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SQL Lesson 6: Multi-table queries with JOINs
Up to now, we've been working with a single table, but entity data in the real world is often broken down into pieces and stored across multiple orthogonal tables using a process known as normalization[1].
Database normalization
Database normalization is useful because it minimizes duplicate data in any single table, and allows for data in the database to grow independently of each other (ie. Types of car engines can grow independent of each type of car). As a trade-off, queries get slightly more complex since they have to be able to find data from different parts of the database, and performance issues can arise when working with many large tables.
In order to answer questions about an entity that has data spanning multiple tables in a normalized database, we need to learn how to write a query that can combine all that data and pull out exactly the information we need.
Multi-table queries with JOINs
Tables that share information about a single entity need to have a primary key that identifies that entity uniquely across the database. One common primary key type is an auto-incrementing integer (because they are space efficient), but it can also be a string, hashed value, so long as it is unique.
Using the JOIN clause in a query, we can combine row data across two separate tables using this
unique key. The first of the joins that we will introduce is the INNER JOIN.
Select query with INNER JOIN on multiple tables
SELECT column, another_table_column, …
FROM mytable
INNER JOIN another_table
ON mytable.id = another_table.id
WHERE condition(s)
ORDER BY column, … ASC/DESC
LIMIT num_limit OFFSET num_offset;
The INNER JOIN is a process that matches rows from the first table and the second table which have
the same key (as defined by the ON constraint) to create a result row with the combined columns
from both tables. After the tables are joined, the other clauses we learned previously are then
applied.
Did you know?
You might see queries where the INNER JOIN is written simply as a JOIN.
These two are equivalent, but we will continue to refer to these joins as inner-joins because they
make the query easier to read once you start using other types of joins, which will be introduced
in the following lesson.
Exercise
We've added a new table to the Pixar database so that you can try practicing some joins. The BoxOffice
table stores information about the ratings and sales of each particular Pixar movie, and the Movie_id
column in that table corresponds with the Id column in the Movies table 1-to-1. Try and solve
the tasks below using the INNER JOIN introduced above.
Table: movies (Read-only)
Table: boxoffice (Read-only)
Query results
Exercise 6 — Tasks
- Find the domestic and international sales for each movie
- Show the sales numbers for each movie that did better internationally rather than domestically
- List all the movies by their ratings in descending order
Stuck? Read this task's Solution.
Solve all tasks to continue to the next lesson.
Finish above Tasks
Solve all tasks to continue to the next lesson.