-- 查询语句
SELECT * FROM student; -- 效率略低，一般不建议使用
SELECT tid,tname FROM student;
SELECT tname AS 姓名 FROM student;
SELECT * FROM student WHERE 1=1;
SELECT tname FROM student; -- 指定查询个别列
SELECT USER,PASSWORD,HOST FROM mysql.user; -- 跨库查询mysql数据库的user表内的用户名、密码、主机
-- 创建学生信息表
CREATE TABLE stu(
sno INT UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
sname VARCHAR(10) NOT NULL,
sgender ENUM(‘男‘,‘女‘) DEFAULT ‘男‘,
saddress VARCHAR(50),
sscore TINYINT UNSIGNED,
sdept VARCHAR(20) NOT NULL
)ENGINE=MYISAM AUTO_INCREMENT=2014270001 DEFAULT CHARSET=utf8;
INSERT INTO stu VALUES(NULL,‘董华‘,‘男‘,‘河南省郑州市‘,80,‘计算机网络‘);
INSERT INTO stu VALUES(NULL,‘代珂‘,‘男‘,‘河南省平顶山‘,85,‘计算机安全‘);
INSERT INTO stu VALUES(NULL,‘小江‘,‘男‘,‘河南省商丘‘,70,‘计算机网络‘);
INSERT INTO stu VALUES(NULL,‘小宾‘,‘男‘,‘河南省商丘市‘,80,‘计算机安全‘);
INSERT INTO stu VALUES(NULL,‘小丽‘,‘女‘,‘河南省安阳市‘,90,‘化工食品‘);
INSERT INTO stu VALUES(NULL,‘小华‘,‘男‘,‘北京市‘,48,‘化工食品‘);
INSERT INTO stu VALUES(NULL,‘凯利‘,‘女‘,‘上海市‘,74,‘生物工程‘);
INSERT INTO stu VALUES(NULL,‘王菲‘,‘女‘,‘上海市‘,81,‘生物工程‘);
INSERT INTO stu VALUES(NULL,‘丽萨‘,‘女‘,‘河北省天津市‘,59,‘生物工程‘);
INSERT INTO stu VALUES(NULL,‘李莉‘,‘女‘,‘河北省天津市‘,0,‘化工食品‘);
INSERT INTO stu VALUES(NULL,‘张闯‘,‘男‘,‘河北省天津市‘,NULL,‘计算机网络‘);
INSERT INTO stu VALUES(NULL,‘张秋千‘,‘男‘,‘安徽省合肥市‘,NULL,‘计算机安全‘);
INSERT INTO stu VALUES(NULL,‘张李秋‘,‘男‘,‘安徽省‘,67,‘建筑工程‘);
INSERT INTO stu VALUES(NULL,‘秋香‘,NULL,‘安徽省‘,95,‘建筑工程‘);
-- 条件查询
-- 空条件判断 IS NULL 或 IS NOT NULL
SELECT * FROM stu WHERE sscore IS NULL;
SELECT sno,sname,sscore FROM stu WHERE sscore IS NOT NULL;
-- IN() 在...范围内 或 NOT IN()不在...范围内
SELECT * FROM stu WHERE sdept IN(‘计算机网络‘,‘计算机安全‘);
SELECT * FROM stu WHERE sdept NOT IN(‘计算机网络‘,‘计算机安全‘);
-- BETWEEN AND 在...之间 或 NOT BETWEEN AND 不在...之间
SELECT * FROM stu WHERE sscore NOT BETWEEN 70 AND 90;
SELECT * FROM stu WHERE sscore BETWEEN 70 AND 90;
-- NOT 取反运算符 ADN 与运算符 OR 或运算符
SELECT * FROM stu WHERE sdept = ‘建筑工程‘ OR sdept = ‘生物工程‘;
SELECT * FROM stu WHERE sgender = ‘男‘ AND sdept = ‘计算机网络‘;
-- LIKE ‘张%‘ 模糊查找运算符(%:代表0-n多个字符，_代表一个字符)
SELECT * FROM stu WHERE sname LIKE ‘李%‘;
SELECT * FROM stu WHERE sname LIKE ‘李_‘;
SELECT * FROM stu WHERE sname LIKE ‘%李%‘;
SELECT * FROM stu WHERE sname NOT LIKE ‘%李%‘;
-- 排序操作 DESC代表降序排序，ASC代表升序排序，默认升序排序
SELECT * FROM stu ORDER BY sscore DESC;
SELECT * FROM stu ORDER BY sscore ASC;
-- LIMIT 限制结果显示行数(用法：LIMIT 5 显示5行 或 LIMIT 0,5 从0位置开始显示5行,0代表数据库内起始行数),LIMIT 在SQL语句最后面
SELECT * FROM stu LIMIT 5;
SELECT * FROM stu LIMIT 0,5;
-- IFNULL() 与 IF()两个函数的使用及区别:IFNULL(exp1,exp2) = IF(exp1 IS NULL,exp2,exp3)
SELECT sno AS 学号,sname AS 姓名,sgender AS 性别,IFNULL(sscore,‘旷考‘) AS 成绩 FROM stu;
SELECT sno AS 学号,sname AS 姓名,sgender AS 性别,IF(sscore IS NULL,‘旷考‘,sscore) AS 成绩 FROM stu;
-- 集合函数(聚集函数)
AVG() -- 求平均值 (计算平均时，不包含空值)
SUM() -- 求和
COUNT() -- 求总记录数
MAX() -- 最大值
MIN() -- 最小值
SELECT AVG(sscore) 平均分,SUM(sscore) 总分,MAX(sscore) 最高分,MIN(sscore) 最低分,COUNT(*) 总人数,COUNT(sscore) 考试人数 FROM stu;
