Introduction to SQL
Author(s) | The Carpentries Helena Rasche Donny Vrins Bazante Sanders Avans |
Reviewers |
OverviewQuestions:Objectives:
How can I get data from a database?
How can I sort a query’s results?
How can I remove duplicate values from a query’s results?
How can I select subsets of data?
How can I calculate new values on the fly?
How do databases represent missing information?
What special handling does missing information require?
Requirements:
Explain the difference between a table, a record, and a field.
Explain the difference between a database and a database manager.
Write a query to select all values for specific fields from a single table.
Write queries that display results in a particular order.
Write queries that eliminate duplicate values from data.
Write queries that select records that satisfy user-specified conditions.
Explain the order in which the clauses in a query are executed.
Write queries that calculate new values for each selected record.
Explain how databases represent missing information.
Explain the three-valued logic databases use when manipulating missing information.
Write queries that handle missing information correctly.
Time estimation: 3 hoursLevel: Introductory IntroductorySupporting Materials:Published: Oct 11, 2021Last modification: Nov 16, 2023License: Tutorial Content is licensed under Creative Commons Attribution 4.0 International License. The GTN Framework is licensed under MITpurl PURL: https://gxy.io/GTN:T00107version Revision: 7
Best viewed in a Jupyter NotebookThis tutorial is best viewed in a Jupyter notebook! You can load this notebook one of the following ways
Launching the notebook in Jupyter in Galaxy
- Instructions to Launch JupyterLab
- Open a Terminal in JupyterLab with File -> New -> Terminal
- Run
wget https://training.galaxyproject.org/training-material/topics/data-science/tutorials/sql-basic/data-science-sql-basic.ipynb
- Select the notebook that appears in the list of files on the left.
Downloading the notebook
- Right click one of these links: Jupyter Notebook (With Solutions), Jupyter Notebook (Without Solutions)
- Save Link As..
This tutorial will introduce you to Structured Query Language (SQL) which can be used to query databases!
CommentThis tutorial is significantly based on the Carpentries Databases and SQL lesson, which is licensed CC-BY 4.0.
Abigail Cabunoc and Sheldon McKay (eds): “Software Carpentry: Using Databases and SQL.” Version 2017.08, August 2017, github.com/swcarpentry/sql-novice-survey, https://doi.org/10.5281/zenodo.838776
Adaptations have been made to make this work better in a GTN/Galaxy environment.
AgendaIn this tutorial, we will cover:
# This preamble sets up the sql "magic" for jupyter. Use %%sql in your cells to write sql!
!python3 -m pip install ipython-sql sqlalchemy
!wget -c http://swcarpentry.github.io/sql-novice-survey/files/survey.db
import sqlalchemy
engine = sqlalchemy.create_engine("sqlite:///survey.db")
%load_ext sql
%sql sqlite:///survey.db
%config SqlMagic.displaycon=False
Selecting Data
A relational database is a way to store and manipulate information. Databases are arranged as table. Each table has columns (also known as fields) that describe the data, and rows (also known as records) which contain the data.
When we are using a spreadsheet, we put formulas into cells to calculate new values based on old ones. When we are using a database, we send commands (usually called queries) to a database manager: a program that manipulates the database for us. The database manager does whatever lookups and calculations the query specifies, returning the results in a tabular form that we can then use as a starting point for further queries.
Queries are written in a language called SQL, SQL provides hundreds of different ways to analyze and recombine data. We will only look at a handful of queries, but that handful accounts for most of what scientists do.
Many database managers — Oracle, IBM DB2, PostgreSQL, MySQL, Microsoft Access, and SQLite — understand SQL but each stores data in a different way, so a database created with one cannot be used directly by another. However, every database manager can import and export data in a variety of formats like .csv, SQL, so it is possible to move information from one to another.
Before we get into using SQL to select the data, let’s take a look at the tables of the database we will use in our examples:
Person: people who took readings.
id | personal | family |
---|---|---|
dyer | William | Dyer |
pb | Frank | Pabodie |
lake | Anderson | Lake |
roe | Valentina | Roerich |
danforth | Frank | Danforth |
Site: locations where readings were taken.
name | lat | long |
---|---|---|
DR-1 | -49.85 | -128.57 |
DR-3 | -47.15 | -126.72 |
MSK-4 | -48.87 | -123.4 |
Visited: when readings were taken at specific sites.
