Fork, clone, and branch(lesson).
By the end of this lesson, students should be able to:
- Create join tables to represent many-to-many relationships.
- Insert rows in join tables to create many-to-many relationships.
- Select data about many-to-many relationships using join tables.
There are a variety of relationships that cannot be easily captured by a simple reference from one table to another. We'll explore how to represent many-to-many relationships using join tables to represent that relationship.
Join tables contain two foreign keys defining a bi-directional relationship between two other tables. Each row in the join table relates one row in the left table with one row in the right table (left and right are arbitrary). The same left row reference may appear with many different right row references and vice versa. A join table usually contains additional columns to hold more detail about the relationship.
We'll use sql-join-tables as the working database. Just as we did in the previous lesson we'll create it using CREATE DATABASE with sql-crud specified as the TEMPLATE.
$ psqlpsql (9.4.5)
Type "help" for help.
and=# CREATE DATABASE "sql-join-tables" TEMPLATE "sql-crud";
CREATE DATABASE
and=# \c sql-join-tables
You are now connected to database "sql-join-tables" as user "and".
sql-join-tables=#First we'll create an addresses table to connect people to cities.
Then we'll insert addresses connecting a few people to some cities.
Last we'll build and run some queries using the addresses join table.
We'll create a skills table and fill it with data from data/skills.csv. Then we'll create an endorsements table to connect people to skills.
Next we'll insert endorsements connecting people to skills.
Last we'll build and run some queries using the endorsements join table.
We'll create a companies table and fill it with data from data/companies.csv. Then we'll create a jobs table to connect people to companies. The jobs table should contain start_date and end_date columns.
Next we'll insert jobs connecting people to companies. Give at least two people non-overlapping jobs at more than one company. Have at least two companies employ at least four people.
Last we'll build and run some queries using the jobs join table. For at least one company find all the people who currently work there. For at least one person find all of the companies they've worked for and order the result by start date.