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:
| Company |
Amount |
| W3Schools |
5500 |
| IBM |
4500 |
| W3Schools |
7100 |
And This SQL:
SELECT Company, SUM(Amount) FROM Sales |
Returns this result:
| Company |
SUM(Amount) |
| W3Schools |
17100 |
| IBM |
17100 |
| W3Schools |
17100 |
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:
| Company |
SUM(Amount) |
| W3Schools |
12600 |
| IBM |
4500 |
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:
| Company |
Amount |
| W3Schools |
5500 |
| IBM |
4500 |
| W3Schools |
7100 |
This SQL:
SELECT Company,SUM(Amount) FROM Sales
GROUP BY Company
HAVING SUM(Amount)>10000 |
Returns this result
| Company |
SUM(Amount) |
| W3Schools |
12600 |
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