第七章 查询数据

其他 2018-12-21 16:44:31 阅读次数: 0

7.1 基本查询语句
使用SELECT语句。其基本格式：

SELECT 
{*|<字段列表>}
[
FROM <表1>，<表2>...
[WHERE <表达式>
HAVING____
GROUP BY__
ORDER BY__
LIMIT__
]
SELECT[字段1，字段2，...]
FROM[表或视图]
WHERE[查询条件]

WHERE子句是可选项，如果选择该选项，将限定查询行必须满足的查询条件。
下面使用一个例子说明如何使用SELECT从单个表中获取数据。
首先定义数据表，输入如下语句：

mysql> CREATE TABLE fruits(
    -> f_id char(10) NOT NULL,
    -> s_id INT NOT NULL,
    -> f_name char(255) NOT NULL,
    -> f_price decimal(8,2) NOT NULL,
    -> PRIMARY KEY(f_id)
    -> );
Query OK, 0 rows affected (0.36 sec)

插入数据：

mysql> INSERT INTO fruits(f_id,s_id,f_name,f_price)
    -> VALUES('a1',101,'apple',5.2),
    -> ('b1',101,'blackberry',10.2),
    -> ('bs1',102,'orange',11.2),
    -> ('bs2',105,'melon',8.2),
    -> ('t1',102,'banana',10.3),
    -> ('t2',102,'grape',5.3),
    -> ('o2',103,'coconut',9.2),
    -> ('c0',101,'cherry',3.2),
    -> ('a2',103,'apricot',2.2),
    -> ('l2',104,'lemon',6.4),
    -> ('b2',104,'berry',7.6),
    -> ('m1',106,'mango',15.6),
    -> ('m2',105,'xbabay',2.6),
    -> ('t4',107,'xbababa',3.6),
    -> ('m3',105,'xxtt',11.6),
    -> ('b5',107,'xxxx',3.6);
Query OK, 16 rows affected (0.09 sec)
Records: 16  Duplicates: 0  Warnings: 0

使用SELECT语句查询f_id字段：

mysql> SELECT f_id FROM fruits;
+------+
| f_id |
+------+
| a1   |
| a2   |
| b1   |
| b2   |
| b5   |
| bs1  |
| bs2  |
| c0   |
| l2   |
| m1   |
| m2   |
| m3   |
| o2   |
| t1   |
| t2   |
| t4   |
+------+
16 rows in set (0.13 sec)

7.2 单表查询

SELECT * FROM db;比列出所有属性名效率低，因为*有一个转换的过程。

在MySQL中，可通过SELECT __ FROM __ WHERE__来查询指定记录。
WHERE后面的操作符有：
IN:用来查询满足指定范围内的条件的记录；
BETWEEN AND：用来查询某个范围内的值；
LIKE：使用通配符进行匹配查找；
（通配符%用于匹配在指定位置的任意数目的字符。b%：b开头；%g%：包含g；b%y：b开头，y结尾。）
（_通配符，只能匹配任意单个字符，即指定个数）
AND：限定只有满足所有查询条件的记录才会被返回，AND可以连接两个甚至两个以上的查询条件。
OR：只需满足其中一个条件即可返回。（注意：AND的优先级大于OR）

7.2.10查询结果不重复
DISTINCT:使查询结果不重复

mysql> SELECT DISTINCT Id,name FROM worker;
+----+--------+
| Id | name   |
+----+--------+
|  1 | jimy   |
|  2 | Tom    |
|  3 | Keiven |
|  4 | Michal |
|  5 | Nick   |
+----+--------+
5 rows in set (0.00 sec)

7.2.11 对查询结果排序

  • 单列排序

mysql> SELECT name FROM worker ORDER BY name;
+--------+
| name   |
+--------+
| jimy   |
| Keiven |
| Michal |
| Nick   |
| Tom    |
+--------+
5 rows in set (0.10 sec)
  • 多列排序
mysql> SELECT name,Id FROM worker ORDER BY name,Id;
+--------+----+
| name   | Id |
+--------+----+
| jimy   |  1 |
| Keiven |  3 |
| Michal |  4 |
| Nick   |  5 |
| Tom    |  2 |
+--------+----+
5 rows in set (0.00 sec)

