SQL INNER JOIN 關鍵字
在表中存在至少一個匹配時，INNER JOIN 關鍵字返回行。
INNER JOIN 與 JOIN 是相同的。

SQL LEFT JOIN 關鍵字
LEFT JOIN 關鍵字會從左表 (table_name1) 那里返回所有的行，即使在右表 (table_name2) 中沒有匹配的行。
在某些數據庫中， LEFT JOIN 稱為 LEFT OUTER JOIN。

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也就是；

? ??INNER JOIN只返回左表和右表都有數據的行；LEFT JOIN 返回左表所有的行；

? ??INNER JOIN? =? JOIN

? ??LEFT JOIN? =? LEFT OUTER JOIN

Sqlite中示例如下；

全部的CMD操作如下；

Microsoft Windows [版本 6.1.7601]
Copyright (c) 2010 Microsoft Corporation. ?All rights reserved.

S:\sqlite>sqlite3 wctest1.db
SQLite version 3.7.15.2 2013-01-09 11:53:05
Enter ".help" for instructions
Enter SQL statements terminated with a ";"
sqlite> create table Persons(
? ?...> Id_P int primary key not null,
? ?...> LastName varchar(255),
? ?...> FirstName varchar(255),
? ?...> Address varchar(255),
? ?...> City varchar(255));
sqlite> INSERT INTO Persons VALUES (1, 'Adams', 'John', 'Oxford Street', 'London
');
sqlite> INSERT INTO Persons VALUES (2, 'Bush', 'George', 'Fifth Avenue', 'New Yo
rk');
sqlite> INSERT INTO Persons VALUES (3, 'Carter', 'Thomas','Changan Street', 'Bei
jing');
sqlite> create table Orders(
? ?...> ?Id_O int primary key not null,
? ?...> ?OrderNo varchar(50),
? ?...> ?Id_P int);
sqlite> INSERT INTO Orders VALUES (1, '77895', 3);
sqlite> INSERT INTO Orders VALUES (2, '44678', 3);
sqlite> INSERT INTO Orders VALUES (3, '22456', 1);
sqlite> INSERT INTO Orders VALUES (4, '24562', 1);
sqlite> INSERT INTO Orders VALUES (5, '34764', 65);
sqlite> select * from Persons;
1|Adams|John|Oxford Street|London
2|Bush|George|Fifth Avenue|New York
3|Carter|Thomas|Changan Street|Beijing
sqlite> select * from Orders;
1|77895|3
2|44678|3
3|22456|1
4|24562|1
5|34764|65
sqlite> SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo
? ?...> FROM Persons
? ?...> INNER JOIN Orders
? ?...> ON Persons.Id_P=Orders.Id_P
? ?...> ORDER BY Persons.LastName;
Adams|John|22456
Adams|John|24562
Carter|Thomas|77895
Carter|Thomas|44678
sqlite> SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo
? ?...> FROM Persons
? ?...> left JOIN Orders
? ?...> ON Persons.Id_P=Orders.Id_P
? ?...> ORDER BY Persons.LastName;
Adams|John|22456
Adams|John|24562
Bush|George|
Carter|Thomas|44678
Carter|Thomas|77895
sqlite>

為了看的清楚些；看一下參照的原來的數據；

內連接；

左連接；

SQL：

create table Persons( Id_P int primary key not null, LastName varchar(255), FirstName varchar(255), Address varchar(255), City varchar(255));INSERT INTO Persons VALUES (1, 'Adams', 'John', 'Oxford Street', 'London'); INSERT INTO Persons VALUES (2, 'Bush', 'George', 'Fifth Avenue', 'New York'); INSERT INTO Persons VALUES (3, 'Carter', 'Thomas','Changan Street', 'Beijing');create table Orders( Id_O int primary key not null, OrderNo varchar(50), Id_P int);INSERT INTO Orders VALUES (1, '77895', 3); INSERT INTO Orders VALUES (2, '44678', 3); INSERT INTO Orders VALUES (3, '22456', 1); INSERT INTO Orders VALUES (4, '24562', 1); INSERT INTO Orders VALUES (5, '34764', 65);SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo FROM Persons INNER JOIN Orders ON Persons.Id_P=Orders.Id_P ORDER BY Persons.LastName;SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo FROM Persons left JOIN Orders ON Persons.Id_P=Orders.Id_P ORDER BY Persons.LastName;

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總結

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