id | site | dated |
---|---|---|
619 | DR-1 | 1927-02-08 |
622 | DR-1 | 1927-02-10 |
734 | DR-3 | 1930-01-07 |
735 | DR-3 | 1930-01-12 |
751 | DR-3 | 1930-02-26 |
752 | DR-3 | None |
837 | MSK-4 | 1932-01-14 |
844 | DR-1 | 1932-03-22 |
Survey: the actual readings. The field quant
is short for quantitative and indicates what is being measured. Values are rad
, sal
, and temp
referring to ‘radiation’, ‘salinity’ and ‘temperature’, respectively.
taken | person | quant | reading |
---|---|---|---|
619 | dyer | rad | 9.82 |
619 | dyer | sal | 0.13 |
622 | dyer | rad | 7.8 |
622 | dyer | sal | 0.09 |
734 | pb | rad | 8.41 |
734 | lake | sal | 0.05 |
734 | pb | temp | -21.5 |
735 | pb | rad | 7.22 |
735 | None | sal | 0.06 |
735 | None | temp | -26.0 |
751 | pb | rad | 4.35 |
751 | pb | temp | -18.5 |
751 | lake | sal | 0.1 |
752 | lake | rad | 2.19 |
752 | lake | sal | 0.09 |
752 | lake | temp | -16.0 |
752 | roe | sal | 41.6 |
837 | lake | rad | 1.46 |
837 | lake | sal | 0.21 |
837 | roe | sal | 22.5 |
844 | roe | rad | 11.25 |
Notice that three entries — one in the Visited
table,
and two in the Survey
table — don’t contain any actual
data, but instead have a special None
entry:
we’ll return to these missing values.
For now,
let’s write an SQL query that displays scientists’ names.
We do this using the SQL command SELECT
,
giving it the names of the columns we want and the table we want them from.
Our query and its output look like this:
SELECT family, personal FROM Person;
The semicolon at the end of the query tells the database manager that the query is complete and ready to run. We have written our commands in upper case and the names for the table and columns in lower case, but we don’t have to: as the example below shows, SQL is case insensitive.
SeLeCt FaMiLy, PeRsOnAl FrOm PeRsOn;
You can use SQL’s case insensitivity to your advantage. For instance,
some people choose to write SQL keywords (such as SELECT
and FROM
)
in capital letters and field and table names in lower
case. This can make it easier to locate parts of an SQL statement. For
instance, you can scan the statement, quickly locate the prominent
FROM
keyword and know the table name follows. Whatever casing
convention you choose, please be consistent: complex queries are hard
enough to read without the extra cognitive load of random
capitalization. One convention is to use UPPER CASE for SQL
statements, to distinguish them from tables and column names. This is
the convention that we will use for this lesson.
Question: Is a personal and family name column a good design?If you were tasked with designing a database to store this same data, is storing the name data in this way the best way to do it? Why or why not?
Can you think of any names that would be difficult to enter in such a schema?
No, it is generally not. There are a lot of falsehoods that programmers believe about names. The situation is much more complex as you can read in that article, but names vary wildly and generally placing constraints on how names are entered is only likely to frustrate you or your users later on when they need to enter data into that database.
In general you should consider using a single text field for the name and allowing users to specify them as whatever they like (if it is a system with registration), or asking what they wish to be recorded (if you are doing this sort of data collection).
If you are doing scientific research, you might know that names are generally very poor identifiers of a single human, and in that case consider recording their ORCiD which will help you reference that individual when you are publishing later.
This is also a good time to consider what data you really need to collect. If you are working in the EU under GDPR, do you really need their full legal name? Is that necessary? Do you have a plan for ensuring that data is correct when publishing, if any part of their name has changed since?
-- Try solutions here!
While we are on the topic of SQL’s syntax, one aspect of SQL’s syntax
that can frustrate novices and experts alike is forgetting to finish a
command with ;
(semicolon). When you press enter for a command
without adding the ;
to the end, it can look something like this:
SELECT id FROM Person
...>
...>
This is SQL’s prompt, where it is waiting for additional commands or
for a ;
to let SQL know to finish. This is easy to fix! Just type
;
and press enter!
Now, going back to our query, it’s important to understand that the rows and columns in a database table aren’t actually stored in any particular order. They will always be displayed in some order, but we can control that in various ways. For example, we could swap the columns in the output by writing our query as:
SELECT personal, family FROM Person;
or even repeat columns:
SELECT id, id, id FROM Person;
As a shortcut,
we can select all of the columns in a table using *
:
SELECT * FROM Person;
Question: Selecting Site NamesWrite a query that selects only the
name
column from theSite
table.SELECT name FROM Site;
name DR-1 DR-3 MSK-4
-- Try solutions here!