注：在对多列进行排序的时候，首先排序的第一列必须有相同的列值，才会对第二列进行排序。如果第一列所有值都是唯一的，将不再对第二列进行排序。

  • 指定排序方向
    排序默认为升序：ASC；可以自己指定升降序，ORDER BY__ ASC/DESC。
mysql> SELECT Id FROM worker ORDER BY Id DESC;
+----+
| Id |
+----+
|  5 |
|  4 |
|  3 |
|  2 |
|  1 |
+----+
5 rows in set (0.11 sec)

7.2.12 分组查询

  1. 创建分组
    GROUP关键字通常和集合函数一起使用，如：MAX()、MIN()、COUNT()、SUM()、AVG()。
mysql> SELECT Id,COUNT(*) AS Total FROM worker GROUP BY Id;
+----+-------+
| Id | Total |
+----+-------+
|  1 |     1 |
|  2 |     1 |
|  3 |     1 |
|  4 |     1 |
|  5 |     1 |
+----+-------+
5 rows in set (0.00 sec)
  1. 使用HAVING过滤分组
    HAVING 在数据分组之后进行过滤来分组。
  2. 在GROUP BY子句中使用WITH ROOLUP
    使用WITH ROLLUP关键字之后，在所有查询出的分组记录之后增加一条记录，该记录计算查询出的所有记录的总和。
  3. 多字段分组
    即GROUP BY 后面接需要的分组字段。
  4. GROUP BU和ORDER BY 一起使用
    注：使用ROOLUP时，不能同时使用ORDER BY子句进行结果排序。

7.4 内连接查询
内连接（INNER JOIN）使用比较运算符进行表间某些列数据的比较操作，并列出这些表中与连接相匹配的数据行，组合成新的记录。p212.

7.7 为表和字段取别名
使用AS。

7.8 使用正则表达式查询
REGEXP关键字指定正则表达式的字符匹配模式。
^：匹配文本的开始字符
$:匹配文本的结束字符
.：匹配任何单个字符，如b.t：bit、bat等
*：匹配零个或多个在它前面的字符
+：匹配前面的字符一次或多次，如：ba+:b开头后面紧跟至少一个a
<字符串>：匹配包含指定的字符串的文本
[字符集]：匹配字符集中的任意一个字符
[^]：匹配不在括号中的任何字符
字符串{n，}：匹配前面的字符串至少n次
字符串{n，m}：匹配前面的字符串至少n次，至多m次

7.9 综合案例

  1. 创建数据表employee和dept
mysql> CREATE TABLE dept(
    -> d_no INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
    -> d_name VARCHAR(50),
    -> d_location VARCHAR(100)
    -> );
Query OK, 0 rows affected (0.37 sec)

mysql> CREATE TABLE employee(
    -> e_no INT NOT NULL PRIMARY KEY,
    -> e_name VARCHAR(100) NOT NULL,
    -> e_gender CHAR(2) NOT NULL,
    -> dept_no INT NOT NULL,
    -> e_job VARCHAR(100) NOT NULL,
    -> e_salary SMALLINT NOT NULL,
    -> hireDate DATE,
    -> CONSTRAINT dno_fk FOREIGN KEY(dept_no)
    -> REFERENCES dept(d_no)
    -> );
Query OK, 0 rows affected (0.35 sec)
  1. 将指定记录分别插入两个表中
mysql> INSERT INTO dept
    -> VALUES (10,'ACCOUNTING','ShangHai'),
    -> (20,'RESEARCH','BeiJing'),
    -> (30,'SALES','ShenZhen'),
    -> (40,'OPERATIONS','FuJian');
Query OK, 4 rows affected (0.06 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> INSERT INTO employee
    -> VALUES(1001,'SMITH','m',20,'CLERK',800,'2005-11-12'),
    -> (1002,'ALLEN','f',30,'SALESMAN',1600,'2001-05-12'),
    -> (1003,'WARD','f',30,'SALESMAN',1250,'2001-05-12'),
    -> (1004,'JONES','m',20,'MANAGER',2975,'1998-05-18'),
    -> (1005,'MARTIN','m',30,'SALSMAN',1250,'2001-06-12'),
    -> (1006,'BLAKE','f',30,'MANAGER',2850,'1997-02-15'),
    -> (1007,'CLARK','m',10,'MANAGER',2450,'2002-09-12'),
    -> (1008,'SCOTT','m',20,'ANALYST',3000,'2003-05-12'),
    -> (1009,'KING','f',10,'PRESIDENT',5000,'1995-01-01'),
    -> (1010,'TURNER','f',30,'SALESMAN',1500,'1997-10-12'),
    -> (1011,'ADAMS','m',20,'CLERK',1100,'1999-10-05'),
    -> (1012,'JAMES','m',30,'CLERK',950,'2008-06-15');
Query OK, 12 rows affected (0.46 sec)
Records: 12  Duplicates: 0  Warnings: 0
  1. 在employee表中，查询有记录的e_no、e_name和e_salary字段
mysql> SELECT e_no,e_name,e_salary FROM employee;
+------+--------+----------+
| e_no | e_name | e_salary |
+------+--------+----------+
| 1001 | SMITH  |      800 |
| 1002 | ALLEN  |     1600 |
| 1003 | WARD   |     1250 |
| 1004 | JONES  |     2975 |
| 1005 | MARTIN |     1250 |
| 1006 | BLAKE  |     2850 |
| 1007 | CLARK  |     2450 |
| 1008 | SCOTT  |     3000 |
| 1009 | KING   |     5000 |
| 1010 | TURNER |     1500 |
| 1011 | ADAMS  |     1100 |
| 1012 | JAMES  |      950 |
+------+--------+----------+
12 rows in set (0.00 sec)
  1. 在employee表中，查询dept_no=10和20的所有记录
mysql> SELECT * FROM employee WHERE dept_no IN (10,20);
+------+--------+----------+---------+-----------+----------+------------+
| e_no | e_name | e_gender | dept_no | e_job     | e_salary | hireDate   |
+------+--------+----------+---------+-----------+----------+------------+
| 1001 | SMITH  | m        |      20 | CLERK     |      800 | 2005-11-12 |
| 1004 | JONES  | m        |      20 | MANAGER   |     2975 | 1998-05-18 |
| 1007 | CLARK  | m        |      10 | MANAGER   |     2450 | 2002-09-12 |
| 1008 | SCOTT  | m        |      20 | ANALYST   |     3000 | 2003-05-12 |
| 1009 | KING   | f        |      10 | PRESIDENT |     5000 | 1995-01-01 |
| 1011 | ADAMS  | m        |      20 | CLERK     |     1100 | 1999-10-05 |
+------+--------+----------+---------+-----------+----------+------------+
6 rows in set (0.01 sec)
  1. 在employee表中，查询工资范围在800~2500之间的员工信息
mysql> SELECT * FROM employee WHERE e_salary BETWEEN 800 AND 2500;
+------+--------+----------+---------+----------+----------+------------+
| e_no | e_name | e_gender | dept_no | e_job    | e_salary | hireDate   |
+------+--------+----------+---------+----------+----------+------------+
| 1001 | SMITH  | m        |      20 | CLERK    |      800 | 2005-11-12 |
| 1002 | ALLEN  | f        |      30 | SALESMAN |     1600 | 2001-05-12 |
| 1003 | WARD   | f        |      30 | SALESMAN |     1250 | 2001-05-12 |
| 1005 | MARTIN | m        |      30 | SALSMAN  |     1250 | 2001-06-12 |
| 1007 | CLARK  | m        |      10 | MANAGER  |     2450 | 2002-09-12 |
| 1010 | TURNER | f        |      30 | SALESMAN |     1500 | 1997-10-12 |
| 1011 | ADAMS  | m        |      20 | CLERK    |     1100 | 1999-10-05 |
| 1012 | JAMES  | m        |      30 | CLERK    |      950 | 2008-06-15 |
+------+--------+----------+---------+----------+----------+------------+
8 rows in set (0.00 sec)
  1. 在employee表中，查询部门编号为20的部门中的员工信息
mysql> SELECT * FROM employee WHERE dept_no=20;
+------+--------+----------+---------+---------+----------+------------+
| e_no | e_name | e_gender | dept_no | e_job   | e_salary | hireDate   |
+------+--------+----------+---------+---------+----------+------------+
| 1001 | SMITH  | m        |      20 | CLERK   |      800 | 2005-11-12 |
| 1004 | JONES  | m        |      20 | MANAGER |     2975 | 1998-05-18 |
| 1008 | SCOTT  | m        |      20 | ANALYST |     3000 | 2003-05-12 |
| 1011 | ADAMS  | m        |      20 | CLERK   |     1100 | 1999-10-05 |
+------+--------+----------+---------+---------+----------+------------+
4 rows in set (0.00 sec)
  1. 在employee表中，查询每个部门的最高工资的员工信息
mysql> SELECT dept_no,MAX(e_salary) FROM employee GROUP BY dept_no;
+---------+---------------+
| dept_no | MAX(e_salary) |
+---------+---------------+
|      10 |          5000 |
|      20 |          3000 |
|      30 |          2850 |
+---------+---------------+
3 rows in set (0.00 sec)
  1. 查询员工BLAKE所在部门和部门所在地
mysql> SELECT d_no,d_location FROM dept WHERE d_no
    -> =(SELECT dept_no FROM employee WHERE e_name='BLAKE');
+------+------------+
| d_no | d_location |
+------+------------+
|   30 | ShenZhen   |
+------+------------+
1 row in set (0.00 sec)
  1. 使用连接查询，查询所有员工的部门和部门信息
mysql> SELECT e_no,e_name,dept_no,d_name,d_location
    -> FROM employee,dept WHERE dept.d_no=employee.dept_no;
+------+--------+---------+------------+------------+
| e_no | e_name | dept_no | d_name     | d_location |
+------+--------+---------+------------+------------+
| 1007 | CLARK  |      10 | ACCOUNTING | ShangHai   |
| 1009 | KING   |      10 | ACCOUNTING | ShangHai   |
| 1001 | SMITH  |      20 | RESEARCH   | BeiJing    |
| 1004 | JONES  |      20 | RESEARCH   | BeiJing    |
| 1008 | SCOTT  |      20 | RESEARCH   | BeiJing    |
| 1011 | ADAMS  |      20 | RESEARCH   | BeiJing    |
| 1002 | ALLEN  |      30 | SALES      | ShenZhen   |
| 1003 | WARD   |      30 | SALES      | ShenZhen   |
| 1005 | MARTIN |      30 | SALES      | ShenZhen   |
| 1006 | BLAKE  |      30 | SALES      | ShenZhen   |
| 1010 | TURNER |      30 | SALES      | ShenZhen   |
| 1012 | JAMES  |      30 | SALES      | ShenZhen   |
+------+--------+---------+------------+------------+
12 rows in set (0.00 sec)
  1. 在employee表中，计算每个部门各有多少名员工
mysql> SELECT dept_no,COUNT(*) FROM employee GROUP BY dept_no;
+---------+----------+
| dept_no | COUNT(*) |
+---------+----------+
|      10 |        2 |
|      20 |        4 |
|      30 |        6 |
+---------+----------+
3 rows in set (0.00 sec)
  1. 在employee表中，计算不同类型职工的总工资数
mysql> SELECT e_job,SUM(e_salary) FROM employee GROUP BY e_job;
+-----------+---------------+
| e_job     | SUM(e_salary) |
+-----------+---------------+
| ANALYST   |          3000 |
| CLERK     |          2850 |
| MANAGER   |          8275 |
| PRESIDENT |          5000 |
| SALESMAN  |          4350 |
| SALSMAN   |          1250 |
+-----------+---------------+
6 rows in set (0.00 sec)
  1. 在employee表中，计算不同部门的平均工资
mysql> SELECT dept_no,AVG(e_salary) FROM employee GROUP BY dept_no;
+---------+---------------+
| dept_no | AVG(e_salary) |
+---------+---------------+
|      10 |     3725.0000 |
|      20 |     1968.7500 |
|      30 |     1566.6667 |
+---------+---------------+
3 rows in set (0.00 sec)
  1. 在employee表中，查询工资低于1500的员工信息
mysql> SELECT * FROM employee WHERE e_salary<1500;
+------+--------+----------+---------+----------+----------+------------+
| e_no | e_name | e_gender | dept_no | e_job    | e_salary | hireDate   |
+------+--------+----------+---------+----------+----------+------------+
| 1001 | SMITH  | m        |      20 | CLERK    |      800 | 2005-11-12 |
| 1003 | WARD   | f        |      30 | SALESMAN |     1250 | 2001-05-12 |
| 1005 | MARTIN | m        |      30 | SALSMAN  |     1250 | 2001-06-12 |
| 1011 | ADAMS  | m        |      20 | CLERK    |     1100 | 1999-10-05 |
| 1012 | JAMES  | m        |      30 | CLERK    |      950 | 2008-06-15 |
+------+--------+----------+---------+----------+----------+------------+
5 rows in set (0.00 sec)
  1. 在employee表中，将查询记录先按部门编号由高到低排列，再按员工工资由高到低排列
mysql> SELECT e_name,dept_no,e_salary FROM employee
    -> ORDER BY dept_no DESC,e_salary DESC;
+--------+---------+----------+
| e_name | dept_no | e_salary |
+--------+---------+----------+
| BLAKE  |      30 |     2850 |
| ALLEN  |      30 |     1600 |
| TURNER |      30 |     1500 |
| WARD   |      30 |     1250 |
| MARTIN |      30 |     1250 |
| JAMES  |      30 |      950 |
| SCOTT  |      20 |     3000 |
| JONES  |      20 |     2975 |
| ADAMS  |      20 |     1100 |
| SMITH  |      20 |      800 |
| KING   |      10 |     5000 |
| CLARK  |      10 |     2450 |
+--------+---------+----------+
12 rows in set (0.00 sec)
  1. 在employee表中，查询员工姓名以字母‘A’或‘S’开头的员工信息
mysql> SELECT * FROM employee WHERE e_name REGEXP'^[AS]';
+------+--------+----------+---------+----------+----------+------------+
| e_no | e_name | e_gender | dept_no | e_job    | e_salary | hireDate   |
+------+--------+----------+---------+----------+----------+------------+
| 1001 | SMITH  | m        |      20 | CLERK    |      800 | 2005-11-12 |
| 1002 | ALLEN  | f        |      30 | SALESMAN |     1600 | 2001-05-12 |
| 1008 | SCOTT  | m        |      20 | ANALYST  |     3000 | 2003-05-12 |
| 1011 | ADAMS  | m        |      20 | CLERK    |     1100 | 1999-10-05 |
+------+--------+----------+---------+----------+----------+------------+
4 rows in set (0.10 sec)
  1. 在employee表中，查询到目前为止，工龄大于等于十年的员工信息
mysql> SELECT * FROM employee WHERE YEAR(CURDATE())-YEAR(hireDate)>=10;
+------+--------+----------+---------+-----------+----------+------------+
| e_no | e_name | e_gender | dept_no | e_job     | e_salary | hireDate   |
+------+--------+----------+---------+-----------+----------+------------+
| 1001 | SMITH  | m        |      20 | CLERK     |      800 | 2005-11-12 |
| 1002 | ALLEN  | f        |      30 | SALESMAN  |     1600 | 2001-05-12 |
| 1003 | WARD   | f        |      30 | SALESMAN  |     1250 | 2001-05-12 |
| 1004 | JONES  | m        |      20 | MANAGER   |     2975 | 1998-05-18 |
| 1005 | MARTIN | m        |      30 | SALSMAN   |     1250 | 2001-06-12 |
| 1006 | BLAKE  | f        |      30 | MANAGER   |     2850 | 1997-02-15 |
| 1007 | CLARK  | m        |      10 | MANAGER   |     2450 | 2002-09-12 |
| 1008 | SCOTT  | m        |      20 | ANALYST   |     3000 | 2003-05-12 |
| 1009 | KING   | f        |      10 | PRESIDENT |     5000 | 1995-01-01 |
| 1010 | TURNER | f        |      30 | SALESMAN  |     1500 | 1997-10-12 |
| 1011 | ADAMS  | m        |      20 | CLERK     |     1100 | 1999-10-05 |
| 1012 | JAMES  | m        |      30 | CLERK     |      950 | 2008-06-15 |
+------+--------+----------+---------+-----------+----------+------------+
12 rows in set (0.17 sec)

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