Using mysql as a Calculator

1.32.1 Problem

You need a quick way to evaluate an expression.

1.32.2 Solution

Use mysql as a calculator. MySQL doesn't require every SELECT statement to refer to a table, so you can select the results of arbitrary expressions.

1.32.3 Discussion

SELECT statements typically refer to some table or tables from which you're retrieving rows. However, in MySQL, SELECT need not reference any table at all, which means that you can use the mysql program as a calculator for evaluating an expression:

mysql> SELECT (17 + 23) / SQRT(64);
| (17 + 23) / SQRT(64) |
| 5.00000000 |

This is also useful for checking how a comparison works. For example, to determine whether or not string comparisons are case sensitive, try the following query:

mysql> SELECT 'ABC' = 'abc';
| 'ABC' = 'abc' |
| 1 |

The result of this comparison is 1 (meaning "true"; in general, nonzero values are true). This tells you that string comparisons are not case sensitive by default. Expressions that evaluate to false return zero:

mysql> SELECT 'ABC' = 'abcd';
| 'ABC' = 'abcd' |
| 0 |

If the value of an expression cannot be determined, the result is NULL:

mysql> SELECT 1/0;
| 1/0 |
| NULL |

SQL variables may be used to store the results of intermediate calculations. The following statements use variables this way to compute the total cost of a hotel bill:

mysql> SET @daily_room_charge = 100.00;
mysql> SET @num_of_nights = 3;
mysql> SET @tax_percent = 8;
mysql> SET @total_room_charge = @daily_room_charge * @num_of_nights;
mysql> SET @tax = (@total_room_charge * @tax_percent) / 100;
mysql> SET @total = @total_room_charge + @tax;
mysql> SELECT @total;
| @total |
| 324 |

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 © 2008-2020.
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