What is the keyword for self-connection in mysql?

In MySQL, the keyword used for self-join is JOIN...ON. Self-joins are used to: 1. Find duplicate values; 2. Find related records; 3. Hierarchy modeling; 4. Find circular dependencies. For example, the following self-join query finds duplicate email addresses in the customers table: SELECT email_address FROM customers AS c1 JOIN customers AS c2 ON c1.email_address = c2.email_address WHERE c1.customer_id <> c2.customer_id;

Self-join keywords in MySQL

Self-join is a SQL operation that allows a table to be connected to itself. In MySQL, the keyword used to perform a self-join is JOIN...ON.

Self-join syntax:

<code class="sql">SELECT ...
FROM table_name AS alias1
JOIN table_name AS alias2
ON alias1.column_name = alias2.column_name</code>

Among them:

• ##alias1 and alias2 is the alias assigned to the table.
• column_name is the column name on the row used to join the two tables.

Uses of self-join:

Self-join is mainly used for the following purposes:

• Lookup table Duplicate values: Duplicate rows can be easily identified by joining the table with itself.
• Find related records: For example, in the customer table, self-join can be used to find customers with a common ancestor.
• Hierarchical Structure Modeling: By using self-joins, you can model data with a hierarchical structure, such as an organizational structure or product catalog.
• Find circular dependencies: With self-joins, you can detect whether there are circular dependencies in the table, which is crucial to prevent data corruption.

Example:

The following example demonstrates how to use a self-join to find duplicate email addresses in the Customers table:

<code class="sql">SELECT email_address
FROM customers AS c1
JOIN customers AS c2
ON c1.email_address = c2.email_address
WHERE c1.customer_id <> c2.customer_id;</code>

This query will Return rows for customers with duplicate email addresses.

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