MySql自学小记(五) 子查询及连接查询

如下为用到的表departmen,employee,project,

CREATE TABLE department
(
  dpt_name   CHAR(20) NOT NULL,
  people_num INT(10) DEFAULT '10',
  CONSTRAINT dpt_pk PRIMARY KEY (dpt_name)
 );

CREATE TABLE employee
(
  id      INT(10) PRIMARY KEY,
  name    CHAR(20),
  age     INT(10),
  salary  INT(10) NOT NULL,
  phone   INT(12) NOT NULL,
  in_dpt  CHAR(20) NOT NULL,
  UNIQUE  (phone),
  CONSTRAINT emp_fk FOREIGN KEY (in_dpt) REFERENCES department(dpt_name)
 );
 
CREATE TABLE project
(
  proj_num   INT(10) NOT NULL,
  proj_name  CHAR(20) NOT NULL,
  start_date DATE NOT NULL,
  end_date   DATE DEFAULT '2015-04-01',
  of_dpt     CHAR(20) REFERENCES department(dpt_name),
  CONSTRAINT proj_pk PRIMARY KEY (proj_num,proj_name)
 );

想要知道名为“Tom"的员工所在部门做了几个工程。员工信息储存在employee表中，但工程信息储存在project表中。对于这样的情况，我们可以用子查询:

SELECT of_dpt,COUNT(proj_name) AS count_project FROM project 
GROUP BY of_dpt
HAVING of_dpt IN
(SELECT in_dpt FROM employee WHERE name='Tom');

在处理多个表时，子查询只有在结果来自一个表时才有用。但如果需要显示两个表或多个表中的数据，这时就必须使用连接(join)操作。连接的基本思想是把两个或多个表当作一个新的表来操作，如下查询出的是，各员工所在部门的人数:

SELECT id,name,people_num
FROM employee,department
WHERE employee.in_dpt = department.dpt_name
ORDER BY id;

 或者

SELECT id,name,people_num
FROM employee JOIN department
ON employee.in_dpt = department.dpt_name
ORDER BY id;