文章目錄
一、什麼是子查詢?
子查詢是將一個查詢語句嵌套在另一個查詢語句中。內層查詢語句的查詢結果，可以作爲外層查詢語句提供條件。
二、引發子查詢的情況?
1.使用[NOT]IN的子查詢
- 測試IN
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);
- 測試 NOT IN
INSERT employee(username,depId) VALUES('testtest',8);
SELECT id,username FROM employee WHERE depId NOT IN(SELECT id FROM department);
2.使用比較運算符的子查詢
- =、 >、<、>=、<=、<>、!=、<=>
- 創建學員表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);
- 創建獎學金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);
測試：查詢獲得1等獎學金的學員
SELECT level FROM scholarship WHERE id=1;
SELECT id,username,score FROM student WHERE score>=90;
SELECT id,username FROM student WHERE score>=(SELECT level FROM scholarship WHERE id=1);
3.使用[NOT]EXISTS的子查詢
[NOT]EXISTS類似於判斷，返回真或者假- 查詢部門表中
SELECT * FROM department WHERE id=5;
SELECT id,username FROM employee WHERE EXISTS(SELECT * FROM department WHERE id=5);
#判斷括號中條件，條件爲真時才執行外面的語句
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);
4.使用ANY|SOME或者ALL的子查詢
(1)測試ANY|SOME、ALL
- 查詢所有獲得獎學金的學員
mysql> SELECT * FROM scholarship;
+----+-------+
| id | level |
+----+-------+
| 1 | 90 |
| 2 | 80 |
| 3 | 70 |
+----+-------+
#大於等於scholarship中的最小值
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);
- 查詢所有學員中獲得一等獎學金的學員
SELECT id,username,score FROM student WHERE score >=ALL(SELECT level FROM scholarship);
- 查詢學員表中沒有獲得獎學金的學員
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);
(2)’=ANY’ 相當於IN
score>=SOME(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 IN(SELECT level FROM scholarship);
score>=SOME(SELECT level FROM scholarship);
(3)’<> ALL’相當於NOT IN
score>=SOME(SELECT level FROM scholarship);
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);
三、將查詢結果寫入到數據表
INSERT [INTO] tbl_name [(col nam...] SELECT ...
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;
四、建數據表同時將查詢結果寫入到數據表
CREATE TABLE [IF NOT EXISTS] tbl_name [(create_definition...] select_statement