LEFT JOIN: This type of join returns all the rows from the left table and the matching rows from the right table. If there is no match, the result will contain NULL values. The syntax for a left join is:
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<p>SELECT column1, column2</p>
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<p>FROM table1</p>
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<p>LEFT JOIN table2</p>
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<p>ON table1.column = table2.column;</p>
<!-- /wp:paragraph -->Real-life example: A company has a table of employees and a table of departments. To get a list of all employees and their department names, you could use a left join to combine the data from the employees and departments tables.
SELECT employees.name, departments.department_name
FROM employees
LEFT JOIN departments
ON employees.department_id = departments.department_id;
There are several best practices for using left joins in SQL Server:
Use the ON clause to specify the join condition: The ON clause should be used to specify the condition for the join, rather than using the WHERE clause. This makes the query more readable and ensures that the join is only applied to the specified columns.
Use indexes on join columns: To improve query performance, it is important to have indexes on the columns that are used in the join condition. This allows the database engine to quickly locate the matching rows in both tables.
Use aliases for table names: Using aliases for table names can make the query more readable and make it easier to reference the correct table when there are multiple tables in the query.
Be mindful of the order of the tables: The order of the tables in the join can affect the performance of the query. It’s best to put the table that you want all the rows from first in the join.
Watch out for NULL values: When using left joins, it’s important to be aware that the columns from the right table will contain NULL values if there is no match. You may need to handle these NULL values in your application or use the ISNULL or COALESCE function to replace them with a default value.
Use Explicit Columns: When joining tables, it’s best to be explicit and only select the columns that you need. This can help to reduce the amount of data that needs to be retrieved and processed, which can improve query performance.
Example:
SELECT employees.name, departments.department_name
FROM employees
LEFT JOIN departments
ON employees.department_id = departments.department_id;
In this example, the join is done on the department_id column, and only the name and department_name columns are selected. This improves the readability and performance of the query.
Use LEFT JOIN instead of RIGHT JOIN when possible: It’s more common to use LEFT JOIN when you want to return all the rows from one table and matching rows from the other table, rather than using RIGHT JOIN. This is because it’s more intuitive and easier to read, making it more maintainable.
