How to join three tables in SQL query – MySQL Example

Three table JOIN Example SQL

Joining three tables in single SQL query can be very tricky if you are not good with concept of SQL Join. SQL Joins have always been tricky not only for new programmers but for many others, who are in programming and SQL for more than 2 to 3 years. There are enough to confuse someone on SQL JOIN ranging from various types of SQL JOIN like INNER and OUTER join, LEFT and RIGHT outer join, CROSS join etc. Between all of these fundamentals, what is most important about Join is, combining multiple tables. If you need data from multiple tables in one SELECT query you need to use either sub-query or JOIN. Most of times we only join two tables like Employee and Department but some time you may require to join more than two tables and a popular case is joining three tables in SQL. In case of joining three tables table 1 relates to table 2 and then table 2 relates to table 3. If you look at closely you find that table 2 is a joining table which contains primary key from both table 1 and table 2. As I said it can be extremely confusing to understand join of three or more tables. I have found that understanding table relationship as primary key and foreign key helps to alleviate confusion than the classical matching row paradigm. SQL Join is also a very popular topic in SQL interviews and there is always been some questions from Join like Difference between INNER and OUTER JOIN,  SQL query with JOIN e.g. Employee Department relationship and  Difference between LEFT and RIGHT OUTER JOIN etc. In short this is one of the most important topic in SQL both from experience and interview point of view.

Three table JOIN syntax in SQL

Here is a general SQL query syntax to join three or more table. This SQL query should work in all major relation databases e.g. MySQL, Oracle, Microsoft SQLServer, Sybase and PostgreSQL:
 

 SELECT t1.col,t3.col FROMtable1 join table2 ONtable1.primarykey = table2.foreignkey
                                   join table3 ON table2.primarykey =table3.foreignkey

We first join table 1 and table 2 which produce a temporary table with combined data from table1 and table2, which is then joined to table3. This formula can be extended for more than 3 tables to N tables; you just need to make sure that SQL query should have N-1 join statement in order to join N tables. Like for joining two tables we require 1 join statement and for joining 3 tables we need 2 join statements.

SQL Query to JOIN three tables in MySQL

In order to better understand joining of 3 tables in SQL query let's see an example.  Consider popular example of Employee and Department schema. I our case we have used a link table called Register which link or relate both Employee to Department. Primary key of Employee table (emp_id) is foreign key in Register and similarly primary key of Department table (dept_id) is foreign key in Register table.

In order to write an SQL query to print employee name and department name alongside we need to join 3 tables. First JOIN statement will join Employee and Register and create a temporary table which will have dept_id as another column. Now second JOIN statement will join this temp table with Department table on dept_id to get desired result. Here is the complete SELECT SQL query example to join 3 tables and it can be extended to join more than 3 or N tables.

mysql> SELECT * FROM Employee;
+--------+----------+--------+
|emp_id | emp_name | salary |
+--------+----------+--------+
|1     | James    |   2000 |
|2     | Jack     |   4000 |
|3     | Henry    |   6000 |
|4     | Tom      |   8000 |
+--------+----------+--------+
4rows IN SET (0.00 sec)

mysql>SELECT*FROMDepartment;
+---------+-----------+
|dept_id | dept_name |
+---------+-----------+
|101    | Sales     |
|102    | Marketing |
|103    | Finance   |
+---------+-----------+
3rows IN SET (0.00 sec)

mysql>SELECT*FROMRegister;
+--------+---------+
|emp_id | dept_id |
+--------+---------+
|     1 |     101 |
|     2 |     102 |
|     3 |     103 |
|     4 |     102 |
+--------+---------+
4rows IN SET (0.00 sec)

mysql>SELECTemp_name, dept_name FROM Employee e JOIN Register r ON e.emp_id=r.emp_id JOIN Department d ON r.dept_id=d.dept_id;
+----------+-----------+
|emp_name | dept_name |
+----------+-----------+
|James    | Sales     |
|Jack     | Marketing |
|Henry    | Finance   |
|Tom      | Marketing |
+----------+-----------+
4rows IN SET (0.01 sec)

If you want to understand it even better than try joining tables step by step. So instead of joining 3 tables in one go, first join 2 tables and see how the result table will look like. That’s all on How to join three tables in one SQL query in relational database. By the way in this SQL JOIN Example we have used ANSI SQL and it will work in other relational database as well e.g. Oracle, SQL Server, Sybase, PostgreSQL etc. Let us know if you face any issue while running this 3 table JOIN query in any other database.