SELECT * FROM stu;
#添加多行数据
INSERT INTO stu VALUES(1,'aaa'),(2,'ccc'),(3,'小明');
DROP TABLE stu_temp;
CREATE TABLE stu_temp
(
)
SELECT * FROM stu_temp;
#1:stu_temp表 不能存在的
#2:一边取数据一边创建数据表
#3:stu_temp 约束（主键）是从stu中拿不出来的
CREATE TABLE stu_temp(SELECT * FROM stu);
SELECT * FROM info;
#4：只需要某些数据表的基本结构，不需要数据！
# 查询过程中 where条件不成立，查询的就是结构，否则就是数据+结构
CREATE TABLE info_temp(SELECT * FROM info WHERE 1=2);
SELECT * FROM info WHERE 1=1;
SELECT * FROM info_temp;
CREATE TABLE info_temp3(SELECT id,age FROM info);
SELECT * FROM info_temp3;
DELETE FROM info_temp3;
SELECT * FROM info;
#通过age排序 1：升序 2：降序
SELECT * FROM info ORDER BY age ASC;
SELECT * FROM info ORDER BY age DESC;
SELECT * FROM info WHERE age>18 ORDER BY age DESC;
SELECT t.`id` AS 编号,t.`name` 姓名,t.`age` 年龄 FROM info t;
# 只要出现+，会发生两个字段之间的运算
SELECT t.id+'.'+t.`age` AS 连接后 FROM info t;
SELECT * FROM borrow WHERE returndate IS NULL;
#not : 不是的
SELECT * FROM borrow WHERE returndate IS NOT NULL;
#聚合函数
SELECT * FROM score;
#最高分 max()
SELECT MAX(t.`sexam`) AS 最高分 FROM score t;
#最低分 min()
SELECT MIN(t.`sexam`) AS 最低分 FROM score t;
#平均分 avg()
SELECT AVG(t.`sexam`) AS 平均分 FROM score t;
#总行数
SELECT COUNT(*) AS 行数 FROM score;
#总和
SELECT SUM(t.`sexam`) AS 总分成绩 FROM score t;
#字符串函数
#1：拼接字符串函数
SELECT CONCAT('jack','and','rose','一起吃个饭！') AS 字符串;
SELECT * FROM student;
#数据表中，是不区分大小写
SELECT CONCAT(t.`StudentName`,t.`LoginPwd`,t.`Address`) 字符串 FROM student t;
SELECT * FROM student;
#select 都是虚拟结果，不会对原始数据表产生影响
#2:insert函数 替换
SELECT INSERT(t.`Address`,2,3,'abc') AS 替换 FROM student t;
#3：大小写转换
SELECT UPPER(LOWER(UPPER(t.`Email`))) 转换 FROM student t;
#4:截取字符串
SELECT SUBSTRING('abcefghgk',2);
SELECT SUBSTRING('abcefghgk',2,3);
#日期函数
#1:当前日期
SELECT CURDATE();
#2:当前时间
SELECT CURTIME();
#3:当前日期和时间
SELECT NOW();
#4:一年中的第几周
SELECT WEEK(NOW());
#5:年份
SELECT YEAR(NOW());
#6:月份
SELECT MONTH(NOW());
SELECT * FROM student;
SELECT YEAR(t.`BornDate`) AS 年份,MONTH(t.`BornDate`) AS 月份 FROM student t;
SELECT DATEDIFF(NOW(),'2002-02-02');
#数据表中计算年龄
SELECT DATEDIFF(NOW(),a.`BornDate`)/365 AS 生日 FROM student a;
SELECT FLOOR(DATEDIFF(NOW(),a.`BornDate`)/365) AS 生日 FROM student a;
#8:计算日期增量
SELECT ADDDATE(NOW(),50000);
SELECT ADDDATE(NOW(),500000);
#数学函数
#向上取整
SELECT CEIL(RAND()*10);
#向下取整
SELECT FLOOR(RAND()*10);
#limit:分页
SELECT * FROM student;
/*
*/
#显示第1页
SELECT * FROM student LIMIT 0,3;
#显示第2页？（当前页-1）*每页显示条数
SELECT * FROM student LIMIT 3,3;
SELECT * FROM student LIMIT 6,3;
#查询比“谢挺疯”年龄大的学生信息
#用一个sql查询出来，需要子查询
#子查询执行的原理：
#先执行子语句，执行完成后，把查询的结果给父查询
#从上往下写，先要结果，后要过程
SELECT * FROM student t WHERE t.`BornDate`<(
);
SELECT * FROM student;
SELECT * FROM score;
SELECT * FROM SUBJECT;
/*
查询参加最近一次Logic Java考试成绩的学生的最高分和最低分
1:最近
2:Logic Java
*/
SELECT MAX(sexam) 最高分,MIN(sexam) 最低分 FROM score WHERE subjectno=(
AND sdate =(
);
#优化后：数据库性能优化
SELECT MAX(sexam) 最高分,MIN(sexam) 最低分 FROM score WHERE sdate =(
);
SELECT * FROM student;
SELECT * FROM score;
SELECT * FROM SUBJECT;
#查询“Logic Java”课程考试成绩为60分的学生名单
#in多个结果返回给父级查询
SELECT * FROM student WHERE studentno IN (
)
#not:取反
SELECT * FROM student WHERE studentno NOT IN (
)
#查询参加“Logic Java”课程最近一次考试的在读学生名单
SELECT * FROM student WHERE studentno IN(
)
SELECT * FROM student WHERE studentno NOT IN(
)
文章来源: mysql多表子查询