How to use join on in sql

JOIN ON is used to match rows between multiple tables by specified columns and return the connection results. The steps include: specifying the target table, the join type (INNER, LEFT, RIGHT, FULL) and the join condition (matching columns of both tables in the ON clause).

Usage of JOIN ON in SQL

Function of JOIN ON

JOIN ON is used to establish a join between multiple tables, match the values ​​of specific columns, and return the join results as a new table.

Syntax structure

<code class="sql">SELECT 列1, 列2, ...
FROM 表1
JOIN 表2 ON 表1.列名 = 表2.列名;</code>

Steps

1. Specify the target table: The FROM clause specifies the table or view to be joined.
2. Specify the connection type: JOIN The keyword specifies the connection type, such as INNER JOIN, LEFT JOIN, etc.
3. Specify the connection conditions: ON The clause specifies the conditions for the connection, that is, the columns to be matched in the two tables.

Example

The following query uses JOIN ON from the Customers and Orders tables Return customer information and order information in:

<code class="sql">SELECT Customers.CustomerID, Customers.CustomerName, Orders.OrderID, Orders.OrderDate
FROM Customers
JOIN Orders ON Customers.CustomerID = Orders.CustomerID;</code>

Connection type

SQL supports multiple connection types, including:

• INNER JOIN: Return only matching rows from both tables.
• LEFT JOIN: Returns all rows from the left table, plus rows that match the right table (or NULL if there is no match).
• RIGHT JOIN: Returns all rows from the right table, plus rows that match the left table (or NULL if there is no match).
• FULL OUTER JOIN: Returns all rows in both tables, regardless of whether they match or not.

Best Practices

• Use indexes to optimize join performance.
• Explicitly specify the connection column to avoid ambiguity.
• Select the appropriate connection type based on business logic.

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