SELECT DISTINCT
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Introduction to DISTINCT
SELECT DISTINCT removes duplicate rows from a result set, returning only the unique values present in the specified column or columns — useful for quickly listing, say, every distinct city in a customers table.
Example: Introduction to DISTINCT
SELECT DISTINCT city FROM users;Distinct on Multiple Columns
When DISTINCT is applied across multiple columns, MySQL treats the *combination* as the uniqueness key, so a row is only filtered out if every listed column matches another row exactly.
Example: Distinct on Multiple Columns
SELECT DISTINCT city, country FROM users;Counting Unique Values
Pairing DISTINCT with COUNT (as in COUNT(DISTINCT column)) tells you how many unique values exist in a column, which is different from a plain COUNT that would include every duplicate.
Example: Counting Unique Values
SELECT COUNT(DISTINCT city) FROM users;Distinct and Null Values
MySQL treats all NULL values in a column as equal to each other for DISTINCT purposes, collapsing any number of NULLs down into a single NULL row in the result set.
Example: Distinct and Null Values
SELECT DISTINCT city FROM users; -- multiple NULL rows collapse into oneDistinct vs Group By
DISTINCT is best for simple deduplication of a result list; once you need aggregated calculations grouped by category, GROUP BY is the more appropriate and more powerful tool.
Example: Distinct vs Group By
SELECT DISTINCT city FROM users;
SELECT city, COUNT(*) FROM users GROUP BY city;Chapter Quiz — Complete all 6 topics to unlock
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