SQL Group By vs Distinct
Difference between GROUP BY and DISTINCT
GROUP BY is used in combination with aggregation functions. Consider the following table:
orderId userId storeName orderValue orderDate
1 43 Store A 25 20-03-2016
2 57 Store B 50 22-03-2016
3 43 Store A 30 25-03-2016
4 82 Store C 10 26-03-2016
5 21 Store A 45 29-03-2016
The query below uses GROUP BY to perform aggregated calculations.
SELECT
storeName,
COUNT(*) AS total_nr_orders,
COUNT(DISTINCT userId) AS nr_unique_customers,
AVG(orderValue) AS average_order_value,
MIN(orderDate) AS first_order,
MAX(orderDate) AS lastOrder
FROM
orders
GROUP BY
storeName;
and will return the following information
storeName total_nr_orders nr_unique_customers average_order_value first_order lastOrder
Store A 3 2 33.3 20-03-2016 29-03-2016
Store B 1 1 50 22-03-2016 22-03-2016
Store C 1 1 10 26-03-2016 26-03-2016
While DISTINCT is used to list a unique combination of distinct values for the specified columns.
SELECT DISTINCT
storeName,
userId
FROM
orders;
storeName userId
Store A 43
Store B 57
Store C 82
Store A 21
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