数据库的等值连接、不等值连接、自连接、外连接、子查询

内连接：合并具有同一列的两个以上的表的行，结果集中不包含一个表与另一个表不匹配的行。以下为左外连接。内连接分为：等值连接和非等值连接

外连接分为：两个表在连接过程中除了返回满足连接条件的行以外还返回左（或右）表中不满足连接的行，这种连接成为左（或右）外连接。没有匹配的行时，结果表中相应的列为空(NULL)，外连接的WHERE子句条件类似与内连接，但外连接条件中没有匹配行的表的列后面要加外连接运算符，即用圆括号括起来的加号（+）。左外连接和右外连接

以下范例使用的表结构如下：

 

创建以上表对应的sql语句：

（1）

ALTER TABLE departments
DISABLE CONSTRAINT DEPT_MGR_FK;

ALTER TABLE job_history
DISABLE CONSTRAINT JHIST_EMP_FK;

DROP TRIGGER secure_employees;

DROP TRIGGER update_job_history;

DROP PROCEDURE add_job_history;

DROP PROCEDURE secure_dml;

DELETE FROM employees
WHERE manager_id IN (108, 114, 120, 121, 122, 123, 145, 146, 147, 148);

DELETE FROM employees
WHERE employee_id IN (114, 120, 121, 122, 123, 145, 146, 147, 148, 
                      196, 197, 198, 199, 105, 106, 108, 175, 177, 
                      179, 203, 204);

DELETE FROM locations
WHERE location_id NOT IN 
  (SELECT DISTINCT location_id
   FROM departments);

DELETE FROM countries
WHERE country_id NOT IN
  (SELECT country_id
   FROM locations);

DELETE FROM jobs
WHERE job_id NOT IN
  (SELECT job_id
   FROM employees);

DELETE FROM departments
WHERE department_id NOT IN 
  (SELECT DISTINCT department_id
   FROM employees
   WHERE department_id IS NOT NULL);

UPDATE departments
SET manager_id = 124
WHERE department_id = 50;

UPDATE departments
SET manager_id = 149
WHERE department_id = 80;

DELETE FROM locations
WHERE location_id IN (2700, 2400);

UPDATE locations
SET street_address = '460 Bloor St. W.', 
    postal_code = 'ON M5S 1X8'
WHERE location_id = 1800;

ALTER TABLE departments
ENABLE CONSTRAINT DEPT_MGR_FK;

CREATE TABLE job_grades
(grade_level VARCHAR2(3),
 lowest_sal  NUMBER,
 highest_sal NUMBER);

INSERT INTO job_grades
VALUES ('A', 1000, 2999);

INSERT INTO job_grades
VALUES ('B', 3000, 5999);

INSERT INTO job_grades
VALUES('C', 6000, 9999);

INSERT INTO job_grades
VALUES('D', 10000, 14999);

INSERT INTO job_grades
VALUES('E', 15000, 24999);

INSERT INTO job_grades
VALUES('F', 25000, 40000);

INSERT INTO departments VALUES 
        ( 190 
        , 'Contracting'
        , NULL
        , 1700
        );

COMMIT;

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