Question: Query StyleMany people format queries as:
SELECT personal, family FROM person;
or as:
select Personal, Family from PERSON;
What style do you find easiest to read, and why?
-- Try solutions here!
Sorting and Removing Duplicates
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:
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 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):
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;
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”):
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.
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 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:
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;
Question: Finding Distinct DatesWrite a query that selects distinct dates from the
Visited
table.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
-- Try solutions here!
Question: Displaying Full NamesWrite a query that displays the full names of the scientists in the
Person
table, ordered by family name.SELECT personal, family FROM Person ORDER BY family ASC;
personal family Frank Danforth William Dyer Anderson Lake Frank Pabodie Valentina Roerich
-- Try solutions here!
If you are someone with a name which falls at the end of the alphabet, you’ve likely been penalised for this your entire life. Alphabetically sorting names should always be looked at critically and through a lens to whether you are fairly reflecting everyone’s contributions, rather than just the default sort order.
There are many options, either by some metric of contribution that everyone could agree on, or better, consider random sorting, like the GTN uses with our Hall of Fame page where we intentionally order randomly to tell contributors that no one persons contributions matter more than anothers.
The evidence provided in a variety of studies leaves no doubt that an alphabetical author ordering norm disadvantages researchers with last names toward the end of the alphabet. There is furthermore con- vincing evidence that researchers are aware of this and that they react strategically to such alphabetical discrimination, for example with their choices of who to collaborate with. See Weber 2018 for more.
When you are sorting things in SQL, you need to be aware of something called collation which can affect your results if you have values that are not the letters A-Z. Collating is the process of sorting values, and this affects many human languages when storing data in a database.
Here is a Dutch example. In the old days their alphabet contained a
ÿ
which was later replaced withij
, a digraph of two characters squished together. This is commonly rendered asij
however, two separate characters, due to the internet and widespread use of keyboards featuring mainly ascii characters. However, it is still the 25th letter of their alphabet.sqlite> create table nl(value text); sqlite> insert into nl values ('appel'), ('beer'), ('index'), ('ijs'), ('jammer'), ('winkel'), ('zon'); sqlite> select * from nl order by value; appel beer index ijs jammer winkel zon
Find a dutch friend and ask them if this is the correct order for this list. Unfortunately it isn’t. Even though it is
ij
as two separate characters, it should be sorted as if it wasij
orÿ
, beforez
. Like so: appel, beer, index, jammer, winkel, ijs, zonWhile there is not much you can do about it now (you’re just beginning!) it is something you should be aware of. When you later need to know about this, you will find the term ‘collation’ useful, and you’ll find the procedure is different for every database engine.
Filtering
One of the most powerful features of a database is
the ability to filter data,
i.e.,
to select only those records that match certain criteria.
For example,
suppose we want to see when a particular site was visited.
We can select these records from the Visited
table
by using a WHERE
clause in our query:
SELECT * FROM Visited WHERE site = 'DR-1';
The database manager executes this query in two stages.
First,
it checks at each row in the Visited
table
to see which ones satisfy the WHERE
.
It then uses the column names following the SELECT
keyword
to determine which columns to display.
This processing order means that
we can filter records using WHERE
based on values in columns that aren’t then displayed:
SELECT id FROM Visited WHERE site = 'DR-1';
![SQL Filtering in Action]`(../../images/carpentries-sql/sql-filter.svg)
We can use many other Boolean operators to filter our data. For example, we can ask for all information from the DR-1 site collected before 1930:
SELECT * FROM Visited WHERE site = 'DR-1' AND dated < '1930-01-01';
Most database managers have a special data type for dates. In fact, many have two: one for dates, such as “May 31, 1971”, and one for durations, such as “31 days”. SQLite doesn’t: instead, it stores dates as either text (in the ISO-8601 standard format “YYYY-MM-DD HH:MM:SS.SSSS”), real numbers (Julian days, the number of days since November 24, 4714 BCE), or integers (Unix time, the number of seconds since midnight, January 1, 1970). If this sounds complicated, it is, but not nearly as complicated as figuring out historical dates in Sweden.
Storing the year as the last two digits causes problems in databases, and is part of what caused Y2K. Be sure to use the databases’ built in format for storing dates, if it is available as that will generally avoid any major issues.
Similarly there is a “Year 2038 problem”, as the timestamps mentioned above that count seconds since Jan 1, 1970 were running out of space on 32-bit machines. Many systems have since migrated to work around this with 64-bit timestamps.
