Joins and aliases

Overview

Teaching: 30 min
Exercises: 5 min

Questions

• How do I bring data together from separate tables?

• How can I make sure column names from my queries make sense and aren’t too long?

Objectives

• Employ joins to combine data from two tables.

• Apply functions to manipulate individual values.

• Employ aliases to assign new names to items in a query.

Joins

To combine data from two tables we use the SQL JOIN command, which comes after the FROM command.

The JOIN command on its own will result in a cross product, where each row in first table is paired with each row in the second table. Usually this is not what is desired when combining two tables with data that is related in some way.

For that, we need to tell the computer which columns provide the link between the two tables using the word ON. What we want is to join the data with the same species codes.

SELECT *
FROM authors
JOIN dates
ON authors.TCP = dates.TCP;

ON is like WHERE, it filters things out according to a test condition. We use the table.colname format to tell the manager what column in which table we are referring to.

The output of the JOIN command will have columns from first table plus the columns from the second table. For the above command, the output will be a table that has the following column names:

Alternatively, we can use the word USING, as a short-hand. In this case we are telling the manager that we want to combine authors with titles and that the common column is eebo.

SELECT *
FROM authors
JOIN dates
USING (TCP);

The output will only have one TCP column

We often won’t want all of the fields from both tables, so anywhere we would have used a field name in a non-join query, we can use table.colname.

For example, what if we wanted information authors and titles but not their TCP ids.

SELECT authors.Author, dates.Date
FROM dates
JOIN authors
ON dates.TCP = authors.TCP;

Many databases, including SQLite, also support a join through the WHERE clause of a query.
For example, you may see the query above written without an explicit JOIN.

SELECT dates.Date, authors.Author
FROM authors, dates
WHERE authors.TCP = dates.TCP;

For the remainder of this lesson, we’ll stick with the explicit use of the JOIN keyword for joining tables in SQL.

Challenge:

• Write a query that returns the authors and places of every TCP ID captured in the catalogue.

Different join types

We can count the number of records returned by our original join query.

SELECT COUNT(*)
FROM authors
JOIN places
USING (TCP);

Notice that this number is smaller than the number of records present in the catalogue data.

SELECT COUNT(*) 
FROM eebo;

This is because, by default, SQL only returns records where the joining value is present in the join columns of both tables (i.e. it takes the intersection of the two join columns). This joining behaviour is known as an INNER JOIN. In fact the JOIN command is simply shorthand for INNER JOIN and the two terms can be used interchangably as they will produce the same result.

We can also tell the computer that we wish to keep all the records in the first table by using the command LEFT OUTER JOIN, or LEFT JOIN for short.

Challenge:

• Re-write the original query to keep all the entries present in the eebo table. How many records are returned by this query?

Challenge:

• Count the number of records in the places table that have a NULL value in the eebo column.

In SQL a NULL value in one table can never be joined to a NULL value in a second table because NULL is not equal to anything, even itself.

Combining joins with sorting and aggregation

Joins can be combined with sorting, filtering, and aggregation. So, if we wanted the average number of pages for each author in the catalogue, we could do something like

SELECT authors.author, AVG(eebo.PageCount)
FROM eebo
JOIN authors
ON authors.TCP = eebo.TCP
GROUP BY eebo.PageCount;

Challenge:

• Write a query that returns the number of authors of the titles published in each year in descending order.

Challenge:

• Write a query that finds the average pages of each year of publication.

Functions

SQL includes numerous functions for manipulating data. You’ve already seen some of these being used for aggregation (SUM and COUNT) but there are functions that operate on individual values as well. Probably the most important of these are IFNULL and NULLIF. IFNULL allows us to specify a value to use in place of NULL.

We can represent unknown ids with “U” instead of NULL:

SELECT TCP, Place, IFNULL(Place, 'U')
FROM places;

The lone “Place” column is only included in the query above to illustrate where IFNULL has changed values; this isn’t a usage requirement.

Challenge:

• Write a query that returns ‘NP’ instead of NULL for values in the Author column.

Challenge:

• Write a query that calculates the average page length of each title, assuming that unknown lengths are 30 (as above).

IFNULL can be particularly useful in JOIN. When joining the authors and dates tables earlier, some results were excluded because the `` was NULL. We can use IFNULL to include them again, re-writing the NULL to a valid joining value:

SELECT eebo.Place, places.Place
FROM places
JOIN eebo
ON eebo.Place = IFNULL(places.Place, 'AB');

Challenge:

• Write a query that returns the number of titles of the authors caught in each plot, using IFNULL to assume that unknown titles are all of the authors “Nemo”.

The inverse of IFNULL is NULLIF. This returns NULL if the first argument is equal to the second argument. If the two are not equal, the first argument is returned. This is useful for “nulling out” specific values.

We can “null out” vid:

SELECT TCP, Place, NULLIF(Place, 'London')
FROM eebo;

Some more functions which are common to SQL databases are listed in the table below:

Finally, some useful functions which are particular to SQLite are listed in the table below:

Challenge:

Write a query that returns author names, sorted from longest titles name down to shortest.

Aliases

As queries get more complex names can get long and unwieldy (as we saw before). To help make things clearer we can use aliases to assign new names to things in the query.

We can alias both table names:

SELECT dt.Date, auth.Author
FROM dates AS dt
JOIN authors AS auth
ON dt.TCP = auth.TCP;

And column names:

SELECT dt.Date AS yr, auth.Author AS author
FROM dates AS dt
JOIN authors AS auth
ON dt.TCP = auth.TCP;

The AS isn’t technically required, so you could do

SELECT dt.Date yr
FROM dates dt;

but using AS is much clearer so it is good style to include it.

Challenge (optional):

SQL queries help us ask specific questions which we want to answer about our data. The real skill with SQL is to know how to translate our humanities questions into a sensible SQL query (and subsequently visualize and interpret our results).

Have a look at the following questions; these questions are written in plain English. Can you translate them to SQL queries and give a suitable answer?

1. How many entries from each year are there per year?

2. How many years have similar amounts of books published?

   Proposed solutions:

   1. Solution: SELECT date as year, count(*) FROM eebo GROUP BY year ORDER BY year DESC

   2. Solution: SELECT date as year, count(*) AS volumes FROM eebo GROUP BY year ORDER BY volumes DESC

Key Points

• Use the JOIN command to combine data from two tables—the ON or USING keywords specify which columns link the tables.

• Regular JOIN returns only matching rows. Other join commands provide different behavior, e.g., LEFT JOIN retains all rows of the table on the left side of the command.

• IFNULL allows you to specify a value to use in place of NULL, which can help in joins

• NULLIF can be used to replace certain values with NULL in results

• Many other functions like IFNULL and NULLIF can operate on individual values.

• Aliases can help shorten long queries. To write clear and readible queries, use the AS keyword when creating aliases.