[Solved] Why does using ELSE 0 instead of ELSE NULL in a CASE statement inside COUNT affect the result? — Ans: ELSE 0 counts all rows because 0 is not NULL, ELSE NULL excludes rows | SQL

SQL - Advanced Query Patterns

Why does using ELSE 0 instead of ELSE NULL in a CASE statement inside COUNT affect the result?

AELSE 0 causes syntax error, ELSE NULL is correct

BELSE 0 counts all rows because 0 is not NULL, ELSE NULL excludes rows

CELSE 0 counts only zero values, ELSE NULL counts all

DNo difference, both count the same rows

Step-by-Step Solution

Solution:

Step 1: Understand COUNT behavior
COUNT counts only non-null values.
Step 2: Effect of ELSE 0 vs ELSE NULL
ELSE 0 returns zero (non-null), so COUNT counts those rows; ELSE NULL returns null, excluding rows from count.
Final Answer:
ELSE 0 counts all rows because 0 is not NULL, ELSE NULL excludes rows -> Option B
Quick Check:
COUNT excludes NULLs, includes zeros [OK]

Quick Trick: Use ELSE NULL to exclude rows in COUNT, ELSE 0 includes them [OK]

Common Mistakes:

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Why does using ELSE 0 instead of ELSE NULL in a CASE statement inside COUNT affect the result?