Introduction to MySQL JOINs

One of the most common tasks of MySQL database analysts and administrators is extracting records from several tables according to specific conditions. It could be quite a sophisticated operation requiring you to write and execute multiple queries. Or, you can use JOINs.

JOINs help you fetch data from several tables, which share a common field, and get a single result. As MySQL tables are connected logically and linked with a common key value, JOINs can provide you with the data you need from several tables by using just one query.

Why MySQL JOINs are helpful:

  • Speed

    • JOINs allow you to use a single query instead of many simple queries run one by one. The result delivered is the same.

  • Effectiveness

    • In MySQL, JOIN clauses perform much better because they work through indexing.

  • Less load

    • JOINs help users reduce the number of data transfers between a MySQL server and a target application. Respectively, it reduces the server load.

Database developers, admins, and analysts use JOINs every day — to combine sets of data, process and find data patterns in vast databases, normalize table structures, etc. Therefore, anyone who works with databases as part of their functional role needs a solid understanding of how to apply this clause.

This white paper is created to equip you with the best professional practices for using JOINs and help you master them quickly and easily.

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What are the supported types of JOINs in MySQL?

MySQL supports the following JOIN types: INNER JOIN, OUTER JOIN (further divided into LEFT JOIN and RIGHT JOIN), SELF JOIN, and CROSS JOIN. The type of the JOIN defines how tables are related in a query. You can join more than two tables.
INNER JOIN

INNER JOIN

Compares all rows from all tables specified in a query with each other and returns records with matching values as a new result table.
LEFT OUTER JOIN

LEFT OUTER JOIN

Returns the result table with all rows from the left table and only those rows from the right table that match the JOIN condition (non-matching rows are returned with NULLs).
RIGHT OUTER JOIN

RIGHT OUTER JOIN

Returns the result table with all rows from the right table and only those rows from the left table where the JOIN condition is fulfilled (non-matching rows are returned
with NULLs).
CROSS JOIN

CROSS JOIN

Combines each row from one table with each row from another table and thus returns a new result table with all possible row combinations from each table.
FULL OUTER JOIN

FULL OUTER JOIN

MySQL does not directly support FULL OUTER JOIN that returns both matching and non-matching rows from the joined tables, but you can combine LEFT and RIGHT OUTER JOINs to achieve the same result.
SELF JOIN

SELF JOIN

Compares a row with other rows within the same table or extracts hierarchical data - table aliases should be used instead of repeating the same table name in a query.

Why do you need this white paper?

"Your Guide to MySQL JOINs" is a handy white paper for MySQL specialists, namely database analysts and administrators, who need a clear and concise guide to all kinds of JOINs available in MySQL.

With this guide, you get the most efficient practical approaches and professional tips to use JOIN clauses in various cases. All methods are illustrated with real-life scenarios. Solutions to each problem are scrutinized in detail, regardless of whether you like to work with a MySQL GUI tool or write all scripts manually.

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