Calculating New Values
Last updated on 2023-05-08 | Edit this page
Estimated time: 10 minutes
Overview
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:
| 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:
| 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:
| 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 ||:
| personal |
|---|
| 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.
Unions
The UNION operator combines the results of two
queries:
| 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:
| 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 |
Selecting Major Site Identifiers
The site identifiers in the Visited table have two parts
separated by a ‘-’:
| 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”).
Key Points
- Queries can do the usual arithmetic operations on values.
- Use UNION to combine the results of two or more queries.