内部联接

Inner join 仅返回基于一个或多个条件（使用 ON 关键字指定）在两个表中匹配/存在的记录/行。这是最常见的联接类型。inner join 的一般语法是：

SELECT * 
FROM table_1
INNER JOIN table_2
  ON table_1.column_name = table_2.column_name

它也可以简化为 JOIN：

SELECT * 
FROM table_1
JOIN table_2
  ON table_1.column_name = table_2.column_name

例

/* Sample data. */
DECLARE @Animal table (
    AnimalId Int IDENTITY,
    Animal Varchar(20)
);

DECLARE @AnimalSound table (
    AnimalSoundId Int IDENTITY,
    AnimalId Int,
    Sound Varchar(20)
);

INSERT INTO @Animal (Animal) VALUES ('Dog');
INSERT INTO @Animal (Animal) VALUES ('Cat');
INSERT INTO @Animal (Animal) VALUES ('Elephant');

INSERT INTO @AnimalSound (AnimalId, Sound) VALUES (1, 'Barks');
INSERT INTO @AnimalSound (AnimalId, Sound) VALUES (2, 'Meows');
INSERT INTO @AnimalSound (AnimalId, Sound) VALUES (3, 'Trumpets');
/* Sample data prepared. */

SELECT 
    * 
FROM 
    @Animal
    JOIN @AnimalSound
        ON @Animal.AnimalId = @AnimalSound.AnimalId;

AnimalId    Animal               AnimalSoundId AnimalId    Sound
----------- -------------------- ------------- ----------- --------------------
1           Dog                  1             1           Barks
2           Cat                  2             2           Meows
3           Elephant             3             3           Trumpets

使用内连接和左外连接（替换为不存在）

此查询将返回表 1 中的数据，其中与 table2 匹配的字段与表 1 中的密钥和数据在与表 2 中与条件和密钥进行比较时

select * 
  from Table1 t1
    inner join Table2 t2 on t1.ID_Column = t2.ID_Column 
    left  join Table3 t3 on t1.ID_Column = t3.ID_Column 
  where t2.column_name = column_value 
    and t3.ID_Column is null 
  order by t1.column_name;