Hi,
What is the difference between WHERE & HAVING and Proper uses of HAVING ?

HAVING lets you apply a filter on an aggregate.

SELECT COUNT(column)
FROM Products
WHERE Active=1
GROUP By type
HAVING COUNT(*) > 1


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Phrasing it differently:

The clauses are performed in below order:
WHERE
GROUP BY
HAVING

A later clause will operate on the output from the immediate earlier clause.

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HAVING is applied after the results are aggregated (grouped). WHERE is
applied before. If the HAVING clause references only the columns that
are included in the GROUP BY list then it is logically equivalent to
WHERE. Usually, however, HAVING refers to some expression including an
aggregate function - something you can't do in the WHERE clause.

Example (from the Northwind database):

/* Customers who have made more than 20 orders */

SELECT customerid
FROM Orders
GROUP BY customerid
HAVING COUNT(*)>20 ;

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