SQL GROUP BY and HAVING

Aggregate functions (like SUM) often need an added GROUP BY functionality.

GROUP BY...

GROUP BY... was added to SQL because aggregate functions (like SUM) return the aggregate of all column values every time they are called, and without the GROUP BY function it was impossible to find the sum for each individual group of column values.

The syntax for the GROUP BY function is:

SELECT column,SUM(column) FROM table GROUP BY column

GROUP BY Example

This "Sales" Table:

And This SQL:

SELECT Company, SUM(Amount) FROM Sales

Returns this result:

The above code is invalid because the column returned is not part of an aggregate. A GROUP BY clause will solve this problem:

SELECT Company,SUM(Amount) FROM Sales
GROUP BY Company

Returns this result:

HAVING...

HAVING... was added to SQL because the WHERE keyword could not be used against aggregate functions (like SUM), and without HAVING... it would be impossible to test for result conditions.

The syntax for the HAVING function is:

SELECT column,SUM(column) FROM table
GROUP BY column
HAVING SUM(column) condition value

This "Sales" Table:

This SQL:

SELECT Company,SUM(Amount) FROM Sales
GROUP BY Company
HAVING SUM(Amount)>10000

Returns this result

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