MYSQL初级学习笔记六：子查询！（视频序号：初级_42 | 43)

知识点八：子查询(42)

什么是子查询：

　　子查询是将一个查询语句嵌套在另一个查询语句中。内层查询语句的查询结果，可以作为外层查询语句提供条件。

引发子查询的情况：

　　使用[NOT] IN 的子查询

 --删除原本的员工表和部门表，重新建
 DROP TABLE employee,department;

 CREATE TABLE IF NOT EXISTS department(
 id TINYINT UNSIGNED AUTO_INCREMENT KEY,
 depName VARCHAR(20) NOT NULL UNIQUE
 )ENGINE=INNODB;

 INSERT department(depName) VALUES('教学部'),
 ('市场部'),
 ('运营部'),
 ('督导部');

 -- 创建员工表employee(子表)
 -- id ,username ,depId
 CREATE TABLE IF NOT EXISTS employee(
 id SMALLINT UNSIGNED AUTO_INCREMENT KEY,
 username VARCHAR(20) NOT NULL UNIQUE,
 depId TINYINT UNSIGNED
 )ENGINE=INNODB;

 INSERT employee(username,depId) VALUES('king',1),
 ('queen',2),
 ('张三',3),
 ('李四',4),
 ('王五',1);

 -- 由[NOT] IN引发的子查询
 SELECT id FROM department;

 SELECT id,username FROM employee WHERE depId IN(1,2,3,4);

 SELECT id,username FROM employee WHERE depId IN(SELECT id FROM department);

 SELECT id,username FROM employee WHERE depId NOT IN(SELECT id FROM department);

 INSERT employee(username,depId) VALUES('testtest',8);

测试[NOT IN]子查询

　　使用比较运算符的子查询：

　　　　=，>,<,>=,<=,<>,!=,ó

　　使用[NOT] EXISTS的子查询：

 --　创建学员表student
 -- id username score
 CREATE TABLE IF NOT EXISTS student(
 id TINYINT UNSIGNED AUTO_INCREMENT KEY,
 username VARCHAR(20)  NOT NULL UNIQUE,
 score TINYINT UNSIGNED
 );
 INSERT student(username,score) VALUES('king',95),
 ('king1',35),
 ('king2',45),
 ('king3',55),
 ('king4',65),
 ('king5',75),
 ('king6',80),
 ('king7',90),
 ('king8',25);
 -- 创建奖学金scholarship
 -- id ,level

 CREATE TABLE IF NOT EXISTS scholarship(
 id TINYINT UNSIGNED AUTO_INCREMENT KEY,
 level TINYINT UNSIGNED
 );
 INSERT scholarship(level) VALUES(90),(80),(70);

 -- 查询获得1等奖学金的学员有

 SELECT level FROM scholarship WHERE id=1;

 SELECT id,username FROM student WHERE score>=90;

 SELECT id,username FROM student WHERE score>=(SELECT level FROM scholarship WHERE id=1);

 -- 查询部门表中

 SELECT * FROM department WHERE id=5;

 SELECT id,username FROM employee WHERE EXISTS(SELECT * FROM department WHERE id=5);

 SELECT id,username FROM employee WHERE EXISTS(SELECT * FROM department WHERE id=4);

 SELECT id,username FROM employee WHERE NOT EXISTS(SELECT * FROM department WHERE id=41);

比较运算符与[NOT] EXISTS测试

　　使用ANY | SOME 或者ALL的子查询：

运算符                关键字	ANY	SOME	ALL
>，>=	最小值	最小值	最大值
<，<=	最大值	最大值	最小值
=	任意值	任意值
<>，!=			任意值

 -- 查询所有获得奖学金的学员

 SELECT id,username,score FROM student WHERE score>=ANY(SELECT level FROM scholarship);

 SELECT id,username,score FROM student WHERE score>=SOME(SELECT level FROM scholarship);

 -- 查询所有学员中获得一等奖学金的学员
 SELECT id,username,score FROM student WHERE score >=ALL(SELECT level FROM scholarship);

 -- 查询学员表中没有获得奖学金的学员

 SELECT id,username,score FROM student WHERE score<ALL(SELECT level FROM scholarship);

 SELECT id,username,score FROM student WHERE score<ANY(SELECT level FROM scholarship);

 SELECT id,username,score FROM student WHERE score<=ANY(SELECT level FROM scholarship);

 -- 相当于IN
 SELECT id,username,score FROM student WHERE score=ANY(SELECT level FROM scholarship);

 SELECT id,username,score FROM student WHERE score IN(SELECT level FROM scholarship);

 -- 相当于NOT IN
 SELECT id,username,score FROM student WHERE score NOT IN(SELECT level FROM scholarship);

 SELECT id,username,score FROM student WHERE score <> ALL(SELECT level FROM scholarship);

ANY,SOME,ALL测试

将查询结果写入到数据表：

　　INSERT [INTO] tbl_name [(col_name,…)] SELECT …

建数据表同时将查询结果写入到数据表：

　　CREATE TABLE [IF NOT EXISTS] tbl_name

　　[(create_definition,…)]

　　select_statement

 --将查询结果写入到数据表
 CREATE TABLE test1 (
 id TINYINT UNSIGNED AUTO_INCREMENT KEY,
 num TINYINT UNSIGNED
 );
 INSERT test1(id,num)
 SELECT id,score FROM student;

 CREATE TABLE test2 (
 id TINYINT UNSIGNED AUTO_INCREMENT KEY,
 num TINYINT UNSIGNED
 )SELECT id,score FROM student;

 CREATE TABLE test3 (
 id TINYINT UNSIGNED AUTO_INCREMENT KEY,
 score TINYINT UNSIGNED
 )SELECT id,score FROM student;

将查询结果写入到数据表测试