CREATE TABLE `t_dept` (
 `id` INT(11) NOT NULL AUTO_INCREMENT,
 `deptName` VARCHAR(30) DEFAULT NULL,
 `address` VARCHAR(40) DEFAULT NULL,
 PRIMARY KEY (`id`)
) ENGINE=INNODB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
 
CREATE TABLE `t_emp` (
 `id` INT(11) NOT NULL AUTO_INCREMENT,
 `name` VARCHAR(20) DEFAULT NULL,
  `age` INT(3) DEFAULT NULL,
 `deptId` INT(11) DEFAULT NULL,
empno INT  NOT NULL,
 PRIMARY KEY (`id`),
 KEY `idx_dept_id` (`deptId`)
 #CONSTRAINT `fk_dept_id` FOREIGN KEY (`deptId`) REFERENCES `t_dept` (`id`)
) ENGINE=INNODB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
 
 
 
INSERT INTO t_dept(deptName,address) VALUES(‘华山‘,‘华山‘);
INSERT INTO t_dept(deptName,address) VALUES(‘丐帮‘,‘洛阳‘);
INSERT INTO t_dept(deptName,address) VALUES(‘峨眉‘,‘峨眉山‘);
INSERT INTO t_dept(deptName,address) VALUES(‘武当‘,‘武当山‘);
INSERT INTO t_dept(deptName,address) VALUES(‘明教‘,‘光明顶‘);
 INSERT INTO t_dept(deptName,address) VALUES(‘少林‘,‘少林寺‘);
 
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES(‘风清扬‘,90,1,100001);
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES(‘岳不群‘,50,1,100002);
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES(‘令狐冲‘,24,1,100003);
 
 INSERT INTO t_emp(NAME,age,deptId,empno) VALUES(‘洪七公‘,70,2,100004);
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES(‘乔峰‘,35,2,100005);
 
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES(‘灭绝师太‘,70,3,100006);
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES(‘周芷若‘,20,3,100007);
 
 
 
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES(‘张三丰‘,100,4,100008);
 
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES(‘张无忌‘,25,5,100009);
 
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES(‘韦小宝‘,18,NULL,100010);



ALTER TABLE `t_dept` 
ADD  CEO  INT(11)  ;
 # CEO=2 值，都应该是t_emp 中id的值。
UPDATE t_dept SET CEO=2 WHERE id=1;
UPDATE t_dept SET CEO=4 WHERE id=2;
UPDATE t_dept SET CEO=6 WHERE id=3;
UPDATE t_dept SET CEO=8 WHERE id=4;
UPDATE t_dept SET CEO=9 WHERE id=5;
 

SELECT * FROM t_dept d LEFT JOIN t_emp e ON d.`id` = e.`deptId`;

SELECT * FROM t_emp e LEFT JOIN t_dept d ON e.`deptId` = d.`id`;

SELECT * FROM t_emp WHERE deptId IS NULL;

SELECT * FROM t_emp e LEFT JOIN t_dept d ON e.`deptId` = d.`id` WHERE e.`deptId` IS NULL;

SELECT * FROM t_dept d LEFT JOIN t_emp e ON d.`id` = e.`deptId`  WHERE e.id IS NULL;

SELECT * FROM t_emp e LEFT JOIN t_dept d ON e.`deptId` = d.`id`
UNION
SELECT * FROM t_emp e RIGHT JOIN t_dept d ON e.`deptId` = d.`id`; 

SELECT * FROM t_emp e LEFT JOIN t_dept d ON e.`deptId` = d.`id` WHERE e.`deptId` IS NULL
UNION
SELECT * FROM t_emp e RIGHT JOIN t_dept d ON d.`id` = e.`deptId` WHERE e.`id` IS NULL; 

SELECT * FROM t_dept d LEFT JOIN t_emp e ON e.`id` = d.`CEO`;

SELECT AVG(age) FROM t_dept d INNER JOIN t_emp e ON d.`CEO` = e.id;

SELECT e1.id,e1.name,d1.`deptName`,e2.`name`
FROM t_emp e1 LEFT JOIN t_dept d1 ON e1.`deptId` = d1.`id` LEFT JOIN t_emp e2 ON e2.`id` = d1.`CEO`;