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
1 --删除原本的员工表和部门表，重新建 2 DROP TABLE employee,department; 3 4 CREATE TABLE IF NOT EXISTS department( 5 id TINYINT UNSIGNED AUTO_INCREMENT KEY, 6 depName VARCHAR(20) NOT NULL UNIQUE 7 )ENGINE=INNODB; 8 9 INSERT department(depName) VALUES('教学部'), 10 ('市场部'), 11 ('运营部'), 12 ('督导部'); 13 14 -- 创建员工表employee(子表) 15 -- id ,username ,depId 16 CREATE TABLE IF NOT EXISTS employee( 17 id SMALLINT UNSIGNED AUTO_INCREMENT KEY, 18 username VARCHAR(20) NOT NULL UNIQUE, 19 depId TINYINT UNSIGNED 20 )ENGINE=INNODB; 21 22 INSERT employee(username,depId) VALUES('king',1), 23 ('queen',2), 24 ('张三',3), 25 ('李四',4), 26 ('王五',1); 27 28 -- 由[NOT] IN引发的子查询 29 SELECT id FROM department; 30 31 SELECT id,username FROM employee WHERE depId IN(1,2,3,4); 32 33 SELECT id,username FROM employee WHERE depId IN(SELECT id FROM department); 34 35 SELECT id,username FROM employee WHERE depId NOT IN(SELECT id FROM department); 36 37 INSERT employee(username,depId) VALUES('testtest',8);
使用比较运算符的子查询：
=，>,<,>=,<=,<>,!=,ó
使用[NOT] EXISTS的子查询：
1 -- 创建学员表student 2 -- id username score 3 CREATE TABLE IF NOT EXISTS student( 4 id TINYINT UNSIGNED AUTO_INCREMENT KEY, 5 username VARCHAR(20) NOT NULL UNIQUE, 6 score TINYINT UNSIGNED 7 ); 8 INSERT student(username,score) VALUES('king',95), 9 ('king1',35), 10 ('king2',45), 11 ('king3',55), 12 ('king4',65), 13 ('king5',75), 14 ('king6',80), 15 ('king7',90), 16 ('king8',25); 17 -- 创建奖学金scholarship 18 -- id ,level 19 20 CREATE TABLE IF NOT EXISTS scholarship( 21 id TINYINT UNSIGNED AUTO_INCREMENT KEY, 22 level TINYINT UNSIGNED 23 ); 24 INSERT scholarship(level) VALUES(90),(80),(70); 25 26 27 -- 查询获得1等奖学金的学员有 28 29 SELECT level FROM scholarship WHERE id=1; 30 31 SELECT id,username FROM student WHERE score>=90; 32 33 SELECT id,username FROM student WHERE score>=(SELECT level FROM scholarship WHERE id=1); 34 35 -- 查询部门表中 36 37 SELECT * FROM department WHERE id=5; 38 39 SELECT id,username FROM employee WHERE EXISTS(SELECT * FROM department WHERE id=5); 40 41 SELECT id,username FROM employee WHERE EXISTS(SELECT * FROM department WHERE id=4); 42 43 SELECT id,username FROM employee WHERE NOT EXISTS(SELECT * FROM department WHERE id=41);
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 |
1 -- 查询所有获得奖学金的学员 2 3 SELECT id,username,score FROM student WHERE score>=ANY(SELECT level FROM scholarship); 4 5 6 SELECT id,username,score FROM student WHERE score>=SOME(SELECT level FROM scholarship); 7 8 -- 查询所有学员中获得一等奖学金的学员 9 SELECT id,username,score FROM student WHERE score >=ALL(SELECT level FROM scholarship); 10 11 -- 查询学员表中没有获得奖学金的学员 12 13 SELECT id,username,score FROM student WHERE score<ALL(SELECT level FROM scholarship); 14 15 16 SELECT id,username,score FROM student WHERE score<ANY(SELECT level FROM scholarship); 17 18 SELECT id,username,score FROM student WHERE score<=ANY(SELECT level FROM scholarship); 19 20 -- 相当于IN 21 SELECT id,username,score FROM student WHERE score=ANY(SELECT level FROM scholarship); 22 23 SELECT id,username,score FROM student WHERE score IN(SELECT level FROM scholarship); 24 25 -- 相当于NOT IN 26 SELECT id,username,score FROM student WHERE score NOT IN(SELECT level FROM scholarship); 27 28 SELECT id,username,score FROM student WHERE score <> ALL(SELECT level FROM scholarship);
将查询结果写入到数据表：
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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
1 --将查询结果写入到数据表 2 CREATE TABLE test1 ( 3 id TINYINT UNSIGNED AUTO_INCREMENT KEY, 4 num TINYINT UNSIGNED 5 ); 6 INSERT test1(id,num) 7 SELECT id,score FROM student; 8 9 10 CREATE TABLE test2 ( 11 id TINYINT UNSIGNED AUTO_INCREMENT KEY, 12 num TINYINT UNSIGNED 13 )SELECT id,score FROM student; 14 15 CREATE TABLE test3 ( 16 id TINYINT UNSIGNED AUTO_INCREMENT KEY, 17 score TINYINT UNSIGNED 18 )SELECT id,score FROM student;