## In SQL, how and when would you do a group by with multiple columns? Also provide an example. |
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In SQL, the group by statement is used along with aggregate functions like SUM, AVG, MAX, etc. Using the group by statement with multiple columns is useful in many different situations – and it is best illustrated by an example. Suppose we have a table shown below called Purchases. The Purchases table will keep track of all purchases made at a fictitious store.
Now, let’s suppose that the owner of the store wants to find out, on a given date, how many of each product was sold in the store. Then we would write this SQL in order to find that out:
Running the SQL above would return this:
Note that in the SQL we wrote, the group by statement uses multiple columns: “group by item, purchase_date;”. This allows us to group the individual items for a given date – so basically we are dividing the results by the date the items are purchased, and then for a given date we are able to find how many items were purchased for that date. This is why the group by statement with multiple columns is so useful! |

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