Selecting Groups - the HAVING Clause

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The HAVING clause restricts the selection of groups in the same way that a WHERE clause restricts the selection of rows. However, in contrast to the WHERE clause, a HAVING clause may use a set function on the left-hand side of a comparison.

The HAVING clause is most often used together with a GROUP BY clause, but may also be used to impose selection conditions on a column derived from a set function.

Find the currency codes that are used by more than one country:

 SELECT currency_code, COUNT(currency_code)
    FROM countries
    GROUP BY currency_code
    HAVING COUNT(currency_code) > 1;

Returns:

CURRENCY_CODE
AUD	8
CHF	2
DKK	3
EUR	23
IDR	2
...	...

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