If we want to find out what measurements were taken by either Lake or Roerich,
we can combine the tests on their names using OR
:
SELECT * FROM Survey WHERE person = 'lake' OR person = 'roe';
Alternatively,
we can use IN
to see if a value is in a specific set:
SELECT * FROM Survey WHERE person IN ('lake', 'roe');
We can combine AND
with OR
,
but we need to be careful about which operator is executed first.
If we don’t use parentheses,
we get this:
SELECT * FROM Survey WHERE quant = 'sal' AND person = 'lake' OR person = 'roe';
which is salinity measurements by Lake, and any measurement by Roerich. We probably want this instead:
SELECT * FROM Survey WHERE quant = 'sal' AND (person = 'lake' OR person = 'roe');
We can also filter by partial matches. For example, if we want to
know something just about the site names beginning with “DR” we can
use the LIKE
keyword. The percent symbol acts as a
wildcard, matching any characters in that
place. It can be used at the beginning, middle, or end of the string
See this page on wildcards for more information:
SELECT * FROM Visited WHERE site LIKE 'DR%';
Finally,
we can use DISTINCT
with WHERE
to give a second level of filtering:
SELECT DISTINCT person, quant FROM Survey WHERE person = 'lake' OR person = 'roe';
But remember:
DISTINCT
is applied to the values displayed in the chosen columns,
not to the entire rows as they are being processed.
What we have just done is how most people “grow” their SQL queries. We started with something simple that did part of what we wanted, then added more clauses one by one, testing their effects as we went. This is a good strategy — in fact, for complex queries it’s often the only strategy — but it depends on quick turnaround, and on us recognizing the right answer when we get it.
The best way to achieve quick turnaround is often to put a subset of data in a temporary database and run our queries against that, or to fill a small database with synthesized records. For example, instead of trying our queries against an actual database of 20 million Australians, we could run it against a sample of ten thousand, or write a small program to generate ten thousand random (but plausible) records and use that.
Question: Fix This QuerySuppose we want to select all sites that lie within 48 degrees of the equator. Our first query is:
SELECT * FROM Site WHERE (lat > -48) OR (lat < 48);
Explain why this is wrong, and rewrite the query so that it is correct.
Because we used
OR
, a site on the South Pole for example will still meet the second criteria and thus be included. Instead, we want to restrict this to sites that meet both criteria:SELECT * FROM Site WHERE (lat > -48) AND (lat < 48);
-- Try solutions here!
Question: Finding OutliersNormalized salinity readings are supposed to be between 0.0 and 1.0. Write a query that selects all records from
Survey
with salinity values outside this range.SELECT * FROM Survey WHERE quant = 'sal' AND ((reading > 1.0) OR (reading < 0.0));
taken person quant reading 752 roe sal 41.6 837 roe sal 22.5
-- Try solutions here!
Question: Matching PatternsWhich of these expressions are true?
'a' LIKE 'a'
'a' LIKE '%a'
'beta' LIKE '%a'
'alpha' LIKE 'a%%'
'alpha' LIKE 'a%p%'
- True because these are the same character.
- True because the wildcard can match zero or more characters.
- True because the
%
matchesbet
and thea
matches thea
.- True because the first wildcard matches
lpha
and the second wildcard matches zero characters (or vice versa).- True because the first wildcard matches
l
and the second wildcard matchesha
.
-- Try solutions here!
But what about if you don’t care about if it’s
ALPHA
oralpha
in the database, and you are using a language that has a notion of case (unlike e.g. Chinese, Japenese)?Then you can use the
ILIKE
operator for ‘case Insensitive LIKE’. for example the following are true:
'a' ILIKE 'A'
'AlPhA' ILIKE '%lpha'
Calculating New Values
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 taken, 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 taken, 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;
But of course that can also be solved by simply having a single name field which avoids other issues.
Question: Fixing Salinity ReadingsAfter 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.SELECT taken, reading / 100 FROM Survey WHERE person = 'roe' AND quant = 'sal';
taken reading / 100 752 0.416 837 0.225
-- Try solutions here!
Question: UnionsThe
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 theUNION
operator, except thatUNION ALL
will select all values. The difference is thatUNION ALL
will not eliminate duplicate rows. Instead,UNION ALL
pulls all rows from the query specifics and combines them into a table. TheUNION
command does aSELECT DISTINCT
on the results set. If all the records to be returned are unique from your union, useUNION ALL
instead, it gives faster results since it skips theDISTINCT
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 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;
-- Try solutions here!
