當(dāng)我們使用MySQL進(jìn)行關(guān)系型數(shù)據(jù)庫查詢時(shí)，經(jīng)常會(huì)用到連接操作。其中最常用的連接方式為外連接（outer join），而左連接（left join）則是外連接的一種。在左連接中，將左表的所有數(shù)據(jù)都保留，同時(shí)匹配右表中相應(yīng)的數(shù)據(jù)，如果右表中沒有匹配的數(shù)據(jù)，那么結(jié)果集中將會(huì)出現(xiàn)NULL值。這意味著左連接可能會(huì)產(chǎn)生一些多余的數(shù)據(jù)，本文將通過實(shí)例演示說明這一點(diǎn)。

-- 創(chuàng)建兩張表
CREATE TABLE Orders (
OrderID int NOT NULL PRIMARY KEY,
CustomerID int NOT NULL,
OrderDate date
);
CREATE TABLE Customers (
CustomerID int NOT NULL PRIMARY KEY,
CustomerName char(50),
ContactName char(50),
Country char(50)
);
-- 插入一些數(shù)據(jù)
INSERT INTO Customers (CustomerID, CustomerName, ContactName, Country) VALUES (1, 'Alfreds Futterkiste', 'Maria Anders', 'Germany');
INSERT INTO Customers (CustomerID, CustomerName, ContactName, Country) VALUES (2, 'Ana Trujillo Emparedados', 'Ana Trujillo', 'Mexico');
INSERT INTO Customers (CustomerID, CustomerName, ContactName, Country) VALUES (3, 'Antonio Moreno Taquería', 'Antonio Moreno', 'Mexico');
INSERT INTO Customers (CustomerID, CustomerName, ContactName, Country) VALUES (4, 'Around the Horn', 'Thomas Hardy', 'UK');
INSERT INTO Customers (CustomerID, CustomerName, ContactName, Country) VALUES (5, 'Berglunds snabbk?p', 'Christina Berglund', 'Sweden');
INSERT INTO Orders (OrderID, CustomerID, OrderDate) VALUES (1, 3, '2021-01-01');
INSERT INTO Orders (OrderID, CustomerID, OrderDate) VALUES (2, 4, '2021-01-02');
INSERT INTO Orders (OrderID, CustomerID, OrderDate) VALUES (3, 2, '2021-01-03');
INSERT INTO Orders (OrderID, CustomerID, OrderDate) VALUES (4, 4, '2021-01-04');
INSERT INTO Orders (OrderID, CustomerID, OrderDate) VALUES (5, 1, '2021-01-05');
INSERT INTO Orders (OrderID, CustomerID, OrderDate) VALUES (6, 5, '2021-01-06');
INSERT INTO Orders (OrderID, CustomerID, OrderDate) VALUES (7, 2, '2021-01-07');
INSERT INTO Orders (OrderID, CustomerID, OrderDate) VALUES (8, 1, '2021-01-08');
-- 使用左連接查詢
SELECT Customers.CustomerName, Orders.OrderID 
FROM Customers
LEFT JOIN Orders ON Customers.CustomerID = Orders.CustomerID;

上述示例中，我們創(chuàng)建了兩張表Orders和Customers，并向其中插入了一些數(shù)據(jù)。然后，我們使用左連接查詢了兩張表中的數(shù)據(jù)，結(jié)果如下圖所示：

+------------------------+---------+
|   CustomerName         |  OrderID|
+------------------------+---------+
| Alfreds Futterkiste    | 5       |
| Alfreds Futterkiste    | 8       |
| Ana Trujillo Emparedados| 3       |
| Ana Trujillo Emparedados| 7       |
| Antonio Moreno Taquería| 1       |
| Around the Horn        | 2       |
| Around the Horn        | 4       |
| Berglunds snabbk?p     | 6       |
+------------------------+---------+

可以看到，左連接返回了Customers表中的所有數(shù)據(jù)，而Orders表中沒有匹配的數(shù)據(jù)，相應(yīng)的位置則使用了NULL值填充。例如，Ana Trujillo Emparedados在Orders表中只有一條數(shù)據(jù)，但在左連接的結(jié)果集中，她的CustomerName被重復(fù)出現(xiàn)了兩次。這便是左連接可能產(chǎn)生的多余數(shù)據(jù)。需要注意的是，如果需要進(jìn)行一些數(shù)據(jù)清洗或者去除多余的數(shù)據(jù)，我們可以通過應(yīng)用其他條件或函數(shù)來實(shí)現(xiàn)。