SQL aggregate functions allow us to answer the statistical inquiries about a set of data. For example, the users of your sale database system might want know the mean of the sales per year or the maximum and minimum sales of a quarter. The common SQL aggregate functions are AVG, MAX, MIN, COUNT and SUM.
AVG Function: This function is used to calculate the average or the arithmetic mean of data values in a field of a table.
Example: Calculate the average of quantity of products in the TblProduct table below. Its syntax is AVG(fieldname)
TblProduct table:
ProID  ProName  UnitPrice  Qty  CatID 
P001  Angkor  20$  10  Cat001 
P002  Anchor  19$  15  Cat002 
P003  ABC  50$  8  Cat003 
FROM TblProduct;
Result:
AvgQty 
11 
MAX Function: The MAX function is used to find the maximum value among the data value of a field of a table. Its syntax is MAX(fieldname).
Example: Find the maximum price of products in the TblProduct.
SELECT MAX(UnitPrice) AS Max_price
FROM TblProducts;
Result:
Max_price 
50$ 
MIN Function: This function is able to calculate the minimum value of the data value in field of a table. Its syntax is MIN(fieldname).
Example: Find the minumum quantity of products in the TblProduct.
SELECT MIN(Qty) AS Min_qty
FROM TblProducts;
Result:
Min_qty 
8 
Count Function: This function can count the number of records(rows) in a table. To count all rows in the table although some data in rows are missed, you can use COUNT(*). If you want to count only the rows which contain data you can use COUNT(fieldname) syntax.
Example: Count the products that are in the category id of Cat002.
SELECT COUNT(ProID) AS Countrows
FROM TblProducts
WHERE CatID='Cat002';
Result:
Countrows 
1 
SUM Function: This function is used to calculate the total of data values of a field of table. Its syntax is SUM(fieldname).
Example: calculate the total quantity of the products in the TblProduct.
SELECT SUM(Qty) AS Totalqty
FROM TblProduct;
Result:
Totalqty 
33 

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