Question: Selecting Major Site IdentifiersThe 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 functionsubstr(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”).SELECT DISTINCT substr(site, 1, instr(site, '-') - 1) AS MajorSite FROM Visited;
-- Try solutions here!
Missing Data
Real-world data is never complete — there are always holes.
Databases represent these holes using a special value called null
.
null
is not zero, False
, or the empty string;
it is a one-of-a-kind value that means “nothing here”.
Dealing with null
requires a few special tricks
and some careful thinking.
By default, the Python SQL interface does not display NULL values in its output, instead it shows None
.
To start,
let’s have a look at the Visited
table.
There are eight records,
but #752 doesn’t have a date — or rather,
its date is null:
SELECT * FROM Visited;
Null doesn’t behave like other values. If we select the records that come before 1930:
SELECT * FROM Visited WHERE dated < '1930-01-01';
we get two results, and if we select the ones that come during or after 1930:
SELECT * FROM Visited WHERE dated >= '1930-01-01';
we get five,
but record #752 isn’t in either set of results.
The reason is that
null<'1930-01-01'
is neither true nor false:
null means, “We don’t know,”
and if we don’t know the value on the left side of a comparison,
we don’t know whether the comparison is true or false.
Since databases represent “don’t know” as null,
the value of null<'1930-01-01'
is actually null
.
null>='1930-01-01'
is also null
because we can’t answer to that question either.
And since the only records kept by a WHERE
are those for which the test is true,
record #752 isn’t included in either set of results.
Comparisons aren’t the only operations that behave this way with nulls.
1+null
is null
,
5*null
is null
,
log(null)
is null
,
and so on.
In particular,
comparing things to null with = and != produces null:
SELECT * FROM Visited WHERE dated = NULL;
produces no output, and neither does:
SELECT * FROM Visited WHERE dated != NULL;
To check whether a value is null
or not,
we must use a special test IS NULL
:
SELECT * FROM Visited WHERE dated IS NULL;
or its inverse IS NOT NULL
:
SELECT * FROM Visited WHERE dated IS NOT NULL;
Null values can cause headaches wherever they appear. For example, suppose we want to find all the salinity measurements that weren’t taken by Lake. It’s natural to write the query like this:
SELECT * FROM Survey WHERE quant = 'sal' AND person != 'lake';
but this query filters omits the records
where we don’t know who took the measurement.
Once again,
the reason is that when person
is null
,
the !=
comparison produces null
,
so the record isn’t kept in our results.
If we want to keep these records
we need to add an explicit check:
SELECT * FROM Survey WHERE quant = 'sal' AND (person != 'lake' OR person IS NULL);
We still have to decide whether this is the right thing to do or not. If we want to be absolutely sure that we aren’t including any measurements by Lake in our results, we need to exclude all the records for which we don’t know who did the work.
In contrast to arithmetic or Boolean operators, aggregation functions
that combine multiple values, such as min
, max
or avg
, ignore
null
values. In the majority of cases, this is a desirable output:
for example, unknown values are thus not affecting our data when we
are averaging it. Aggregation functions will be addressed in more
detail in the next section.
Question: Sorting by Known DateWrite a query that sorts the records in
Visited
by date, omitting entries for which the date is not known (i.e., is null).SELECT * FROM Visited WHERE dated IS NOT NULL ORDER BY dated ASC;
id site dated 619 DR-1 1927-02-08 622 DR-1 1927-02-10 734 DR-3 1930-01-07 735 DR-3 1930-01-12 751 DR-3 1930-02-26 837 MSK-4 1932-01-14 844 DR-1 1932-03-22
-- Try solutions here!
Question: NULL in a SetWhat do you expect the following query to produce?
SELECT * FROM Visited WHERE dated IN ('1927-02-08', NULL);
What does it actually produce?
You might expect the above query to return rows where dated is either ‘1927-02-08’ or NULL. Instead it only returns rows where dated is ‘1927-02-08’, the same as you would get from this simpler query:
SELECT * FROM Visited WHERE dated IN ('1927-02-08');
The reason is that the
IN
operator works with a set of values, but NULL is by definition not a value and is therefore simply ignored.If we wanted to actually include NULL, we would have to rewrite the query to use the IS NULL condition:
SELECT * FROM Visited WHERE dated = '1927-02-08' OR dated IS NULL;
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Question: Pros and Cons of SentinelsSome database designers prefer to use a sentinel value to mark missing data rather than
null
. For example, they will use the date “0000-00-00” to mark a missing date, or -1.0 to mark a missing salinity or radiation reading (since actual readings cannot be negative). What does this simplify? What burdens or risks does it introduce?
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