Working with Per-Group and Overall Summary Values Simultaneously

7.17.1 Problem

You want to produce a report that requires different levels of summary detail. Or you want to compare per-group summary values to an overall summary value.

7.17.2 Solution

Use two queries that retrieve different levels of summary information. Or use a programming language to do some of the work so that you can use a single query.

7.17.3 Discussion

Sometimes a report involves different levels of summary information. For example, the following report displays the total number of miles per driver from the driver_log table, along with each driver's miles as a percentage of the total miles in the entire table:

| name | miles/driver | percent of total miles |
| Ben | 362 | 16.712834718375 |
| Henry | 911 | 42.059095106187 |
| Suzi | 893 | 41.228070175439 |

The percentages represent the ratio of each driver's miles to the total miles for all drivers. To perform the percentage calculation, you need a per-group summary to get each driver's miles and also an overall summary to get the total miles. Generating the report in SQL involves a couple of queries, because you can't calculate a per-group summary and an overall summary in a single query.[2] First, run a query to get the overall mileage total:

[2] Well... that's not strictly true. With a subselect, you could generate the summary with a single query. But MySQL won't have subselects until Version 4.1.

mysql> SELECT @total := SUM(miles) AS 'total miles' FROM driver_log;
| total miles |
| 2166 |

Then calculate the per-group values and use the overall total to compute the percentages:

mysql> SELECT name,
 -> SUM(miles) AS 'miles/driver',
 -> (SUM(miles)*100)/@total AS 'percent of total miles'
 -> FROM driver_log GROUP BY name;
| name | miles/driver | percent of total miles |
| Ben | 362 | 16.712834718375 |
| Henry | 911 | 42.059095106187 |
| Suzi | 893 | 41.228070175439 |

A different form of multiple-query solution that doesn't involve a variable is to retrieve the overall summary into another table, then join that with the original table:

 -> SELECT SUM(miles) AS total FROM driver_log;
mysql> SELECT driver_log.name,
 -> SUM(driver_log.miles) AS 'miles/driver',
 -> (SUM(driver_log.miles)*100)/t.total AS 'percent of total miles'
 -> FROM driver_log, t GROUP BY driver_log.name;
| name | miles/driver | percent of total miles |
| Ben | 362 | 16.71 |
| Henry | 911 | 42.06 |
| Suzi | 893 | 41.23 |

If you're generating the report from within a program, you can do some of the summary math using your programming language and eliminate one of the queries. Here's an example in Python:

# issue query to calculate per-driver totals
cursor = conn.cursor ( )
cursor.execute ("SELECT name, SUM(miles) FROM driver_log GROUP BY name")
rows = cursor.fetchall ( )
cursor.close ( )

# iterate once through result to calculate overall total miles
total = 0
for (name, miles) in rows:
 total = total + miles

# iterate again to print report
print "name miles/driver percent of total miles"
for (name, miles) in rows:
 print "%-8s %5d %f" 
 % (name, miles, (100*miles)/total)

Another type of problem that uses different levels of summary information occurs when you want to compare per-group summary values with the corresponding overall summary value. Suppose you want to determine which drivers had a lower average miles per day than the group average. Using only SQL, this task can't be performed with a single query, but you can easily do it with two. First, calculate the overall average and save it in a variable:

mysql> SELECT @overall_avg := AVG(miles) FROM driver_log;
| @overall_avg := AVG(miles) |
| 216.6000 |

Then compare each driver's average to the saved value using a HAVING clause:

mysql> SELECT name, AVG(miles) AS driver_avg FROM driver_log
 -> GROUP BY name
 -> HAVING driver_avg < @overall_avg;
| name | driver_avg |
| Ben | 120.6667 |
| Henry | 182.2000 |

Just as when producing a report that uses different levels of summary information, you can solve this problem without using two queries if you're writing a program by using your programming language to do some of the work:

  1. Issue a query to retrieve the per-group summary information.
  2. Iterate through the result set once to calculate the overall summary value.
  3. Iterate through the result set again, comparing each per-group summary value to the overall value and displaying only those records for which the comparison succeeds.

Using the mysql Client Program

Writing MySQL-Based Programs

Record Selection Techniques

Working with Strings

Working with Dates and Times

Sorting Query Results

Generating Summaries

Modifying Tables with ALTER TABLE

Obtaining and Using Metadata

Importing and Exporting Data

Generating and Using Sequences

Using Multiple Tables

Statistical Techniques

Handling Duplicates

Performing Transactions

Introduction to MySQL on the Web

Incorporating Query Resultsinto Web Pages

Processing Web Input with MySQL

Using MySQL-Based Web Session Management

Appendix A. Obtaining MySQL Software

Appendix B. JSP and Tomcat Primer

Appendix C. References

MySQL Cookbook
MySQL Cookbook
ISBN: 059652708X
EAN: 2147483647
Year: 2005
Pages: 412
Authors: Paul DuBois

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