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';

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 visited_id, round(5 * (reading - 32) / 9, 2) FROM Survey WHERE quant = 'temp';

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 visited_id, round(5 * (reading - 32) / 9, 2) as Celsius FROM Survey WHERE quant = 'temp';

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

SELECT personal || ' ' || family FROM Person;

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 visited_id, reading / 100 FROM Survey WHERE person_id = 'roe' AND quant = 'sal';

visited_id	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

The UNION ALL command is equivalent to the UNION operator, except that UNION ALL will select all values. The difference is that UNION ALL will not eliminate duplicate rows. Instead, UNION ALL pulls all rows from the query specifics and combines them into a table. The UNION command does a SELECT DISTINCT on the results set. If all the records to be returned are unique from your union, use UNION ALL instead, it gives faster results since it skips the DISTINCT step. For this section, we shall use UNION.

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:

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

Selecting Major Site Identifiers

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

SELECT DISTINCT site_id FROM Visited;

site_id
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_id, 1, instr(site_id, '-') - 1) AS MajorSite FROM Visited;

Key Points

• Queries can do the usual arithmetic operations on values.

• Use UNION to combine the results of two or more queries.