8.1 Types of joins (inner, outer, cross)

Joins are essential for combining data from multiple tables in SQL. They allow you to create meaningful connections between related information, enhancing your ability to extract valuable insights from your database.

Understanding different join types is crucial for effective data retrieval. From inner joins for matching rows to outer joins for including all data, and cross joins for combinations, each type serves a specific purpose in data analysis and reporting.

Inner and Outer Joins

Inner Join

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• Returns only the rows that have matching values in both tables being joined
• Combines rows from two or more tables based on a related column between them
• Syntax:

  SELECT columns FROM table1 [INNER JOIN](https://www.fiveableKeyTerm:inner_join) table2 ON table1.column = table2.column

• Requires a condition to match rows from the tables being joined, typically using the ON clause
• Can be used to combine related data from multiple tables into a single result set (customer orders with customer details)

Left and Right Outer Joins

• Left outer join returns all the rows from the left table and the matched rows from the right table
  • Unmatched rows in the right table are returned as NULL values
  • Syntax:

    SELECT columns FROM table1 LEFT [OUTER] JOIN table2 ON table1.column = table2.column

• Right outer join returns all the rows from the right table and the matched rows from the left table
  • Unmatched rows in the left table are returned as NULL values
  • Syntax:

    SELECT columns FROM table1 RIGHT [OUTER] JOIN table2 ON table1.column = table2.column

• Useful when you want to include all rows from one table and only the matching rows from another table (all customers with their orders, including customers without orders)

Full Outer Join and Join Conditions

• Full outer join returns all rows from both tables, with NULL values for unmatched rows
  • Combines the results of both left and right outer joins
  • Syntax:

    SELECT columns FROM table1 FULL [OUTER] JOIN table2 ON table1.column = table2.column

• ON clause specifies the join condition, which is the criteria for matching rows from the tables
  • Typically used to compare columns from both tables to determine the matching rows
• USING clause can be used instead of ON when the join condition involves columns with the same name
  • Syntax:

    SELECT columns FROM table1 JOIN table2 USING (column)

  • Simplifies the join syntax when the join condition involves identically named columns (joining

    customer.id

    with

    order.customer_id

    using

    USING (id)

    )

Cross and Natural Joins

Cross Join

• Returns the Cartesian product of the rows from the tables being joined
• Combines each row from the first table with each row from the second table
• Does not require a join condition or any matching columns between the tables
• Syntax:

  SELECT columns FROM table1 [CROSS JOIN](https://www.fiveableKeyTerm:cross_join) table2

• Can be useful for generating all possible combinations of rows from two tables (all possible combinations of products and colors)

Natural Join

• Joins tables based on columns with the same name and data type in both tables
• Implicitly matches the columns and eliminates duplicate columns in the result set
• Syntax:

  SELECT columns FROM table1 NATURAL JOIN table2

• Requires the joined tables to have one or more identically named columns that can be used for the join
• Can simplify the join syntax by automatically matching columns, but may produce unexpected results if unintended columns have the same name

Special Join Types

Self Join

• Joins a table to itself by treating the same table as both the left and right tables in the join
• Useful for comparing rows within the same table based on a certain condition
• Requires aliasing the table with different names to distinguish between the two instances of the same table
• Syntax:

  SELECT columns FROM table1 AS t1 JOIN table1 AS t2 ON t1.column = t2.column

• Can be used to find hierarchical relationships or compare rows within the same table (finding employees and their managers within an employee table)

Equi-join and Non-equi Join

• Equi-join is a join that uses an equality operator (=) in the join condition
  • Matches rows from the tables based on the equality of the specified columns
  • Most common type of join and is the default when using INNER JOIN, LEFT JOIN, or RIGHT JOIN
• Non-equi join is a join that uses comparison operators other than equality (>, <, >=, <=) in the join condition
  • Matches rows from the tables based on the specified comparison criteria
  • Syntax:

    SELECT columns FROM table1 JOIN table2 ON table1.column > table2.column

  • Can be used to join tables based on range conditions or inequality comparisons (finding products with a price greater than the average price)