SQL (Structured Query Language) allows us to select, update, create or delete records on a single table in a relational database. But what if we had two tables that had different information about the same person, and we wanted to use all of that information to display it on a single table? We would need to use a JOIN clause in that case. The SQL JOIN clause allows us 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.
Database professionals use normalizations for improving data integrity, and to keep data redundancy low so that they can decrease the number of data anomalies in the application when deleting or updating a record.
Understanding the Types of Joins in SQL
According to ANSI standard SQL, there are five different types of SQL JOIN:
- INNER JOIN: This join returns records that have common values in both tables.
- LEFT (OUTER) JOIN: This returns all records from the left table combined with matching records from the right table. In case there are no matching records in the right table, it will return NULL values.
- RIGHT (OUTER) JOIN: This returns all records from the right table and the matching records from the left table. In case there are no matching records in the left table, it will return NULL values.
- FULL (OUTER) JOIN: It returns the combined records of both left and right outer join and returns all rows from both the tables. For the records for which there is no match, the final table will contain NULL values.
- CROSS JOIN: A cross join returns a Cartesian product of rows from both tables, by joining every row from the first table with every row from the second table.
5 Best Practices For Writing SQL Joins:
Now we know, the most common method for joining and fetching data from multiple tables is using the JOIN operator. But mastering JOIN to write the perfect query requires a lot of hard work and practice on solving real-world challenges, and also, having a solid technical foundation. However, remembering a few important tips might help you write better SQL Join queries. Here are a few:
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.
