សៀវភៅណែនាំអ្នកប្រើប្រាស់ សំណួរ និងចម្លើយចម្លើយ
ថ្ងៃទី 28 ខែកញ្ញា ឆ្នាំ 2025 - ម៉ោង 08:18 យប់
In Tr, JOIN operations are used to access data from multiple tables by combining rows that have related columns. There are different types of ចូលរួម operations that you can use:
1. **INNER JOIN:** This type of join returns rows when there is at least one match in both tables being joined.
2. **OUTER JOIN:** Outer joins can be of three types:
- **LEFT OUTER JOIN (or LEFT JOIN):** Returns all rows from the left table and the matched rows from the right table. If there is no match, NULL values are returned for the right table columns.
- **RIGHT OUTER JOIN (or RIGHT JOIN):** Returns all rows from the right table and the matched rows from the left table. If there is no match, NULL values are returned for the left table columns.
- **FULL OUTER JOIN (or FULL JOIN):** Returns rows when there is a match in one of the tables. It combines the results of both LEFT JOIN and RIGHT JOIN.
3. **CROSS JOIN:** This type of join returns the Cartesian product of the two tables involved. It matches every row in the first table with every row in the second table.
4. **Self-Join:** A self-join is a regular join but the table is joined with itself.
When writing T-SQL queries with JOIN operations, you need to specify the tables you are joining, the columns you are joining on, and the type of join you want to perform. Here is an example of an INNER JOIN query:
```sql
SELECT Orders.OrderID, Customers.CustomerName
FROM Orders
INNER JOIN Customers ON Orders.CustomerID = Customers.CustomerID;
```
In this query, the `Orders` table is joined with the `Customers` table using the `CustomerID` column to retrieve the `OrderID` and `CustomerName`.
By understanding and using JOIN operations effectively in Transact-SQL, you can retrieve data from multiple tables and perform complex queries to analyze and manipulate your database information.
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