PHP

Sorting and Grouping Output

We will now discuss techniques to manage the order and grouping of the output.

ORDER BY

The ORDER BY clause sorts the data after the query has been evaluated. Consider an example:

SELECT surname, firstname FROM customer
  WHERE title='Mr'
  AND city = 'Portsea'
  ORDER by surname;

This query finds all customers who have a title Mr and live in Portsea. It then presents the results sorted alphabetically by ascending surname:

+-----------+-----------+
| surname   | firstname |
+-----------+-----------+
| Dalion    | Anthony   |
| Galti     | Jim       |
| Keisling  | Mark      |
| Leramonth | James     |
| Mellili   | Derryn    |
| Mockridge | James     |
| Nancarral | Joshua    |
| Ritterman | James     |
+-----------+-----------+
8 rows in set (0.01 sec)

Sorting can be on multiple attributes. For example:

SELECT surname, firstname, initial FROM customer
  WHERE zipcode='3001' OR
        zipcode='3000'
  ORDER BY surname, firstname, initial;

This presents a list of customers in areas with zipcode='3000' or zipcode='3001', sorted first by ascending surname, then (for those customers with the same surname) by firstname, and (for those customers with the same surname and first name), by initial. So, for example, the output may be:

+-----------+-----------+---------+
| surname   | firstname | initial |
+-----------+-----------+---------+
| Keisling  | Belinda   | C       |
| Leramonth | Alexa      | D       |
| Leramonth | Joshua    | H       |
| Leramonth | Joshua    | R       |
| Young     | Bob       | A       |
+-----------+-----------+---------+
5 rows in set (0.11 sec)

By default, the ORDER BY clause sorts in ascending order, or ASC. To sort in reverse or descending order, DESC can be used. Consider an example:

SELECT * FROM customer
  WHERE city='Melbourne'
  ORDER BY surname DESC;
GROUP BY

The GROUP BY clause is different from ORDER BY because it doesn't sort the data for output. Instead, it sorts the data early in the query process, for the purpose of grouping or aggregation. An example shows the difference:

SELECT city, COUNT(*) FROM customer
  GROUP BY city;

This query outputs a sorted list of cities and, for each city, the COUNT of the number of customers who live in that city. The effect of COUNT(*) is to count the number of rows per group. In this example, it doesn't matter what is counted; COUNT(surname) has exactly the same result.

Here are the first few lines output by the query:

+--------------+----------+
| city         | COUNT(*) |
+--------------+----------+
| Alexandra    |       14 |
| Armidale     |        7 |
| Athlone      |        9 |
| Bauple       |        6 |
| Belmont      |       11 |
| Bentley      |       10 |
| Berala       |        9 |
| Broadmeadows |       11 |

The query aggregates or groups all the rows for each city into sets, and the COUNT(*) operation counts the number in each set. So, for example, there are 14 customers who live in Alexandra.

The GROUP BY clause can find different properties of the aggregated rows. Here's an example:

SELECT city, MAX(salary) FROM customer
  GROUP BY city;

This query first groups the rows by city and then shows the maximum salary in each city. The first few rows of the output are as follows:

+-----------+-------------+
| city      | MAX(salary) |
+-----------+-------------+
| Alexandra |      109000 |
| Armidale  |       75000 |
| Athlone   |       84000 |
| Bauple    |       33000 |

The GROUP BY clause should be used only when the query is designed to find a characteristic of a group of rows, not the details of individual rows.

There are several functions that can be used in aggregation with the GROUP BY clause. Five particularly useful functions are:

AVG( )

Finds the average value of a numeric attribute in a set

MIN( )

Finds a minimum value of a string or numeric attribute in a set

MAX( )

Finds a maximum value of a string or numeric attribute in a set

SUM( )

Finds the sum total of a numeric attribute

COUNT( )

Counts the number of rows in a set

The SQL standard places a constraint on the GROUP BY clause that MySQL doesn't enforce. In the standard, all attributes that are selected (i.e., appear after the SELECT statement) must appear in the GROUP BY clause. Most examples in this chapter don't meet this unnecessary constraint of the SQL standard.

HAVING

The HAVING clause permits conditional aggregation of data into groups. For example, consider the following query:

SELECT city, count(*), max(salary)
  FROM customer
  GROUP BY city
  HAVING count(*) > 10;

The query groups rows by city, but only for cities that have more than 10 resident customers. For those groups, the city, count( ) of customers, and maximum salary of a customer in that city is output. Cities with less than 10 customers are omitted from the result set. The first few rows of the output are as follows:

+--------------+----------+-------------+
| city         | count(*) | max(salary) |
+--------------+----------+-------------+
| Alexandra    |       14 |      109000 |
| Belmont      |       11 |       71000 |
| Broadmeadows |       11 |       51000 |
| Doveton      |       13 |       77000 |
| Eleker       |       11 |       97000 |
| Gray         |       12 |       77000 |

The HAVING clause must contain an attribute or expression from the SELECT clause.

The HAVING clause is used exclusively with the GROUP BY clause. It is slow and should never be used instead of a WHERE clause.

DISTINCT

The DISTINCT operator presents only one example of each row from a query. Consider an example:

SELECT DISTINCT surname FROM customer;

This shows one example of each different customer surname in the customer table. This example has exactly the same effect as:

SELECT surname FROM customer GROUP BY surname;

The DISTINCT clause is usually slow to run, much like the GROUP BY clause. We discuss how indexes and query optimization can speed queries later in this chapter.

Limiting Result Sets in MySQL

An additional operator is available in MySQL that limits the size of the result sets. For example, the following query returns only the first five rows from the wine table:

SELECT * FROM wine LIMIT 5;

This saves query evaluation time and reduces the size of the result set that must be buffered by the DBMS. The LIMIT operator is MySQL-specific.

by BrainBellupdated
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