Introduction to Combining Tables

Introduction

For several reasons, it can be useful to combine data from different tables.

This is so that data is merged into one table.

We will look at this from the following example.

• There are 2 tables:
  • employees: Each row contains data of an employee.
  • addresses: Each row contains an address.
• There is a one-to-many relationship between employees and addresses.
  • Each employee can have different types of addresses.
    • For example, home and work address.

For example, to send a letter to each employee, you want to combine both tables.

If you combine the tables into one table, you get something like this:

employee_id	first_name	last_name	address_type	address
10920	Sulema	Carter	work	1108 AA, Provincialeweg 172, Amsterdam
10920	Sulema	Carter	home	4791 KR, De Flank 91, Klundert
17173	Venessa	Wolf	home	7054 BM, Zaaltjesdijk 145, Westendorp
30959	Johan	Maurer	home	2411 CZ, Wilhelminastraat 151, Bodegraven
...	...	...	...	...

Now we need to do this with an SQL query.

• There are different ways to merge data from tables.
• For this, you use different statements in SQL.

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1. Different Methods

The "different SQL statements" for combining tables are listed below:

These will be discussed step by step in the following submodules.

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Summary

• You can merge data from 2 tables.
• There are different ways to do this.
• For each method, there is a specific SQL statement.
  • LEFT JOIN
  • RIGHT JOIN
  • INNER JOIN
  • FULL JOIN
  • UNION