [OK]
Common Mistakes:
MISTAKES
  • Assuming COUNT(column) counts all rows
  • Confusing NULL and NOT NULL counts
  • Thinking COUNT(column) counts NULLs

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