Perform MySQL LEFT JOIN on two tables?

Let us first create two tables and join them with foreign key constraint. The query to create the first table is as follows −

mysql> create table ParentTable
   -> (
   -> UniqueId int NOT NULL AUTO_INCREMENT PRIMARY KEY,
   -> EmployeeName varchar(10)
   -> );
Query OK, 0 rows affected (0.56 sec)

Insert some records in the first table using insert command. The query is as follows −

mysql> insert into ParentTable(EmployeeName) values('John');
Query OK, 1 row affected (0.15 sec)
mysql> insert into ParentTable(EmployeeName) values('Carol');
Query OK, 1 row affected (0.32 sec)
mysql> insert into ParentTable(EmployeeName) values('Sam');
Query OK, 1 row affected (0.18 sec)
mysql> insert into ParentTable(EmployeeName) values('Bob');
Query OK, 1 row affected (0.19 sec)

Now you can display all records from the table using select statement. The query is as follows −

mysql> select *from ParentTable;

The following is the output −

+----------+--------------+
| UniqueId | EmployeeName |
+----------+--------------+
|        1 | John         |
|        2 | Carol        |
|        3 | Sam          |
|        4 | Bob          |
+----------+--------------+
4 rows in set (0.00 sec)

The query to create the second table with foreign key constraints is as follows −

mysql> create table ChildTable
   -> (
   -> UniqueId int NOT NULL PRIMARY KEY,
   -> EmployeeAddress varchar(100),
   -> CONSTRAINT fk_uniqueId FOREIGN KEY(UniqueId) references ParentTable(UniqueId)
   -> );
Query OK, 0 rows affected (0.54 sec)

Now insert some records in the second table using insert command. The query is as follows −

mysql> insert into ChildTable values(1,'15 West Shady Lane Starkville, MS 39759');
Query OK, 1 row affected (0.19 sec)
mysql> insert into ChildTable values(2,'72 West Rock Creek St. Oxford, MS 38655');
Query OK, 1 row affected (0.18 sec)
mysql> insert into ChildTable(UniqueId) values(3);
Query OK, 1 row affected (0.41 sec)
mysql> insert into ChildTable values(4,'119 North Sierra St. Marysville, OH 43040');
Query OK, 1 row affected (0.16 sec)

Display all records from the table using select statement. The query is as follows −

mysql> select *from ChildTable;

The following is the output −

+----------+-------------------------------------------+
| UniqueId | EmployeeAddress                           |
+----------+-------------------------------------------+
|        1 | 15 West Shady Lane Starkville, MS 39759   |
|        2 | 72 West Rock Creek St. Oxford, MS 38655   |
|        3 | NULL                                      |
|        4 | 119 North Sierra St. Marysville, OH 43040 |
+----------+-------------------------------------------+
4 rows in set (0.00 sec)

Let us now join the tables using left join. The query is as follows −

mysql> select
ParentTable.UniqueId,ParentTable.EmployeeName,ChildTable.EmployeeAddress from ParentTable left join
   -> ChildTable on ParentTable.UniqueId=ChildTable.UniqueId;

The following is the output −

+----------+--------------+-------------------------------------------+
| UniqueId | EmployeeName | EmployeeAddress                           |
+----------+--------------+-------------------------------------------+
|        1 | John         | 15 West Shady Lane Starkville, MS 39759   |
|        2 | Carol        | 72 West Rock Creek St. Oxford, MS 38655   |
|        3 | Sam          | NULL                                      |
|        4 | Bob          | 119 North Sierra St. Marysville, OH 43040 |
+----------+--------------+-------------------------------------------+
4 rows in set (0.00 sec)