基本查询：
SELECT * FROM students;
条件查询：
SELECT * FROM students WHERE score >= 80;
投影查询：
SELECT id, score, name FROM students;
排序：
SELECT id, name, gender, score FROM students ORDER BY score;
分页查询：
SELECT id, name, gender, score FROM students ORDER BY score DESC;
聚合查询：对于统计总数、平均数这类计算，SQL提供了专门的聚合函数，使用聚合函数进行查询，就是聚合查询，它可以快速获得结果。
SELECT COUNT(*) FROM students;
SUM计算某一列的合计值，该列必须为数值类型
AVG计算某一列的平均值，该列必须为数值类型MAX计算某一列的最大值MIN计算某一列的最小值多表查询：
SELECT * FROM students, classes;
SELECT students.id sid, students.name, students.gender, students.score, classes.id cid, classes.name cname FROM students, classes;
SELECT s.id sid, s.name, s.gender, s.score, c.id cid, c.name cname FROM students s, classes c WHERE s.gender = 'M' AND c.id = 1;
连接查询：
SELECT s.id, s.name, s.class_id, c.name class_name, s.gender, s.score FROM students s INNER JOIN classes c ON s.class_id = c.id;
INNER JOIN：
LEFT OUTER JOIN是选出左表存在的记录：
RIGHT OUTER JOIN是选出右表存在的记录：
FULL OUTER JOIN则是选出左右表都存在的记录：
添加记录：
INSERT INTO students (class_id, name, gender, score) VALUES (1, '大宝', 'M', 87), (2, '二宝', 'M', 81);
SELECT * FROM students;
更新数据：
UPDATE students SET name='大牛', score=66 WHERE id=1; -- 查询并观察结果: SELECT * FROM students WHERE id=1;
UPDATE students SET score=score+10 WHERE score<80; -- 查询并观察结果: SELECT * FROM students;
删除数据：
DELETE FROM students WHERE id=1; -- 查询并观察结果: SELECT * FROM students;
