linux下mysql Select查询命令

SELECT查询格式:

SELECT 字段 FROM 表 WHERE 条件;

例如: SELECT * FROM 表名;           #查询表所有内容

SELECT 字段1,字段2... FROM 表名;       #投影,仅显示指定字段中的内容

SELECT [DISTINCT] * FROM 表名 WHERE 条件; #选择,仅显示符合条件的所有字段部分行内容

SELECT * FROM students;

SELECT Name,Age FROM students;

SELECT * FROM students WHERE Age>=20;

SELECT Name,Age FROM students WHERE Age>=20;

SELECT Name,Age FROM students WHERE Gender=‘F‘;

SELECT DISTINCT Gender FROM students; #DISTINCT仅显示Gender字段中的相同值只显示一次

2.查询语句类型：简单查询,多表查询,子查询

FROM子句： 要查询的关系    表、多个表、其它SELECT语句

WHERE子句：布尔关系表达式,

2.1.比较操作符如下: =、>、>=、<=、<

SELECT Name,Age FROM students WHERE Age+1>20;

2.2.逻辑关系：AND OR NOT

SELECT Name,Age,Gender FROM students WHERE Age>20 AND Gender=‘F‘;

SELECT Name,Age,Gender FROM students WHERE Age>20 OR Gender=‘F‘;

SELECT Name,Age,Gender FROM students WHERE NOT Age>20 ‘;

SELECT Name,Age,Gender FROM students WHERE NOT Age>20 AND NOT Gender=‘F‘;

SELECT Name,Age,Gender FROM students WHERE NOT ( Age>20 OR Gender=‘F‘ );

2.3.BETWEEN ... AND ...#在AND两值之间

SELECT Name,Age FROM students WHERE Age>=20 AND Age<=25;

SELECT Name,Age FROM students WHERE Age BETWEEN 20 AND 25;

2.4.LIKE ‘‘

%: 任意长度任意字符

_：任意单个字符

SELECT Name FROM students WHERE Name LIKE ‘Y%‘;

SELECT Name FROM students WHERE Name LIKE ‘Y____‘;

SELECT Name FROM students WHERE Name LIKE ‘%willow%‘;

2.5.REGEXP, RLIKE(正规表达式)

SELECT Name FROM students WHERE Name RLIKE ‘^[LNY].*$‘;

2.6.IN(值1,值2,...)

SELECT Name,Age FROM students WHERE Age IN (20,30,40);

2.7.IS NULL

SELECT Name FROM students WHERE CID2 IS NULL;

2.8.IS NOT NULL

SELECT Name FROM students WHERE CID2 IS NOT NULL;

2.9.ORDER BY 字段名 {ASC|DESC}  #排序,默认为升序

SELECT Name FROM students WHERE CID2 IS NULL ORDER BY Name;

SELECT Name FROM students WHERE CID2 IS NULL ORDER BY Name DESC;

2.10.AS,引用字段别名或表别名

SELECT Name AS student_name FROM students;

2.11.LIMIT子句：LIMIT [偏移行,]显示行

SELECT Name AS student_name FROM students LIMIT 2;

SELECT Name AS student_name FROM students LIMIT 2,3;

2.12.聚合：SUM(), MIN(), MAX(), AVG(), COUNT()

SELECT AVG(age) FROM students;

SELECT MAX(age) FROM students;

SELECT MIN(age) FROM students;

SELECT SUM(age) FROM students;

SELECT COUNT(age) FROM students;

2.13.GROUP BY: 分组

SELECT AVG(age) FROM students WHERE Gender=‘M‘;

SELECT AVG(age) FROM students WHERE GROUP BY Gender;

SELECT COUNT(CID1) AS Persons,CID1 FROM students GROUP BY CID1;

2.14.HAVING 条件   (必须与GROUP BY一起使用)

SELECT COUNT(CID1) AS Persons,CID1 FROM students GROUP BY CID1 HAVING Persons>=2;

3.多表查询：

3.1.交叉连接：笛卡尔乘积

SELECT * FROM students,courses;

3.2.自然连接：

SELECT * FROM students,courses WHERE students.CID1 = courses.CID;

SELECT students.Name,courses.Cname FROM students,courses WHERE students.CID1 = courses.CID;

SELECT s.Name,c.Cname FROM students AS s,courses AS c WHERE s.CID1 = c.CID;

3.3.左外连接：... LEFT JOIN ... ON ...#以左表为基准

SELECT s.Name,c.Cname FROM students AS s LEFT JOIN courses AS c ON s.CID1 = c.CID;

3.4.右外连接: ... RIGHT JOIN ... ON ...#以右表为基准

SELECT s.Name,c.Cname FROM students AS s RIGHT JOIN courses AS c ON s.CID1 = c.CID;

3.5.自连接：同一张表中的自连接

SELECT c.Name As student,s.Name AS teacher FROM students AS c,students AS s WHERE c.TID=s.SID;

4.子查询：

4.1.比较操作中使用子查询：子查询只能返回单个值；

SELECT Name FROM students WHERE Age > (SELECT AVG(age) FROM students);

4.2.IN(): 使用子查询；

SELECT Name FROM students WHERE Age IN (SELECT Age FROM tutors);

4.3.在FROM中使用子查询；

SELECT Name,Age FROM (SELECT Name,Age FROM students) AS s WHERE s.Age >= 20;

4.4.联合查询：UNION

(SELECT Name,Age FROM students) UNION (SELECT Tname,Age FROM tutors);

5.综合复杂查询案例:

5.1.找出courses表中没有被students选中的CID2学习的课程的课程名称；

SELECT Cname FROM courses WHERE CID NOT IN (SELECT DISTINCT CID2 FROM students WHERE CID2 IS NOT NULL);

5.2.找出没有教授任何课程的老师

SELECT Tname FROM tutors WHERE TID NOT IN (SELECT DISTINCT TID FROM courses);

找出students表中CID1有两个(含)以上同学学习了的同一个门课程的课程名称

SELECT Cname FROM courses WHERE CID IN (SELECT CID1 FROM students GROUP BY CID1 HAVING COUNT(CID1) >= 2);

5.3.找出每一位老师及其所教授的课程；没有教授的课程的保持为NULL

SELECT t.Tname,c.Cname FROM tutors AS t LEFT JOIN courses AS c ON t.TID=c.TID;

5.4.找出每一个课程及其相关的老师，没有老师教授的课程将其老师显示为NULL

SELECT t.Tname,c.Cname FROM tutors AS t RIGHT JOIN courses AS c ON t.TID=c.TID;

5.5.找出每位同学CID1课程的课程名及其讲授了相关课程的老师的名称

SELECT Name,Cname,Tname FROM students,courses,tutors WHERE students.CID1=courses.CID;

6.视图:就是存储下来的SELECT语句,即基于基表的查询结果；

6.1.创建视图:

CREATE VIEW 视图名称 AS SELECT语句;

CREATE VIEW sct_viewname AS SELECT Name,Cname,Tname FROM students,courses,tutors WHERE students.CID1=courses.CID;

SELECT * FROM sct_viewname;

SHOW TABLES;可查看到此视图当成表

SHOW TABLE STATUS\G

6.2.删除视图:

DROP VIEW 视图名称

6.3.查看创建视图命令

SHOW CREATE VIWE 视图名称;

6.4.查看创建表命令

SHOW CREATE TABLE 表名称;