Sorting and Removing Duplicates

Estimated time: 20 minutes

Overview

Questions

• How can I sort a query’s results?
• How can I remove duplicate values from a query’s results?

Objectives

• Write queries that display results in a particular order.
• Write queries that eliminate duplicate values from data.

In beginning our examination of the Antarctic data, we want to know:

• what kind of quantity measurements were taken at each site;
• which scientists took measurements on the expedition;

To determine which measurements were taken at each site, we can examine the Survey table. Data is often redundant, so queries often return redundant information. For example, if we select the quantities that have been measured from the Survey table, we get this:

SQL

SELECT quant FROM Survey;

quant
rad
sal
rad
sal
rad
sal
temp
rad
sal
temp
rad
temp
sal
rad
sal
temp
sal
rad
sal
sal
rad

This result makes it difficult to see all of the different types of quant in the Survey table. We can eliminate the redundant output to make the result more readable by adding the DISTINCT keyword to our query:

SQL

SELECT DISTINCT quant FROM Survey;

quant
rad
sal
temp

If we want to determine which visit (stored in the taken column) have which quant measurement, we can use the DISTINCT keyword on multiple columns. If we select more than one column, distinct sets of values are returned (in this case pairs, because we are selecting two columns):

SQL

SELECT DISTINCT taken, quant FROM Survey;

taken	quant
619	rad
619	sal
622	rad
622	sal
734	rad
734	sal
734	temp
735	rad
735	sal
735	temp
751	rad
751	temp
751	sal
752	rad
752	sal
752	temp
837	rad
837	sal
844	rad

Notice in both cases that duplicates are removed even if the rows they come from didn’t appear to be adjacent in the database table.

Our next task is to identify the scientists on the expedition by looking at the Person table. As we mentioned earlier, database records are not stored in any particular order. This means that query results aren’t necessarily sorted, and even if they are, we often want to sort them in a different way, e.g., by their identifier instead of by their personal name. We can do this in SQL by adding an ORDER BY clause to our query:

SQL

SELECT * FROM Person ORDER BY id;

id	personal	family
danfort	Frank	Danforth
dyer	William	Dyer
lake	Anderson	Lake
pb	Frank	Pabodie
roe	Valentina	Roerich

By default, when we use ORDER BY, results are sorted in ascending order of the column we specify (i.e., from least to greatest).

We can sort in the opposite order using DESC (for “descending”):

A note on ordering

While it may look that the records are consistent every time we ask for them in this lesson, that is because no one has changed or modified any of the data so far. Remember to use ORDER BY if you want the rows returned to have any sort of consistent or predictable order.

SQL

SELECT * FROM person ORDER BY id DESC;

id	personal	family
roe	Valentina	Roerich
pb	Frank	Pabodie
lake	Anderson	Lake
dyer	William	Dyer
danfort	Frank	Danforth

(And if we want to make it clear that we’re sorting in ascending order, we can use ASC instead of DESC.)

In order to look at which scientist measured quantities during each visit, we can look again at the Survey table. We can also sort on several fields at once. For example, this query sorts results first in ascending order by taken, and then in descending order by person within each group of equal taken values:

SQL

SELECT taken, person, quant FROM Survey ORDER BY taken ASC, person DESC;

taken	person	quant
619	dyer	rad
619	dyer	sal
622	dyer	rad
622	dyer	sal
734	pb	rad
734	pb	temp
734	lake	sal
735	pb	rad
735	-null-	sal
735	-null-	temp
751	pb	rad
751	pb	temp
751	lake	sal
752	roe	sal
752	lake	rad
752	lake	sal
752	lake	temp
837	roe	sal
837	lake	rad
837	lake	sal
844	roe	rad

This query gives us a good idea of which scientist was involved in which visit, and what measurements they performed during the visit.

Looking at the table, it seems like some scientists specialized in certain kinds of measurements. We can examine which scientists performed which measurements by selecting the appropriate columns and removing duplicates.

SQL

SELECT DISTINCT quant, person FROM Survey ORDER BY quant ASC;

quant	person
rad	dyer
rad	pb
rad	lake
rad	roe
sal	dyer
sal	lake
sal	-null-
sal	roe
temp	pb
temp	-null-
temp	lake

Finding Distinct Dates

Write a query that selects distinct dates from the Visited table.

Show me the solution

SQL

SELECT DISTINCT dated FROM Visited;

dated
1927-02-08
1927-02-10
1930-01-07
1930-01-12
1930-02-26
1932-01-14
1932-03-22

Displaying Full Names

Write a query that displays the full names of the scientists in the Person table, ordered by family name.

Show me the solution

SQL

SELECT personal, family FROM Person ORDER BY family ASC;

personal	family
Frank	Danforth
William	Dyer
Anderson	Lake
Frank	Pabodie
Valentina	Roerich

Key Points

• The records in a database table are not intrinsically ordered: if we want to display them in some order, we must specify that explicitly with ORDER BY.
• The values in a database are not guaranteed to be unique: if we want to eliminate duplicates, we must specify that explicitly as well using DISTINCT.