What does join mean in sql

JOIN in SQL is used to combine rows from multiple tables and match rows based on common columns or expressions. JOIN types are: INNER JOIN: Rows matching values ​​LEFT JOIN: All rows in the left table match rows in the right table RIGHT JOIN: All rows in the right table match rows in the left table FULL JOIN: All rows in the left and right tables

Meaning of JOIN in SQL

JOIN is a keyword in SQL that is used to combine rows from two or more tables Together, create a new result set. It does this by matching rows on a common column or expression.

Segmentation of JOIN Types

There are four main JOIN types:

• INNER JOIN: Only Returns the combination of rows from two tables with matching values.
• LEFT JOIN: Returns all rows from the left table, plus right table rows that match the right table if they exist.
• RIGHT JOIN: Returns all rows from the right table, and left table rows that match the left table if they exist.
• FULL JOIN: Returns all rows from the left and right tables, including those with no matching values.

JOIN Syntax

The JOIN operation uses the following syntax:

<code class="sql">SELECT 列名
FROM 表1
JOIN 表2 ON 表1.公共列 = 表2.公共列</code>

Application of JOIN

Combining data from multiple tables
• Creating one-to-many or many-to-many relationships
• Looking for occurrences of Matching records in different tables
• Filtering and filtering results

Example

Customers and Orders. To find the number of orders for each customer, you can use the following JOIN query:

<code class="sql">SELECT c.CustomerName, COUNT(o.OrderID) AS OrderCount
FROM Customers c
JOIN Orders o ON c.CustomerID = o.CustomerID
GROUP BY c.CustomerName</code>

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