单表查询

Posted 2020-12-03 2734156755z

单表查询
查询所有列

1 SELECT * FROM product;

查询指定列

1 SELECT pro_name,price,pinpai FROM product;

添加常量列

1 SELECT pro_name AS ‘产品名称‘,price FROM product;

创建学生表

 1 CREATE TABLE stu(
 2      sid  INT,
 3      sname  VARCHAR(10),
 4      sex  VARCHAR(2),
 5      servlet  DOUBLE,
 6      jsp   DOUBLE,
 7      html  DOUBLE 
 8 )
 9 SHOW TABLES;
10 INSERT INTO stu VALUES(1,‘佩奇‘,‘女‘,100,60,80);
11 INSERT INTO stu VALUES(2,‘乔治‘,‘男‘,25,58,100);
12 INSERT INTO stu VALUES(3,‘薛之谦‘,‘男‘,100,100,100);
13 INSERT INTO stu VALUES(4,‘李荣浩‘,‘男‘,90,90,90);
14 INSERT INTO stu(sid,sname,servlet,html) VALUES(5,‘于文文‘,90,90);
15 INSERT INTO stu(sid,sname,servlet,html) VALUES(6,‘‘,90,10);
16 SELECT * FROM stu;

查询时合并列

1 SELECT sname,(servlet+jsp+html) AS ‘总成绩‘ FROM stu;

查询时去除重复记录

1 SELECT DISTINCT sex FROM stu;

条件查询

1 SELECT * FROM stu WHERE sex=‘男‘ AND sname=‘薛之谦‘;
2 SELECT * FROM stu WHERE sex=‘女‘ OR sname=‘薛之谦‘;

查询大于70分的学生

1 SELECT * FROM stu WHERE servlet>60;

查询jsp成绩不等于60分的学生

1 SELECT * FROM stu WHERE jsp<>60;

查询html成绩在60和100之间的学生

1 SELECT * FROM stu WHERE html BETWEEN 60 AND 100;
2 SELECT * FROM stu WHERE html >= 60 AND html<=100;

查询sex为null的学生

1 SELECT * FROM stu WHERE sex IS NULL;
2 SELECT * FROM stu WHERE sex IS NOT NULL;

 查询sname是空字符串的学生

1 SELECT * FROM stu WHERE sname=‘‘;
2 SELECT * FROM stu WHERE sname<>‘‘;

查询sex不为空的学生

1 SELECT * FROM stu WHERE sex IS NOT NULL AND sex<>‘‘;

查询所有产品中带有索尼的产品

1 SELECT * FROM product WHERE pro_name LIKE ‘%索尼%‘;

 查询班级中所有两个字的同学

1 SELECT * FROM stu WHERE sname LIKE ‘__‘;

 聚合函数：查询stu表中所有学生的servlet总成绩

1 SELECT SUM(servlet) FROM stu;

聚合函数：查询stu表中所有学生的servlet平均成绩

1 SELECT AVG(servlet) FROM stu;

 聚合函数：查询stu表中jsp课程中的最高分

1 SELECT MAX(JSP) FROM stu;

最低分

1 SELECT MIN(jsp) FROM stu;

查询stu表中有多少人

1 SELECT COUNT(*) FROM stu;（每列统计 取最大值）

 分页

1 SELECT * FROM stu LIMIT 0,2;

查询排序

1 SELECT * FROM stu ORDER BY html ASC;（升序）
2 SELECT * FROM stu ORDER BY html DESC;(倒序)

查询stu表中所有男同学的html成绩排序

1 SELECT * FROM stu WHERE sex=‘男‘ ORDER BY html;
2 SELECT * FROM stu ORDER BY jsp ASC,html DESC;

查询男女人数

1 SELECT sex,COUNT(*) FROM stu GROUP BY sex; 

查询总人数大于2的性别

1 SELECT SEX,COUNT(*) FROM STU GROUP BY sex HAVING COUNT(*)>2;