JOIN clause allows for the return of results from more than one table by joining them together with other results based on common column values specified using an
INNER JOIN is the default
JOIN and it will only return results matching the condition specified by
SELECT * FROM books JOIN authors ON books.author_id = authors.id;
An outer join will combine rows from different tables even if the join condition is not met. In a
LEFT JOIN, every row in the left table is returned in the result set, and if the join condition is not met, then
NULL values are used to fill in the columns from the right table.
SELECT column_name(s) FROM table1 LEFT JOIN table2 ON table1.column_name = table2.column_name;
A primary key column in a SQL table is used to uniquely identify each record in that table. A primary key cannot be
NULL. In the example,
customer_id is the primary key. The same value cannot re-occur in a primary key column. Primary keys are often used in
A foreign key is a reference in one table’s records to the primary key of another table. To maintain multiple records for a specific row, the use of foreign key plays a vital role. For instance, to track all the orders of a specific customer, the table
order (illustrated at the bottom of the image) can contain a foreign key.
CROSS JOIN Clause
CROSS JOIN clause is used to combine each row from one table with each row from another in the result set. This
JOIN is helpful for creating all possible combinations for the records (rows) in two tables.
The given query will select the
pants_color columns from the result set, which will contain all combinations of combining the rows in the
pants tables. If there are 3 different shirt colors in the
shirts table and 5 different pants colors in the
pants table then the result set will contain 3 x 5 = 15 rows.
SELECT shirts.shirt_color, pants.pants_color FROM shirts CROSS JOIN pants;
UNION clause is used to combine results that appear from multiple
SELECT statements and filter duplicates.
For example, given a
first_names table with a column
name containing rows of data “James” and “Hermione”, and a
last_names table with a column
name containing rows of data “James”, “Hermione” and “Cassidy”, the result of this query would contain three
names: “Cassidy”, “James”, and “Hermione”.
SELECT name FROM first_names UNION SELECT name FROM last_names
WITH clause stores the result of a query in a temporary table (
temporary_movies) using an alias.
Multiple temporary tables can be defined with one instance of the
WITH temporary_movies AS ( SELECT * FROM movies ) SELECT * FROM temporary_movies WHERE year BETWEEN 2000 AND 2020;