COUNT DISTINCT and COUNT both deal with counting, but their focus is different. Simultaneously, COUNT tallies every instance of values in a column, even duplicates, while COUNT DISTINCT focuses solely on unique values. To put it simply, COUNT gives you the overall headcount, whereas COUNT DISTINCT provides the count of distinct, unique entries.
2. Can COUNT DISTINCT be used with multiple columns?
Absolutely! COUNT DISTINCT is versatile; it allows you to count unique combinations across multiple columns. This proves useful when you aim to identify the number of distinct pairs or sets of values present across various columns.
3. Is COUNT DISTINCT efficient with large datasets?
In general, COUNT DISTINCT is efficient, but the size of your dataset can influence its performance. When dealing with large datasets, it's a good idea to ensure relevant columns are properly indexed to maintain efficiency.
4. How does COUNT DISTINCT handle NULL values?
COUNT DISTINCT handles NULL values with ease. It exclusively counts distinct non-NULL values, ensuring no duplicates with NULLs. If you prefer excluding NULLs from the count, add a condition like WHERE column_name IS NOT NULL in your query.
5. Can you use COUNT DISTINCT with GROUP BY?
Absolutely! Combining COUNT DISTINCT with GROUP BY allows you to get unique counts within specific groups. It is suitable for more detailed and organized data analysis, giving you unique counts for each group.