Sorting and Removing Duplicates
Last updated on 2023-05-08 | Edit this page
- How can I sort a query’s results?
- How can I remove duplicate values from a query’s results?
- 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
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
table, we get this:
SELECT quant FROM Survey;
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:
SELECT DISTINCT quant FROM Survey;
If we want to determine which visit (stored in the
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):
SELECT DISTINCT taken, quant FROM Survey;
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:
SELECT * FROM Person ORDER BY id;
By default, when we use
ORDER BY, results are sorted in
ascending order of the column we specify (i.e., from least to
We can sort in the opposite order using
SELECT * FROM person ORDER BY id DESC;
(And if we want to make it clear that we’re sorting in ascending
order, we can use
ASC instead of
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
person within each group of equal
SELECT taken, person, quant FROM Survey ORDER BY taken ASC, person DESC;
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.
SELECT DISTINCT quant, person FROM Survey ORDER BY quant ASC;
SELECT DISTINCT dated FROM Visited;
SELECT personal, family FROM Person ORDER BY family ASC;
- 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.