PHP and MySQL Web Development 4th Ed-tqw-_darksiderg

256 Chapter 10 Working with Your MySQL Database
In addition, you can sort on more than one column.You can also use column aliases or
even their position numbers (for example, 3 is the third column in the table) instead of
names.
Grouping and Aggregating Data
You may often want to know how many rows fall into a particular set or the average
value of some column—say, the average dollar value per order. MySQL has a set of
aggregate functions that are useful for answering this type of query.
These aggregate functions can be applied to a table as a whole or to groups of data
within a table.The most commonly used ones are listed in Table 10.3.
Table 10.3
Name
Aggregate Functions in MySQL
Description
AVG(column)
COUNT(items)
MIN(column)
MAX(column)
STD(column)
STDDEV(column)
SUM(column)
Average of values in the specified column.
If you specify a column, this will give you the number of non-NULL
values in that column. If you add the word DISTINCT in front of the
column name, you will get a count of the distinct values in that column
only. If you specify COUNT(*), you will get a row count regardless
of NULL values.
Minimum of values in the specified column.
Maximum of values in the specified column.
Standard deviation of values in the specified column.
Same as STD(column).
Sum of values in the specified column.
Let’s look at some examples, beginning with the one mentioned earlier.You can calculate
the average total of an order like this:
select avg(amount)
from orders;
The output is something like this:
+-------------+
| avg(amount) |
+-------------+
| 54.985002 |
+-------------+
To get more detailed information, you can use the GROUP BY clause. It enables you to
view the average order total by group—for example, by customer number.This information
tells you which of your customers place the biggest orders: