Linux命令：MySQL系列之五--SELECT单表查询多表查询升级及删除，插入

Posted 2020-09-14

SELECT：查询

    SELECT select-list FROM tb WHERE qualification  根据标准qualification查找对应的行

  查询语句类型：  qualification条件  field领域  distinct独特的

    简单查询：

    多表查询：

    子查询：

SELECT * FROM tb_name: 查询tb_name表的所有信息

SELECT field1,field2 FROM tb_name: 投影显示所设定的领域条目（field），一个field就是一列

SELECT [DISTINCT] * FROM tb_name WHERE qualification；从tb_name表中选择符合条件的独特的不重复的条目。

FROM子句：表、多个表、其他SELECT语句

WHERE子句：布尔关系表达式  =、>、<、<=、>= 表示大于等于，小于等于，大于，小于，等于。

逻辑关系：AND(与&&)   OR(或||)    NOT(非!)    XOR(异或)

mysql>SELECT Name,Age,Gender FROM students WHERE NOT Age>20 AND NOT Gender=‘M‘; #选择查询年龄不大于20，且性别不是男的数据,也可以写成下面的条件

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

特殊关系：BETWEEN ... AND ...  在两者之间之间。

   LIKE ‘‘

       %：任意长度任意字符

       _：任意单个字符

Usage: SELECT Name FROM students WHERE Name LIKE ‘Y%‘; 查找Name字段以Y开头的数据。

    SELECT Name FROM students WHERE Name LIKE ‘%ing%‘;查找Name字段中必须包括ing的字段

   SELECT Name FROM students WHERE Name LIKE ‘Y___‘;查找Name字段中Y后面至少跟着3个字符的

 

REGEXP或者RLIKE 支持正则表达式：

   Usage:SELECT Name,Age FROM students WHERE Name RLIKE ‘^[XY].*$‘;

   查找Name字段行首为X或Y的数据

 IN离散条件的查找：

   Usage: SELECT Name,Age FROM students WHERE Age IN (18,20,25);

    查找Age字段为18或20或25岁的相关数据

有空值的时候，比较：IS NULL ,NOT NULL  ORDER BY ... 以...某字段升序排列

         ORDER BY ... 以...某字段降序排列

   Usage: SELECT Name,Age FROM students WHERE Name IS NULL;

     查找Name字段为空值的数据  

   SELECT Name,Age FROM students WHERE Name IS NULL;

     查找Name字段不为空值的数据

按照升序或者降序排列出查找的数据：ORDER BY field_name {ASC|DESC}; 

   ASC升序排列(默认值)DESC降序排列

   SELECT Name,Age FROM students WHERE CID IS NOT NULL ORDER BY Name;

     查找CID字段不为空的数据并按字段Name的升序排列；

  SELECT Name,Age FROM students WHERE CID IS NOT NULL ORDER BY Name DESC;

    查找CID字段不为空的数据并按字段Name的降序排列；

字段别名：AS

     Usage: SELECT Name AS Student_Name FROM student;

mysql> SELECT Name FROM student;

+------------+

| Name       |

+------------+

| Li Lianjie |

| Cheng Long |

| Yang Guo   |

| Guo Jing   |

+------------+

4 rows in set (0.00 sec)

mysql> SELECT Name AS Student_Name FROM student;

+--------------+

| Student_Name | AS别名为Student_Name

+--------------+

| Li Lianjie   |

| Cheng Long   |

| Yang Guo     |

| Guo Jing     |

+--------------+

4 rows in set (0.00 sec)

LIMIT子句：LIMIT [offset,]Count  offset偏移多少，Count显示多少

  Usage: SELECT Name AS Student_Name FROM student LIMIT 3;

       查找显示Name别名为Student_Name,只显示前3个数据

  Usage: SELECT Name AS Student_Name FROM student LIMIT 2,2;

   查找显示Name别名为Student_Name,偏移掉前2个数据不显示，显示第2个数据后的2个数据

mysql> SELECT Name AS Student_Name FROM student LIMIT 3;

+--------------+

| Student_Name |

+--------------+

| Li Lianjie   |

| Cheng Long   |

| Yang Guo     |

+--------------+

3 rows in set (0.00 sec)

mysql> SELECT Name AS Student_Name FROM student LIMIT 2,2;

+--------------+

| Student_Name |

+--------------+

| Yang Guo     |

| Guo Jing     |

+--------------+

2 rows in set (0.00 sec)

聚合运算：SUM()求和,MIN()最小值,MAX()最大值,AVG()平均值,COUNT()个数；

  新建立一张表：

mysql> CREATE TABLE class(ID INT NOT NULL PRIMARY KEY AUTO_INCREMENT,Name CHAR(20) NOT NULL UNSIGNED, Age TINYINT NOT NULL);

mysql> INSERT INTO class (Name,Age) VALUES (‘Yang Guo‘,22),(‘Guo Jing‘,46),(‘Xiao Longnv‘,18),(‘Huang Rong‘,40);

mysql> DESC class;

+-------+------------+------+-----+---------+----------------+

| Field | Type       | Null | Key | Default | Extra          |

+-------+------------+------+-----+---------+----------------+

| ID    | int(11)    | NO   | PRI | NULL    | auto_increment |

| Name  | char(20)   | NO   |     | NULL    |                |

| Age   | tinyint(4) | NO   |     | NULL    |                |

+-------+------------+------+-----+---------+----------------+

3 rows in set (0.00 sec)

mysql> SELECT * FROM class;

+----+-------------+-----+

| ID | Name        | Age |

+----+-------------+-----+

|  1 | Yang Guo    |  22 |

|  2 | Guo Jing    |  46 |

|  3 | Xiao Longnv |  18 |

|  4 | Huang Rong  |  40 |

+----+-------------+-----+

4 rows in set (0.00 sec)

mysql> SELECT SUM(Age) FROM class;

+----------+

| SUM(Age) |

+----------+

|      126 |

+----------+

1 row in set (0.02 sec)

mysql> SELECT MAX(Age) FROM class;

+----------+

| MAX(Age) |

+----------+

|       46 |

+----------+

1 row in set (0.00 sec)

mysql> SELECT MIN(Age) FROM class;

+----------+

| MIN(Age) |

+----------+

|       18 |

+----------+

1 row in set (0.00 sec)

mysql> SELECT AVG(Age) FROM class;

+----------+

| AVG(Age) |

+----------+

|  31.5000 |

+----------+

1 row in set (0.00 sec)

分组：GROUP BY ... HAVING qualification   根据...分组 并且满足条件qualification

 对于GROUP BY的条件选择需用HAVING作为条件筛选，而不是用WHERE

mysql> SELECT Age,Gender FROM class1 GROUP BY Gender; #以性别分组显示

+-----+--------+

| Age | Gender |

+-----+--------+

|  18 | F      |

|  22 | M      |

+-----+--------+

2 rows in set (0.00 sec)

mysql> SELECT AVG(Age),Gender FROM class1 GROUP BY Gender; #求男同学和女同学的平均年龄。

+----------+--------+

| AVG(Age) | Gender |

+----------+--------+

|  29.0000 | F      |

|  34.0000 | M      |

+----------+--------+

2 rows in set (0.00 sec)

mysql> SELECT Name,AVG(Age) FROM class1 GROUP BY Gender HAVING Name RLIKE ‘Y.*‘; 

#求以Gender分组的年龄平局值，且只显示以Y开头的Name字段的平均值和姓名

+----------+----------+

| Name     | AVG(Age) |

+----------+----------+

| Yang Guo |  34.0000 |

+----------+----------+

1 row in set (0.00 sec)

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