OverviewTeaching: 25 min
Exercises: 20 minQuestions
How do you query databases using SQL?Objectives
understand how SQL can be used to query databases
Writing my first query
Let’s start by using the articles table. Here we have data on every article that has been published, including the title of the article, the authors, date of publication, etc.
Let’s write an SQL query that selects only the title column from the articles table.
SELECT title FROM articles;
We have capitalized the words SELECT and FROM because they are SQL keywords. This makes no difference to the SQL interpreter as it is case-insensitive, but it helps for readability and is therefore considered good style.
If we want more information, we can add a new column to the list of fields,
SELECT title, authors, issns, year FROM articles;
Or we can select all of the columns in a table using the wildcard ‘*’
SELECT * FROM articles;
If we want only the unique values so that we can quickly see the ISSNs of
journals included in the collection, we use
SELECT DISTINCT issns FROM articles;
If we select more than one column, then the distinct pairs of values are returned
SELECT DISTINCT issns, day, month, year FROM articles;
We can also do calculations with the values in a query. For example, if we wanted to look at the relative popularity of an article, so we divide by 10 (because we know the most popular article has 10 citations).
SELECT first_author, citation_count/10.0 FROM articles;
When we run the query, the expression
citation_count / 10.0 is evaluated for each
row and appended to that row, in a new column. Expressions can use any fields,
any arithmetic operators (
/) and a variety of built-in
functions. For example, we could round the values to make them easier to read.
SELECT first_author, title, ROUND(author_count/16.0, 2) FROM articles;
Write a query that returns the title, first_author, citation_count, author_count, month and year
Databases can also filter data – selecting only the data meeting certain
criteria. For example, let’s say we only want data for a specific ISSN
for the Theory and Applications of Mathematics & Computer Science journal,
which has a ISSN code 2067-2764|2247-6202. We need to add a
WHERE clause to our query:
SELECT * FROM articles WHERE issns='2067-2764|2247-6202';
We can use more sophisticated conditions by combining tests with
For example, suppose we want the data on Theory and Applications of Mathematics
& Computer Science published after June:
SELECT * FROM articles WHERE (issns='2067-2764|2247-6202') AND (month > 06);
Parentheses are used merely for readability in this case, but can be required to disambiguate formulas for the SQL interpreter.
If we wanted to get data for the Humanities and Religions journals, which have
2077-1444, we could combine the tests using OR:
SELECT * FROM articles WHERE (issns = '2076-0787') OR (issns = '2077-1444');
Write a query that returns the title, first_author, issns, month and year for all single author papers with more than 4 citations
Building more complex queries
Now, lets combine the above queries to get data for the 3 journals from
June on. This time, let’s use IN as one way to make the query easier
to understand. It is equivalent to saying
WHERE (issns = '2076-0787') OR (issns
= '2077-1444') OR (issns = '2067-2764|2247-6202'), but reads more neatly:
SELECT * FROM articles WHERE (month > 06) AND (issns IN ('2076-0787', '2077-1444', '2067-2764|2247-6202'));
We started with something simple, then added more clauses one by one, testing their effects as we went along. For complex queries, this is a good strategy, to make sure you are getting what you want. Sometimes it might help to take a subset of the data that you can easily see in a temporary database to practice your queries on before working on a larger or more complicated database.
When the queries become more complex, it can be useful to add comments. In SQL,
comments are started by
--, and end at the end of the line. For example, a
commented version of the above query can be written as:
-- Get post June data on selected journals -- These are in the articles table, and we are interested in all columns SELECT * FROM articles -- Sampling month is in the column `month`, and we want to include -- everything after June WHERE (month > 06) -- selected journals have the `issns` 2076-0787, 2077-1444, 2067-2764|2247-6202 AND (issns IN ('2076-0787', '2077-1444', '2067-2764|2247-6202'));
Although SQL queries often read like plain English, it is always useful to add comments; this is especially true of more complex queries.
We can also sort the results of our queries by using
For simplicity, let’s go back to the articles table and alphabetize it by issns.
SELECT * FROM articles ORDER BY issns ASC;
ASC tells us to order it in Ascending order.
We could alternately use
DESC to get descending order.
SELECT * FROM articles ORDER BY first_author DESC;
ASC is the default.
We can also sort on several fields at once. To truly be alphabetical, we might want to order by genus then species.
SELECT * FROM articles ORDER BY issns DESC, first_author ASC;
Write a query that returns title, first_author, issns and citation_count from the articles table, sorted with the most cited article at the top and alphabetically
Order of execution
Another note for ordering. We don’t actually have to display a column to sort by it. For example, let’s say we want to order the articles by their ISSN, but we only want to see Authors and Titles.
SELECT authors, title FROM articles WHERE issns = '2067-2764|2247-6202' ORDER BY date ASC, first_author ASC;
We can do this because sorting occurs earlier in the computational pipeline than field selection.
The computer is basically doing this:
- Filtering rows according to WHERE
- Sorting results according to ORDER BY
- Displaying requested columns or expressions.
Clauses are written in a fixed order:
BY. It is possible to write a query as a single line, but for readability,
we recommend to put each clause on its own line.
Let’s try to combine what we’ve learned so far in a single query. Using the articles table write a query to display the three date fields,
citation_count, for articles published after June, ordered alphabetically by first author name. Write the query as a single line, then put each clause on its own line, and see how more legible the query becomes!
SQL is ideal for querying databases