SQL JOINs

SQL JOINs are a way of combining records from two or more tables in a database.

A JOIN is a means of combining fields from two tables using values that are common to both.

Types of JOINs

SQL defines three major types of JOINs.

  • INNER JOIN

    An INNER JOIN is the most common type of JOIN and is the default type of JOIN. Using the INNER keyword is optional. This JOIN selects all rows from both participating tables as long as there is a match between the columns.

  • OUTER JOIN

    OUTER JOINs can be sub-classified into LEFT, RIGHT, and FULL OUTER JOINs. Using the OUTER keyword is optional. This returns all rows from both the participating tables that satisfy the JOIN condition, along with rows that do not satisfy the JOIN condition.

  • CROSS JOIN

    In a CROSS JOIN, each row from table1 is matched with each row from table2, creating a Cartesian product.

A common (but disputed) way of visualising JOINs is with Venn diagrams:

Colours Database

Let's try out the JOINs with simple data to help visualise how they work.

createdb colours
psql colours
CREATE TABLE a (
  name VARCHAR(255)
);

CREATE TABLE b (
  name VARCHAR(255)
);

INSERT INTO 
a (name) 
VALUES ('red'), ('yellow'), ('green'), ('yellow');

INSERT INTO 
b (name) 
VALUES ('yellow'), ('red'), ('fuchsia'), ('blue');

[INNER] JOIN

An INNER JOIN will return all matching rows in BOTH tables.

LEFT [OUTER] JOIN

A LEFT JOIN will return all rows from the left table with the matching rows from the right table. If there is no match, the right side will be set to null.

RIGHT [OUTER] JOIN

A RIGHT JOIN is just a reverse LEFT JOIN.

FULL [OUTER] JOIN

A FULL JOIN will return all matched and unmatched rows from both tables, setting null on either side if there's no match.

CROSS JOIN

A CROSS JOIN will return every combination of rows from both tables.

[INNER] JOIN

Let's consider two tables, EMPLOYEES and DEPARTMENTS:

EMPLOYEES:

The table has five columns: ID, NAME, AGE, ADDRESS, and SALARY.

DEPARTMENTS:

The table has three columns: ID, DEPT, and EMP_ID.

Based on the tables above, we can write an INNER JOIN as follows:

SELECT EMP_ID, NAME, DEPT FROM EMPLOYEES JOIN DEPARTMENTS ON EMPLOYEES.ID = DEPARTMENTS.EMP_ID

The query will produce the following result:

As we can see from the query, we compare the tables based on one column in each:

  • ID in the EMPLOYEES table.

  • EMP_ID in the DEPARTMENTS table.

The returned data includes information about EMP_ID, NAME, and DEPT.

Further Reading

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