Given the table employees with the column department containing values: ['HR', 'IT', NULL, 'IT', 'HR', 'Finance', NULL], what is the result of the query SELECT COUNT(DISTINCT department) FROM employees;?

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SQL - Aggregate Functions

Given the table employees with the column department containing values: ['HR', 'IT', NULL, 'IT', 'HR', 'Finance', NULL], what is the result of the query SELECT COUNT(DISTINCT department) FROM employees;?

Step-by-Step Solution

Solution:

Step 1: Identify distinct non-NULL values in department
The distinct non-NULL values are 'HR', 'IT', and 'Finance'. That's 3 unique values.
Step 2: Understand COUNT(DISTINCT) behavior
COUNT(DISTINCT column) counts unique non-NULL values only, so NULLs are excluded.
Step 3: Count distinct values
There are 3 distinct non-NULL values, so the count is 3.
Final Answer:
3 -> Option A
Quick Check:
COUNT(DISTINCT) excludes NULLs [OK]

Quick Trick: COUNT(DISTINCT) counts unique non-NULL values only [OK]

Common Mistakes:

MISTAKES

Including NULL as a distinct value
Counting total rows instead of distinct
Confusing COUNT(*) with COUNT(DISTINCT)

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Given the table employees with the column department containing values: ['HR', 'IT', NULL, 'IT', 'HR', 'Finance', NULL], what is the result of the query SELECT COUNT(DISTINCT department) FROM employees;?

Step 1: Identify distinct non-NULL values in department

Step 2: Understand COUNT(DISTINCT) behavior

Step 3: Count distinct values

Final Answer:

Quick Check:

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