SQL的子查询与JOIN的小试牛刀

//学生表
CREATE TABLE student(
ID INT PRIMARY KEY,
s_name VARCHAR(16) NOT NULL,
class_id INT NOT NULL);

INSERT INTO student VALUES(1, "qf", 3), (2, "lap", 3), (3, "qfa", 8)

//班级表
CREATE TABLE class(
classID INT PRIMARY KEY ,
className VARCHAR(16) NOT NULL
);

INSERT INTO class VALUES (3, "liaoshihua"), (8, "changan")

//以下两行的功能是类似的: 前者用JOIN, 后者用子查询
SELECT s.ID, s.s_name, c.className FROM student s LEFT JOIN class c ON s.class_id = c.classID

SELECT s.ID, s.s_name, (SELECT c.className FROM class c WHERE s.class_id = c.classID) AS className FROM student s //这里的AS语句要用在查询的结果之后,修饰查询的结果.