Structured Query Language (SQL) is a relational database language used by programmers and developers to create, manipulate, and manage databases. Joining tables is one of the most common operations performed in SQL. In this article, we will discuss two types of Joins – SQL LEFT JOIN and LEFT OUTER JOIN – their differences, and how they are used.
SQL Join:
A join is a way to combine data from two or more tables in a database. Joining tables is helpful in extracting information from multiple tables in a single query. In SQL, there are different types of Joins – INNER JOIN, OUTER JOIN, LEFT JOIN, RIGHT JOIN, FULL OUTER JOIN, and CROSS JOIN. In this article, we will focus on the LEFT JOIN and LEFT OUTER JOIN.
SQL LEFT JOIN:
LEFT JOIN or LEFT OUTER JOIN is used to combine data from two tables even if one of the tables doesn’t have any matching rows. The LEFT JOIN retrieves all the rows from the left table and the matching rows from the right table. If there are no matching rows in the right table, then the result will be a NULL value. The syntax for SQL LEFT JOIN is as follows:
SELECT column_names
FROM table1
LEFT JOIN table2
ON table1.col_name = table2.col_name;
In the above syntax, column_names are the columns we want to retrieve from the tables, table1 is the left table, table2 is the right table, col_name is the column we want to join, and ON is the condition for joining.
In simple terms, the Left Join retrieves all the rows from the left table, and matching rows from the right table. If there are no matching rows in the right table, the result will be NULL.
SQL LEFT OUTER JOIN:
LEFT OUTER JOIN is similar to LEFT JOIN, but it also retrieves non-matching rows from the left table. The left outer join is used when we want to retrieve all the rows from the left table, regardless of whether there is a matching row in the right table. If there is no matching row in the right table, then the result will be NULL. The syntax for SQL LEFT OUTER JOIN is as follows:
SELECT column_names
FROM table1
LEFT OUTER JOIN table2
ON table1.col_name = table2.col_name;
In the above syntax, column_names are the columns we want to retrieve from the tables, table1 is the left table, table2 is the right table, col_name is the column we want to join, and ON is the condition for joining.
In simple terms, the Left Outer Join retrieves all the rows from the left table, whether there is a matching row in the right table or not. If there is no matching row in the right table, the result will be NULL.
Difference between SQL Left Join and Left Outer Join:
The main difference between LEFT JOIN and LEFT OUTER JOIN is that LEFT JOIN retrieves all the matching rows from the left table and the right table, whereas LEFT OUTER JOIN retrieves all the matching rows from the left table and the right table, as well as non-matching rows from the left table.
For example, suppose we have two tables – employees and departments. The employees table contains the columns employee_id, employee_name, and department_id. The departments table contains the columns department_id and department_name. If we want to retrieve the name of all the employees and their respective department names, we can use the following SQL queries:
LEFT JOIN:
SELECT employee_name, department_name
FROM employees
LEFT JOIN departments
ON employees.department_id = departments.department_id;
LEFT OUTER JOIN:
SELECT employee_name, department_name
FROM employees
LEFT OUTER JOIN departments
ON employees.department_id = departments.department_id;
In the above example, if we use LEFT JOIN, we will get all the matching rows from the employees and departments tables. If there are no matching rows in the departments table, the value of the department_name field will be NULL. On the other hand, if we use LEFT OUTER JOIN, we will get all the matching rows from the employees and departments tables, as well as non-matching rows from the employees table. If there are no matching rows in the departments table, the value of the department_name field will still be NULL, but we will get all the rows from the employees table.
Conclusion:
In summary, SQL JOIN is used to combine data from two or more tables in a database. LEFT JOIN and LEFT OUTER JOIN are two types of Joins that differ in the way they retrieve data from the tables. LEFT JOIN retrieves all the matching rows from the left table and the right table, and the non-matching rows from the left table, whereas LEFT OUTER JOIN retrieves all the matching rows from the left table and the right table, as well as non-matching rows from the left table. As a programmer or developer, it’s essential to understand the differences between these two SQL Joins and when to use them to optimize the performance and functionality of the database.
Keywords: SQL JOIN, LEFT JOIN, LEFT OUTER JOIN, SQL queries, database, developers, programmers, INNER JOIN, OUTER JOIN, FULL OUTER JOIN, CROSS JOIN, tables, department, employees, matching rows, non-matching rows, NULL value, syntax, condition, performance, functionality.