Databases and SQL

Calculating New Values

Overview

Teaching: 5 min
Exercises: 5 min
Questions
  • How can I calculate new values on the fly?

Objectives
  • Write queries that calculate new values for each selected record.

After carefully re-reading the expedition logs, we realize that the radiation measurements they report may need to be corrected upward by 5%. Rather than modifying the stored data, we can do this calculation on the fly as part of our query:

SELECT 1.05 * reading FROM Survey WHERE quant='rad';
1.05 * reading
10.311
8.19
8.8305
7.581
4.5675
2.2995
1.533
11.8125

When we run the query, the expression 1.05 * reading is evaluated for each row. Expressions can use any of the fields, all of usual arithmetic operators, and a variety of common functions. (Exactly which ones depends on which database manager is being used.) For example, we can convert temperature readings from Fahrenheit to Celsius and round to two decimal places:

SELECT taken, round(5*(reading-32)/9, 2) FROM Survey WHERE quant='temp';
taken round(5*(reading-32)/9, 2)
734 -29.72
735 -32.22
751 -28.06
752 -26.67

As you can see from this example, though, the string describing our new field (generated from the equation) can become quite unwieldy. SQL allows us to rename our fields, any field for that matter, whether it was calculated or one of the existing fields in our database, for succinctness and clarity. For example, we could write the previous query as:

SELECT taken, round(5*(reading-32)/9, 2) as Celsius FROM Survey WHERE quant='temp';
taken Celsius
734 -29.72
735 -32.22
751 -28.06
752 -26.67

We can also combine values from different fields, for example by using the string concatenation operator ||:

SELECT personal || ' ' || family FROM Person;
personal   ’ ‘   family
William Dyer        
Frank Pabodie        
Anderson Lake        
Valentina Roerich        
Frank Danforth        

Fixing Salinity Readings

After further reading, we realize that Valentina Roerich was reporting salinity as percentages. Write a query that returns all of her salinity measurements from the Survey table with the values divided by 100.

Solution

SELECT taken, reading / 100 FROM Survey WHERE person='roe' AND quant='sal';
taken reading / 100
752 0.416
837 0.225

Unions

The UNION operator combines the results of two queries:

SELECT * FROM Person WHERE id='dyer' UNION SELECT * FROM Person WHERE id='roe';
id personal family
dyer William Dyer
roe Valentina Roerich

Use UNION to create a consolidated list of salinity measurements in which Valentina Roerich’s, and only Valentina’s, have been corrected as described in the previous challenge. The output should be something like:

taken reading
619 0.13
622 0.09
734 0.05
751 0.1
752 0.09
752 0.416
837 0.21
837 0.225

Solution

SELECT taken,reading FROM Survey WHERE person!='roe' AND quant='sal' UNION SELECT taken,reading / 100 FROM Survey WHERE person='roe' AND quant='sal' ORDER BY taken ASC;

Selecting Major Site Identifiers

The site identifiers in the Visited table have two parts separated by a ‘-‘:

SELECT DISTINCT site FROM Visited;
site
DR-1
DR-3
MSK-4

Some major site identifiers (i.e. the letter codes) are two letters long and some are three. The “in string” function instr(X, Y) returns the 1-based index of the first occurrence of string Y in string X, or 0 if Y does not exist in X. The substring function substr(X, I, [L]) returns the substring of X starting at index I, with an optional length L. Use these two functions to produce a list of unique major site identifiers. (For this data, the list should contain only “DR” and “MSK”).

Solution

SELECT DISTINCT substr(site, 1, instr(site, '-') - 1) AS MajorSite FROM Visited;

Key Points