Aggregation means combining or summarizing multiple rows into a single row. Up to this point, our queries always returned every individual row that met the query constraints. Aggregation allows us to combine rows and find the count, compute statistical summaries, or join text to create a single row.
If you only need to count how many rows match some criteria, use the
COUNT() aggregation method.
SELECT COUNT(*) FROM ...
The above technique,
COUNT(*), returns the count of rows. If you want to count only those rows for which a particular column is not null, then apply count to that column:
SELECT COUNT(col_name) FROM ...
You can also count just the distinct rows:
SELECT COUNT(DISTINCT col_name) FROM ...
Finding the sum, min, max, average
These are self-explanatory.
SELECT SUM(col_name) FROM ... SELECT SUM(DISTINCT col_name) FROM ... SELECT MIN(col_name) FROM ... SELECT MIN(DISTINCT col_name) FROM ... SELECT MAX(col_name) FROM ... SELECT MAX(DISTINCT col_name) FROM ... SELECT AVG(col_name) FROM ... SELECT AVG(DISTINCT col_name) FROM ...
Grouping allows us to aggregate multiple times in the same query. Rows can be grouped before finding the count/min/max/etc. according to a
GROUP BY clause. Typically, you’ll include the group by columns in your query so you can see the group value as well as the aggregated value.
SELECT group_col_name, COUNT(agg_col_name) FROM ... WHERE ... GROUP BY group_col_name
For example, consider the shopping cart table:
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We can query for the total number of items in each customer’s cart by grouping by the customer:
SELECT customer, SUM(count) FROM shopping_cart GROUP BY customer
The result would be:
A query using aggregation can only select columns that are either aggregated with a function like count/min/max/etc. or included in a group by clause. Additional columns cannot be included, because it wouldn’t be able to determine their value. For example, this does not work, because it’s not clear what the value for ‘product’ should be:
-- doesn't work! SELECT customer, product, SUM(count) FROM shopping_cart GROUP BY customer
MySQL gives this error:
ERROR 1055 (42000): Expression #2 of SELECT list is not in GROUP BY clause and contains nonaggregated column ‘shopping_cart.product’ which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by