SELECT COUNT(DISTINCT sdept) 专业 FROM stu; -- DISTINCT 去除重复
SELECT sdept,AVG(sscore) FROM stu GROUP BY sdept; -- 分组查询,一般前面是分组名称，紧跟着是集合函数(聚集函数)
-- 条件中的那个查询是子查询，显示出最高分的学生信息
SELECT * FROM stu WHERE sscore =(
SELECT MAX(sscore) FROM stu -- 子查询，当子查询查到的数据只有一个时，可以用‘=‘,多于一条时只能用‘IN‘
);
SELECT * FROM stu WHERE sscore IN(
SELECT MAX(sscore) FROM stu -- 子查询
);
-- 统计男、女生分别有几人
SELECT IFNULL(sgender,‘保密‘),COUNT(*) FROM stu GROUP BY sgender;
-- 统计各个专业的平均成绩，显示平均成绩不小于60分的专业的前两条数据
SELECT sdept,AVG(sscore) FROM stu WHERE 1=1 GROUP BY sdept HAVING AVG(sscore)>=60 ORDER BY AVG(sscore) DESC LIMIT 2;
WHERE 查询条件
GROUP BY 分组
HAVING 是分组条件（使用前提是使用了分组）
ORDER BY 排序
LIMIT 取几条记录
-- 多表连接查询 查询结果是多个表中的信息
CREATE TABLE student(
sid INT UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
sname VARCHAR(10) NOT NULL,
sage TINYINT UNSIGNED,
stid SMALLINT UNSIGNED
)ENGINE=MYISAM DEFAULT CHARSET=utf8;
CREATE TABLE teacher(
tid SMALLINT UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
tname VARCHAR(10) NOT NULL
)ENGINE=MYISAM DEFAULT CHARSET=utf8;
INSERT INTO teacher VALUES(NULL,‘张老师‘);
INSERT INTO teacher VALUES(NULL,‘王老师‘);
INSERT INTO teacher VALUES(NULL,‘赵老师‘);
INSERT INTO teacher VALUES(NULL,‘李老师‘);
SELECT * FROM teacher;
INSERT INTO student VALUES(NULL,‘曹操‘,20,1);
INSERT INTO student VALUES(NULL,‘张飞‘,27,4);
INSERT INTO student VALUES(NULL,‘关羽‘,29,2);
INSERT INTO student VALUES(NULL,‘刘备‘,50,2);
INSERT INTO student VALUES(NULL,‘孙权‘,40,4);
INSERT INTO student VALUES(NULL,‘诸葛亮‘,24,3);
INSERT INTO student VALUES(NULL,‘姚盖‘,25,1);
INSERT INTO student VALUES(NULL,‘大乔‘,25,5);
SELECT * FROM student;
TRUNCATE student;
-- 查询学生学号、姓名、年龄及代课老师的姓名：
-- 等值链接查询
SELECT sid,sname,sage,tname FROM student,teacher WHERE student.stid=teacher.tid;
-- 左链接查询
SELECT sid,sname,sage,IFNULL(tname,‘无‘) FROM student s LEFT JOIN teacher t ON s.stid=t.tid;
-- 右链接查询
SELECT sid,sname,sage,IFNULL(tname,‘无‘) FROM student s RIGHT JOIN teacher t ON s.stid=t.tid;
-- 表的自身链接查询
SELECT a.sid,a.sname,b.sname FROM student a,student b WHERE a.sid<b.sid;
-- INSERT 插入语句
CREATE TABLE t1(
tid INT UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
tname VARCHAR(10) NOT NULL,
tage TINYINT UNSIGNED
)ENGINE=MYISAM DEFAULT CHARSET=utf8;
INSERT INTO t1 VALUES(NULL,‘jack‘,18); -- 插入一条数据
INSERT INTO t1 VALUES(NULL,‘jane‘,20),(NULL,‘lili‘,10); -- 插入多条数据
INSERT INTO t1 (tname,tage) VALUES(‘张三‘，20); -- 插入一条数据，字段对应
INSERT INTO t1 SELECT * FROM t1; -- 将查询的数据插入数据库(前提是没有主键约束)
REPLACE INTO t1 VALUES(1,‘库克‘,35); -- 如果主键相同则替换插入，不同则插入
-- DELETE 删除语句
DELETE FROM t1 WHERE tid=1; -- 删除t1表内tid为1的数据
DELETE FROM t1; -- 删除t1表内的所有数据(再次插入数据，主键接着之前的增加)
TRUNCATE t1; -- 截断表内的所有数据，清空数据，保留结构(再次插入数据的话，主键重新开始),不触发触发器
-- drop 删除语句
DROP DATABASE db;-- 删除数据库
DROP TABLE t1;-- 删除表
-- UPDATE 更新修改语句
UPDATE t1 SET tname = ‘小可‘,tage=18 WHERE tid =2;