What are the methods for conducting multiple table queries in MySQL?
There are several methods for performing multi-table queries in MySQL.
- By using INNER JOIN, you can connect two or more tables together and retrieve data that meets the join conditions. For example:
SELECT Orders.OrderID, Customers.CustomerName
FROM Orders
INNER JOIN Customers ON Orders.CustomerID = Customers.CustomerID;
- Using LEFT JOIN: Using LEFT JOIN keyword can return all rows from the left table, as well as the rows from the right table that meet the connection condition. If there are no matching rows in the right table, the rows from the left table will still be returned. For example:
SELECT Customers.CustomerName, Orders.OrderID
FROM Customers
LEFT JOIN Orders ON Customers.CustomerID = Orders.CustomerID;
- With RIGHT JOIN: Using the RIGHT JOIN keyword allows you to return all rows from the right table, as well as rows from the left table that meet the joining condition. If there are no matching rows in the left table, it will still return rows from the right table. For example:
SELECT Customers.CustomerName, Orders.OrderID
FROM Customers
RIGHT JOIN Orders ON Customers.CustomerID = Orders.CustomerID;
- By using CROSS JOIN: The CROSS JOIN keyword can return the Cartesian product of two tables, which includes all possible combinations of the two tables.例如：
SELECT Customers.CustomerName, Orders.OrderID
FROM Customers
CROSS JOIN Orders;
- With UNION: By using the UNION keyword, multiple SELECT statements can be combined into one result set. It is important to note that each SELECT statement must return the same number of columns with the same data types when using UNION. For example:
SELECT CustomerName FROM Customers
UNION
SELECT ProductName FROM Products;
- 使用嵌套的查询可以实现多表查询的目的。一个例子是：
SELECT CustomerName, OrderID
FROM Customers
WHERE CustomerID IN (SELECT CustomerID FROM Orders);
Here are some commonly used MySQL methods for querying multiple tables. Choose the appropriate method based on your specific needs and query conditions.
