2.
Aggregate Functions
ISO standard defines five aggregate functions:
1. COUNT returns number of values in specified
column.
2. SUM returns sum of values in specified column.
3. AVG returns average of values in specified
column.
4. MIN returns smallest value in specified column.
5. MAX returns largest value in specified column.
Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh 2
3.
Things to remember
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4.
Count Function
The COUNT() function returns the number of rows that
matches a specified criteria.
The COUNT(column_name) function returns the number of
values (NULL values will not be counted) of the specified
column:
Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh 4
5.
Count Function
Now we want to count the number of orders from "Customer
Nilsen".
We use the following SQL statement:
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6.
Count Function
The COUNT(*) function returns the number of records in a
table:
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7.
Count Function
which is the total number of rows in the table.
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8.
Count Function
The COUNT(DISTINCT column_name) function returns the
number of distinct values of the specified column:
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9.
Count Function
which is the number of unique customers (Hansen, Nilsen, and
Jensen) in the "Orders" table.
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10.
Sum Function
Now we want to find the sum of all "OrderPrice" fields".
We use the following SQL statement:
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11.
Avg Function
Now we want to find the average value of the "OrderPrice"
fields.
We use the following SQL statement:
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12.
Max Function
Now we want to find the largest value of the "OrderPrice"
column.
We use the following SQL statement:
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13.
Min Function
Now we want to find the smallest value of the "OrderPrice"
column.
We use the following SQL statement:
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14.
Group By Clause
The GROUP BY statement is used in conjunction with the
aggregate functions to group the result-set by one or more
columns.
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15.
Group By Clause
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Now we want to find the total sum (total order) of each
customer.
We will have to use the GROUP BY statement to group the
customers.
We use the following SQL statement:
16.
Group By Clause
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17.
Group By Clause
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If we omit the group by clause from the statement-
18.
Things to remember
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19.
Having Clause
The HAVING clause was added to SQL because the WHERE
keyword could not be used with aggregate functions.
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20.
Having Clause
Now we want to find if any of the customers have a total
order of less than 2000.
We use the following SQL statement:
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21.
Having Clause
Now we want to find if the customers "Hansen" or "Jensen"
have a total order of more than 1500.
We add an ordinary WHERE clause to the SQL statement:
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22.
Things to remember
A HAVING condition can refer only to an
expression in the SELECT list, or to an
expression involving an aggregate function.
If you specify an expression in the HAVING
clause that isn't in the SELECT list, or that
isn't an aggregate expression, you will get
an error.
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23.
Reference
World Wide Web Schools,
http://www.w3schools.com/sql/sql_functions.asp
[05/04/2009]
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