Postgres - Understanding Group By, Aggregation Functions and Having Clause in PostgreSQL

Understanding Group By, Aggregation Functions and Having Clause in SQL

The SQL GROUP BY clause and SQL HAVING clause are powerful clauses, especially when analyzing large amounts of data.
The GROUP BY clause is a clause in the SELECT statement. It allows you to create groups of values when using aggregating functions.
Aggregate functions without a GROUP BY will return a single value.
If you want to find the aggregate value for each value of X, you can GROUP BY x to find it.

It allows you to find the answers to problems such as:
The total number of sales per month
The average salary of an employee per department
The number of students enrolled per year
It goes towards the end of your SELECT query.

Summary:

Grouping is needed when working with aggregate functions.
The SQL GROUP BY clause allows you to specify the columns to group by.
The HAVING clause allows you to filter records after the GROUP BY is applied.
ROLLUP lets you group by sets of columns, from right to left.
CUBE lets you group by all combinations of columns.
GROUPING SETS lets you specify the sets or subtotals you require.
Composite grouping means you can use brackets to force several columns to be treated as a single unit for grouping.
Concatenated grouping means you can specify different group types within the same GROUP BY clause, separated by a comma.
The GROUPING function shows 1 if a row is a subtotal row and 0 if it is not.
The GROUPING_ID function shows a number indicating what level of a group the row relates to.
The GROUP_ID function shows 0 if the record is unique or 1 if it is a duplicate.
That’s how the GROUP BY clause and related keywords work

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