What function counts the number of rows in a column?
Explanation
COUNT() function returns the number of rows that match specified criteria.
Functions NUMBER(), TOTAL(), and ADD() do not exist in SQL.
Theory
• #### SQL HAVING Example

``````
|OrderID |EmployeeID|ShipperID  |
+--------+----------+-----------+
|10248	 | 5        |	3       |
+--------+----------+-----------+
|10249	 | 6        |	1       |
+--------+----------+-----------+
|10250	 | 4        |	2       |
``````
And a selection from the "Employees" table:
``````
|EmployeeID |LastName |
+-----------+---------+
|1          |Davolio  |
+-----------+---------+
|2          |Fuller   |
+-----------+---------+
|3          |Leverling|
``````
Now we want to find if any of the employees has registered more than 10 orders.
We use the following SQL statement:
``````SELECT Employees.LastName, COUNT(Orders.OrderID) AS NumberOfOrders FROM (Orders
INNER JOIN Employees
ON Orders.EmployeeID=Employees.EmployeeID)
GROUP BY LastName
HAVING COUNT(Orders.OrderID) > 10;
``````
• #### SQL HAVING Example

``````
|OrderID |EmployeeID|ShipperID  |
+--------+----------+-----------+
|10248	 | 5        |	3       |
+--------+----------+-----------+
|10249	 | 6        |	1       |
+--------+----------+-----------+
|10250	 | 4        |	2       |
``````
And a selection from the "Employees" table:
``````
|EmployeeID |LastName |
+-----------+---------+
|1          |Davolio  |
+-----------+---------+
|2          |Fuller   |
+-----------+---------+
|3          |Leverling|
``````
Now we want to find if the employees "Davolio" or "Fuller" have registered more than 25 orders.
We add an ordinary WHERE clause to the SQL statement:
``````SELECT Employees.LastName, COUNT(Orders.OrderID) AS NumberOfOrders FROM Orders
INNER JOIN Employees
ON Orders.EmployeeID=Employees.EmployeeID
WHERE LastName='Davolio' OR LastName='Fuller'
GROUP BY LastName
HAVING COUNT(Orders.OrderID) > 25;
``````
• #### SQL Aggregate Functions

SQL aggregate functions return a single value, calculated from values in a column.
Useful aggregate functions:
• AVG() - Returns the average value
• COUNT() - Returns the number of rows
• FIRST() - Returns the first value
• LAST() - Returns the last value
• MAX() - Returns the largest value
• MIN() - Returns the smallest value
• SUM() - Returns the sum