【一步一步学习mysql】数据表的基本操作

USE db_name;，打开已创建的数据库。

创建数据表

CREATE TABLE [IF NOT EXISTS] table_name(
	column_name data_type,
    ...
)

mysql> use imax;
Database changed
mysql> CREATE TABLE user( name VARCHAR(20), age TINYINT UNSIGNED, salary FLOAT(8, 2) UNSIGNED );
Query OK, 0 rows affected (0.31 sec)

查看数据表

SHOW TABLES;

SHOW COLUMNS FROM tbl_name;

mysql> SHOW COLUMNS FROM user;
+--------+---------------------+------+-----+---------+-------+
| Field  | Type                | Null | Key | Default | Extra |
+--------+---------------------+------+-----+---------+-------+
| name   | varchar(20)         | YES  |     | NULL    |       |
| age    | tinyint(3) unsigned | YES  |     | NULL    |       |
| salary | float(8,2) unsigned | YES  |     | NULL    |       |
+--------+---------------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

插入

方式1

INSERT [INTO] tbl_name[(col_name, ...)] {VALUES|VALUE}(val, ...);

mysql> INSERT user values('tom', 25, 7888.98);
Query OK, 1 row affected (0.07 sec)

mysql> INSERT user(name, salary) VALUES('jack', 8997.98);
Query OK, 1 row affected (0.11 sec)

注意：INSERT中，如果表明没有指定字段名则默认所有的字段都要赋值。

这里我就有个疑问了：我们现在的表中场景很简单没有自增的字段，如果有的话，怎么赋值？

• 将user表增加一个id字段，并进行数据的插入。

mysql> ALTER TABLE user ADD id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT FIRST;
Query OK, 0 rows affected (0.45 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> SHOW CREATE TABLE user;
+-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                                                    |
+-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| user  | CREATE TABLE `user` (
  `id` smallint(5) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(20) DEFAULT NULL,
  `age` tinyint(3) unsigned DEFAULT NULL,
  `salary` float(8,2) unsigned DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8 |
+-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT * FROM user;
+----+------+------+---------+
| id | name | age  | salary  |
+----+------+------+---------+
|  1 | tom  |   25 | 7888.98 |
|  2 | jack | NULL | 8997.98 |
+----+------+------+---------+
2 rows in set (0.00 sec)

• 插入数据(使用DEFAULT|NULL即可)

mysql> INSERT user VALUES(DEFAULT, 'xxx', 29, 19999);
Query OK, 1 row affected (0.06 sec)
mysql> SELECT * FROM user;
+----+------+------+----------+
| id | name | age  | salary   |
+----+------+------+----------+
|  1 | tom  |   25 |  7888.98 |
|  2 | jack | NULL |  8997.98 |
|  3 | xxx  |   29 | 19999.00 |
+----+------+------+----------+
3 rows in set (0.00 sec)

补充：如果字段具有默认值的场景下，也可以使用DEFAULT来赋值。

方式2

INSERT [INTO] tbl_name SET col_name={DEFAULT|expr}, ...

mysql> INSERT user SET name='yyy', salary=9999;

方式3

INSERT [INTO] tbl_name [(col_name, ...)] SELECT ...

说明：此方法可以将查询结果插入到指定的数据表。

# 创建一个新表top，存放工资前3的信息
mysql> CREATE TABLE top (
    -> id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
    -> salary FLOAT UNSIGNED);
Query OK, 0 rows affected (0.29 sec)

# 使用 INSERT SELECT 插入数据
mysql> INSERT top(salary) SELECT  salary FROM user ORDER BY salary DESC LIMIT 3;
Query OK, 3 rows affected (0.06 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from top;
+----+---------+
| id | salary  |
+----+---------+
| 11 |   10999 |
| 12 | 9997.98 |
| 13 | 8888.98 |
+----+---------+
3 rows in set (0.00 sec)

更新

UPDATE table_reference SET col_name1={expr1|DEFAULT}[, col_name2={expr2|DEFAULT}] ... [WHERE where_condition];

