5 Best Practices For Writing SQL JOIN Query

The SQL JOIN clause allows users to take records from two or more tables at once and retrieve data based on a common field. It also helps to keep our database normalized. However, poorly written joins can lead to inefficient queries, slow performance, and even incorrect results.

Here, we will explore the 5 best practices for writing SQL joins, and understand different techniques to optimize SQL JOIN queries. By writing efficient SQL JOIN queries, we can ensure that the joins are efficient, scalable, and accurate, ultimately improving the overall performance and reliability of your database applications.

Database professionals use normalizations to improve data integrity and keep data redundancy low so that they can decrease the number of data anomalies in the application when deleting or updating a record. Following the given SQL JOIN performance tips, you can also improve the database performance.

Understanding the Types of Joins in SQL

According to ANSI standard SQL, there are five types of SQL JOIN:

Best Practices For Writing SQL JOINs

Here are top 5 practices for writing optimized join queries. These practices are also used by database professionals to get the best results for their applications.

1. Using Table Aliases  

An Alias is a temporary custom name or a shorthand for a table or column name. Aliases are very useful, as they improve the readability and maintainability of your query by reducing the amount of typing required. There are two types of aliases, table alias, and column alias, the syntax for which are provided below:  

The syntax for a column aliasing:  

SELECT column_name AS alias_name  

FROM table_name  

WHERE condition  

The syntax for a table aliasing:  

SELECT column_names  

FROM table_name alias_name  

WHERE condition  

Using table/column alias with joins in queries is always a good practice because with queries getting complex, names can get long and complex too. Hence to create neat queries, using aliases is always a good practice.  

2. Knowing Different Joins Before Using Them  

It is always a good practice to know the different JOINS before starting to write a query. The expected outputs from a particular query should be clear before applying a JOIN. For example, The INNER JOIN clause will return only matching records that are the same in both tables whereas, the LEFT JOIN will return all the records from the left table and only the matching records from the respective right table.

The RIGHT JOIN, on the other hand, will return all records from the right table and the matching records from the respective left table. In case there are no matching records, it will return NULL values. If remembering the JOINS and the syntax seems like a tough task, it's always good to keep a SQL cheat sheet handy while writing complex queries.  

3. Carefully Designing the JOIN Condition  

Before writing a SQL Query, it's always important to know what output your query should provide you with. One common column may not uniquely identify the records, in which scenario you might need to use two or more JOIN conditions, or you might need to apply non-equi joins with conditional operators. Hence, it is necessary to define a correct JOIN condition before expecting and getting the correct output from the joined tables.  

4. Using Explicit JOIN Syntax  

Now that you know, the JOIN clause is used to combine or join data from two or more tables; it is highly recommended to use Explicit JOINS by using the JOIN and ON clause.  

Explicit JOIN syntax has several benefits. You can encounter many SQL queries where tables are joined using implicit JOINS by listing table names in the FROM clause and using the WHERE clause to specify the join condition.

This creates a lot of confusion. Hence, to see the difference between the JOIN conditions and filtering conditions explicit JOIN is used. When the type of join is explicitly stated in a query, it becomes easier to understand and expect what output the query might bring. So, it's better to use "JOIN" to specify the tables to join and "ON" to specify the join condition.  

5. Getting To Know Multiple JOINS  

Once you’ve become familiar with writing JOIN queries, it’s time you learn about multiple joins. In SQL, besides joining two tables you can also join three or more tables which are done with the help of multiple JOINs. It allows you to join more than two tables.  

This is a pretty common operation but there are some rules you need to take care of before writing the queries: each JOIN has its conditions written after the "ON" clause; and to increase the readability of the query, putting each JOIN on a new line is a good approach.

However, you should use the logical orders of the tables while using multiple JOINS so that it satisfies your data requirement and minimize the data flow between various operators of the execution plan.

Conclusion

Just writing a SQL query is not sufficient when working on huge databases as developers have to get the result in least amount of time and spending least amount of resources. In this article, we learnt the best 5 practices to write SQL JOIN queries.

SQL JOIN is widely used in database applications, hence we should always write the most optimized join query. Following these practices will help you increase the performance of database application and reduce the waiting time for results.