结论

从下面的测试结果很容易看出，左关联的查询语句中，只要有 WHERE 的过滤条件，那么该语句将被转为内关联。

一、创建表

演示使用，随便创建两张表

--班级表CREATE TABLE T_CLASS(  class_id NUMBER not null,  class_name VARCHAR2(100))--学生表CREATE TABLE T_STUDENT(  student_id NUMBER not null,  class_id NUMBER not null,  student_name VARCHAR2(100),  age NUMBER,  sex CHAR(1) )

二、创建数据

--班级数据insert into T_CLASS (CLASS_ID, CLASS_NAME)values (1, '一班');insert into T_CLASS (CLASS_ID, CLASS_NAME)values (2, '二班');insert into T_CLASS (CLASS_ID, CLASS_NAME)values (3, '三班');insert into T_CLASS (CLASS_ID, CLASS_NAME)values (4, '四班');insert into T_CLASS (CLASS_ID, CLASS_NAME)values (5, '五班');--学生数据insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)values (1, 1, '李1', 3, '1');insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)values (2, 1, '李2', 2, '1');insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)values (3, 1, '李3', 3, '1');insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)values (4, 2, '李4', 4, '1');insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)values (5, 2, '李5', 3, '2');insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)values (6, 2, '李6', 3, '1');insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)values (7, 3, '李7', 6, '2');insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)values (8, 3, '李8', 4, '2');insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)values (9, 2, '李9', 2, '2');insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)values (10, 2, '李10', 3, '1');insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)values (11, 3, '李11', 3, '2');insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)values (12, 2, '李12', 8, '2');insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)values (13, 1, '李13', 6, '2');

三、左关联出现的几种情形

1.无过滤条件

查询语句：

SELECT *  FROM T_CLASS A  LEFT JOIN T_STUDENT B    ON A.CLASS_ID = B.CLASS_ID ORDER BY A.CLASS_ID;

查询结果：

2.A表过滤条件在AND中

查询语句：

SELECT *  FROM T_CLASS A  LEFT JOIN T_STUDENT B    ON A.CLASS_ID = B.CLASS_ID   AND A.CLASS_ID = 1 ORDER BY A.CLASS_ID;

查询结果：

3.A表过滤条件在WHERE中

查询语句：

SELECT *  FROM T_CLASS A  LEFT JOIN T_STUDENT B    ON A.CLASS_ID = B.CLASS_ID WHERE A.CLASS_ID = 1 ORDER BY A.CLASS_ID;

查询结果：

4.B表过滤条件在AND中

查询语句：

SELECT *  FROM T_CLASS A  LEFT JOIN T_STUDENT B    ON A.CLASS_ID = B.CLASS_ID   AND B.AGE = 3 ORDER BY A.CLASS_ID;

查询结果：

5.B表过滤条件在WHERE中

查询语句：

SELECT *  FROM T_CLASS A  LEFT JOIN T_STUDENT B    ON A.CLASS_ID = B.CLASS_ID WHERE B.AGE = 3 ORDER BY A.CLASS_ID;

查询结果：

6.A表过滤条件在AND中，B表过滤条件在WHERE中

查询语句：

SELECT *  FROM T_CLASS A  LEFT JOIN T_STUDENT B    ON A.CLASS_ID = B.CLASS_ID   AND A.CLASS_ID = 1 WHERE B.AGE = 3 ORDER BY A.CLASS_ID;

查询结果：

7.A表过滤条件在WHERE中，B表过滤条件在AND中

查询语句：

SELECT *  FROM T_CLASS A  LEFT JOIN T_STUDENT B    ON A.CLASS_ID = B.CLASS_ID   AND B.AGE = 3 WHERE A.CLASS_ID = 1 ORDER BY A.CLASS_ID;

查询结果：

8.A表和B表过滤条件都在AND中

查询语句：

SELECT *  FROM T_CLASS A  LEFT JOIN T_STUDENT B    ON A.CLASS_ID = B.CLASS_ID   AND B.AGE = 3   AND A.CLASS_ID = 1 ORDER BY A.CLASS_ID;

查询结果：

9.A表和B表过滤条件都在WHERE中

查询语句：

SELECT *  FROM T_CLASS A  LEFT JOIN T_STUDENT B    ON A.CLASS_ID = B.CLASS_ID WHERE B.AGE = 3   AND A.CLASS_ID = 1 ORDER BY A.CLASS_ID;

查询结果：

以下附上所有查询语句：

--1.无过滤条件SELECT *  FROM T_CLASS A  LEFT JOIN T_STUDENT B    ON A.CLASS_ID = B.CLASS_ID ORDER BY A.CLASS_ID;--2.A表过滤条件在AND中SELECT *  FROM T_CLASS A  LEFT JOIN T_STUDENT B    ON A.CLASS_ID = B.CLASS_ID   AND A.CLASS_ID = 1 ORDER BY A.CLASS_ID;--3.A表过滤条件在WHERE中SELECT *  FROM T_CLASS A  LEFT JOIN T_STUDENT B    ON A.CLASS_ID = B.CLASS_ID WHERE A.CLASS_ID = 1 ORDER BY A.CLASS_ID;--4.B表过滤条件在AND中SELECT *  FROM T_CLASS A  LEFT JOIN T_STUDENT B    ON A.CLASS_ID = B.CLASS_ID   AND B.AGE = 3 ORDER BY A.CLASS_ID;--5.B表过滤条件在WHERE中SELECT *  FROM T_CLASS A  LEFT JOIN T_STUDENT B    ON A.CLASS_ID = B.CLASS_ID WHERE B.AGE = 3 ORDER BY A.CLASS_ID;--6.A表过滤条件在AND中，B表过滤条件在WHERE中SELECT *  FROM T_CLASS A  LEFT JOIN T_STUDENT B    ON A.CLASS_ID = B.CLASS_ID   AND A.CLASS_ID = 1 WHERE B.AGE = 3 ORDER BY A.CLASS_ID;--7.A表过滤条件在WHERE中，B表过滤条件在AND中SELECT *  FROM T_CLASS A  LEFT JOIN T_STUDENT B    ON A.CLASS_ID = B.CLASS_ID   AND B.AGE = 3 WHERE A.CLASS_ID = 1 ORDER BY A.CLASS_ID;--8.A表和B表过滤条件都在AND中SELECT *  FROM T_CLASS A  LEFT JOIN T_STUDENT B    ON A.CLASS_ID = B.CLASS_ID   AND B.AGE = 3   AND A.CLASS_ID = 1 ORDER BY A.CLASS_ID;--9.A表和B表过滤条件都在WHERE中SELECT *  FROM T_CLASS A  LEFT JOIN T_STUDENT B    ON A.CLASS_ID = B.CLASS_ID WHERE B.AGE = 3   AND A.CLASS_ID = 1 ORDER BY A.CLASS_ID;