嵌套查詢又稱子查詢，有select子查詢，where子查詢，from子查詢，exists子查詢。

插入數據?

#創建表及插入記錄 CREATE TABLE class (cid int(11) NOT NULL AUTO_INCREMENT,caption varchar(32) NOT NULL,PRIMARY KEY (cid) ) ENGINE=InnoDB CHARSET=utf8;INSERT INTO class VALUES (1, '三年二班'), (2, '三年三班'), (3, '一年二班'), (4, '二年九班');CREATE TABLE course(cid int(11) NOT NULL AUTO_INCREMENT,cname varchar(32) NOT NULL,teacher_id int(11) NOT NULL,PRIMARY KEY (cid),KEY fk_course_teacher (teacher_id),CONSTRAINT fk_course_teacher FOREIGN KEY (teacher_id) REFERENCES teacher (tid) ) ENGINE=InnoDB DEFAULT CHARSET=utf8;INSERT INTO course VALUES (1, '生物', 1), (2, '物理', 2), (3, '體育', 3), (4, '美術', 2);CREATE TABLE score (sid int(11) NOT NULL AUTO_INCREMENT,student_id int(11) NOT NULL,course_id int(11) NOT NULL,num int(11) NOT NULL,PRIMARY KEY (sid),KEY fk_score_student (student_id),KEY fk_score_course (course_id),CONSTRAINT fk_score_course FOREIGN KEY (course_id) REFERENCES course (cid),CONSTRAINT fk_score_student FOREIGN KEY (student_id) REFERENCES student(sid) ) ENGINE=InnoDB DEFAULT CHARSET=utf8;INSERT INTO score VALUES (1, 1, 1, 10), (2, 1, 2, 9), (5, 1, 4, 66), (6, 2, 1, 8), (8, 2, 3, 68), (9, 2, 4, 99), (10, 3, 1, 77), (11, 3, 2, 66), (12, 3, 3, 87), (13, 3, 4, 99), (14, 4, 1, 79), (15, 4, 2, 11), (16, 4, 3, 67), (17, 4, 4, 100), (18, 5, 1, 79), (19, 5, 2, 11), (20, 5, 3, 67), (21, 5, 4, 100), (22, 6, 1, 9), (23, 6, 2, 100), (24, 6, 3, 67), (25, 6, 4, 100), (26, 7, 1, 9), (27, 7, 2, 100), (28, 7, 3, 67), (29, 7, 4, 88), (30, 8, 1, 9), (31, 8, 2, 100), (32, 8, 3, 67), (33, 8, 4, 88), (34, 9, 1, 91), (35, 9, 2, 88), (36, 9, 3, 67), (37, 9, 4, 22), (38, 10, 1, 90), (39, 10, 2, 77), (40, 10, 3, 43), (41, 10, 4, 87), (42, 11, 1, 90), (43, 11, 2, 77), (44, 11, 3, 43), (45, 11, 4, 87), (46, 12, 1, 90), (47, 12, 2, 77), (48, 12, 3, 43), (49, 12, 4, 87), (52, 13, 3, 87);CREATE TABLE student(sid int(11) NOT NULL AUTO_INCREMENT,gender char(1) NOT NULL,class_id int(11) NOT NULL,sname varchar(32) NOT NULL,PRIMARY KEY (sid),KEY fk_class (class_id),CONSTRAINT fk_class FOREIGN KEY (class_id) REFERENCES class (cid) ) ENGINE=InnoDB DEFAULT CHARSET=utf8;INSERT INTO student VALUES (1, '男', 1, '理解'), (2, '女', 1, '鋼蛋'), (3, '男', 1, '張三'), (4, '男', 1, '張一'), (5, '女', 1, '張二'), (6, '男', 1, '張四'), (7, '女', 2, '鐵錘'), (8, '男', 2, '李三'), (9, '男', 2, '李一'), (10, '女', 2, '李二'), (11, '男', 2, '李四'), (12, '女', 3, '如花'), (13, '男', 3, '劉三'), (14, '男', 3, '劉一'), (15, '女', 3, '劉二'), (16, '男', 3, '劉四')CREATE TABLE teacher(tid int(11) NOT NULL AUTO_INCREMENT,tname varchar(32) NOT NULL,PRIMARY KEY (tid) ) ENGINE=InnoDB DEFAULT CHARSET=utf8;INSERT INTO teacher VALUES (1, '張磊老師'), (2, '李平老師'), (3, '劉海燕老師'), (4, '朱云海老師'), (5, '李杰老師');

?select子查詢(外語句先執行，內語句后執行)

例1.?查詢課程名并顯示課程老師的名稱

? select * from course c where c.teacher_id='1'; select * from teacher t where t.tid='1';??-- 最后拼接 select * ,(select tname from teacher t where t.tid=c.teacher_id)name from course c;

where 子查詢（先執行子查詢，再執行外查詢）

例2.查詢學習了體育的學生

-- 選擇了體育學生的id select sc.student_id from course co left join score sc on co.cid=sc.course_id where cname='體育' -- 通過學生表用學生id匹配學生姓名 select st.sname from student st where st.sid in( select sc.student_id from course co left join score sc on co.cid=sc.course_id where cname='體育')

部分結果：?

?from 子查詢(先執行子查詢，再執行外查詢)

例3.列出三年二班學了體育的學生

select a.* from (select st.sname,cl.caption from score sc left join course co on sc.course_id=co.cid left join student st on st.sid=sc.student_id left join class cl on cl.cid=st.class_id where co.cname='體育')a where a.caption='三年二班';

??

?exists 子查詢

(先執行外語句，再執行子查詢，根據子查詢返回結果判斷是否保留外查詢結果)

例4.查詢學了課程di為1的學生的姓名

select * from student st where exists( select *from score sc where course_id='1' and sc.student_id=st.sid );

?

總結

sql查詢本質就是對各種表進行裁剪和拼接，最后得到我們想要的數據。

所有的能通過子查詢完成的查詢都能用連接查詢完成，如例1：查詢課程名并顯示課程老師的名稱

select *from course co left join teacher te on te.tid=co.teacher_id;

?

?所以要靈活運用查詢方式，才能更高效的查詢。

?

總結

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