postgresqlHow do I use the GROUP BY clause in PostgreSQL?

The GROUP BY clause is used in PostgreSQL to group together rows of data from a table that have the same values in one or more columns. It is typically used in conjunction with the SELECT and HAVING clauses.

For example, to group together all the rows in the orders table that have the same customer_id and get the total number of orders for each customer, the following query could be used:

SELECT customer_id, COUNT(*)
FROM orders
GROUP BY customer_id;

The output of this query would be:

 customer_id | count
-------------+-------
          10 |     3
          11 |     2
          12 |     5
(3 rows)

The components of this query are:

• SELECT: This clause is used to specify the columns that will be included in the output. In this case, the customer_id and the count of orders for each customer.
• FROM: This clause is used to specify the table that will be used in the query. In this case, the orders table.
• GROUP BY: This clause is used to group together all the rows in the table that have the same values in the specified column(s). In this case, the customer_id column.
• COUNT: This is an aggregate function used to count the number of rows in each group.

For more information, see the PostgreSQL documentation.

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