CROSS JOIN vs. INNER JOIN in SQL: When to Use Each Join Type?

Cross join (CROSS JOIN) and inner join (INNER JOIN) in SQL: Comprehensive comparison

SQL provides two distinct connection types: CROSS JOIN and INNER JOIN, each connection type has its own application scenarios. This article takes an in-depth look at the differences between these two types of connectivity, highlighting their benefits and use cases.

CROSS JOIN: Cartesian product

A cross join, also known as a Cartesian product, combines every row in the first table with every row in the second table. This will generate a new table containing all possible combinations of rows from the joined table.

Grammar:

<code class="language-sql">SELECT *
FROM Table1 CROSS JOIN Table2</code>

Example:

Consider a cross join between the 'Customers' table and the 'Movies' table:

<code class="language-sql">SELECT 
    Movies.CustomerID, Movies.Movie, Customers.Age, 
    Customers.Gender, Customers.[Education Level], 
    Customers.[Internet Connection], Customers.[Marital Status], 
FROM   
    Customers 
CROSS JOIN 
    Movies</code>

This query will return every possible pairing of customer and movie, resulting in a large number of rows.

INNER JOIN: Limit results

The inner join returns only rows that satisfy the conditions specified in the ON clause. It selects only those rows in both tables that have matching values.

Grammar:

<code class="language-sql">SELECT *
FROM Table1 INNER JOIN Table2 ON Table1.Column = Table2.Column</code>

Example:

Using the same table as before, let’s create an inner join using the CustomerID column:

<code class="language-sql">SELECT 
    Movies.CustomerID, Movies.Movie, Customers.Age, 
    Customers.Gender, Customers.[Education Level], 
    Customers.[Internet Connection], Customers.[Marital Status]
FROM   
    Customers 
INNER JOIN 
    Movies ON Customers.CustomerID = Movies.CustomerID</code>

This query will only return CustomerID matching rows from the 'Customers' table and the 'Movies' table, providing more relevant and filtered results.

Select connection type

The choice of cross joins and inner joins depends on the specific requirements of the query.

• CROSS JOIN: Used when you need all possible combinations of rows in two tables.
• INNER JOIN: Used when you need to retrieve only those rows from two tables that have matching values.

Generally, it is recommended to use inner joins due to its greater efficiency and ability to filter results, while cross joins are useful in specific scenarios where a Cartesian product is required.

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