• 后边没有where条件，就是所有的数据都更新。全员涨工资1000

mysql> SELECT * FROM user;
+----+------+------+----------+
| id | name | age  | salary   |
+----+------+------+----------+
|  1 | tom  |   30 |  7888.98 |
|  2 | jack | NULL |  8997.98 |
|  3 | xxx  |   34 | 19999.00 |
|  4 | yyy  | NULL |  9999.00 |
+----+------+------+----------+
4 rows in set (0.00 sec)

mysql> UPDATE user SET salary=salary+1000;
Query OK, 4 rows affected (0.07 sec)
Rows matched: 4  Changed: 4  Warnings: 0

mysql> SELECT * FROM user;
+----+------+------+----------+
| id | name | age  | salary   |
+----+------+------+----------+
|  1 | tom  |   30 |  8888.98 |
|  2 | jack | NULL |  9997.98 |
|  3 | xxx  |   34 | 20999.00 |
|  4 | yyy  | NULL | 10999.00 |
+----+------+------+----------+
4 rows in set (0.00 sec)

• id是奇数的年龄增加10岁

mysql> UPDATE user SET age=age+10 WHERE id%2=1;
Query OK, 2 rows affected (0.05 sec)
Rows matched: 2  Changed: 2  Warnings: 0

查询

SELECT expr, ... FROM tbl_name;

mysql> select * from user;
+------+------+---------+
| name | age  | salary  |
+------+------+---------+
| tom  |   25 | 7888.98 |
| jack | NULL | 8997.98 |
+------+------+---------+

• 复杂形式(真的好复杂)

SELECT select_expr [, ...]
[
    FROM table_references
    [WHERE where_condition]
    [GROUP BY {col_name|position} [ASC|DESC], ...]
    [HAVING where_condition]
    [ORDER BY {col_name | expr | position} [ASC|DESC], ...]
    [LIMIT {[offset,] row_count|row_count OFFSET offset}]
]

• 查询时间

mysql> SELECT NOW();
+---------------------+
| NOW()               |
+---------------------+
| 2018-08-27 15:17:19 |
+---------------------+
1 row in set (0.00 sec)

• 设置别名

mysql> SELECT name AS username, id AS userid FROM user;
+----------+--------+
| username | userid |
+----------+--------+
| tom      |      1 |
| jack     |      2 |
| yyy      |      4 |
+----------+--------+
3 rows in set (0.00 sec)

• 查询工资大于10000的名字和其id

mysql> SELECT name AS username, id AS userid FROM user WHERE salary>10000;
+----------+--------+
| username | userid |
+----------+--------+
| yyy      |      4 |
+----------+--------+
1 row in set (0.00 sec)

• 根据年龄分类

mysql> SELECT age FROM user GROUP BY age;

• 根据年龄分类，并且输出分类年龄中最高工资和最低工资

mysql> SELECT age, max(salary) as max_salary, min(salary) as min_salary FROM user GROUP BY age;
+------+------------+------------+
| age  | max_salary | min_salary |
+------+------------+------------+
| NULL |   10999.00 |    9997.98 |
|   40 |    8888.98 |    6000.00 |
+------+------------+------------+
2 rows in set (0.00 sec)

• 根据年龄分类，并且输出分类年龄中最高工资和最低工资，将最高工资大于10000的类输出出来

mysql> SELECT age, max(salary) as max_salary, min(salary) as min_salary FROM user GROUP BY age HAVING max_salary>10000;
+------+------------+------------+
| age  | max_salary | min_salary |
+------+------------+------------+
| NULL |   10999.00 |    9997.98 |
+------+------------+------------+
1 row in set (0.00 sec)

• 查询所有信息，按照工资的降序排序

mysql> SELECT * FROM user ORDER BY salary DESC;
+----+------+------+----------+
| id | name | age  | salary   |
+----+------+------+----------+
|  4 | yyy  | NULL | 10999.00 |
|  2 | jack | NULL |  9997.98 |
|  1 | tom  |   40 |  8888.98 |
|  5 | xxx  |   40 |  6000.00 |
+----+------+------+----------+
4 rows in set (0.00 sec)

• 查询所有信息，按照工资的降序排序，只显示TOP3

mysql> SELECT * FROM user ORDER BY salary DESC LIMIT 3;
+----+------+------+----------+
| id | name | age  | salary   |
+----+------+------+----------+
|  4 | yyy  | NULL | 10999.00 |
|  2 | jack | NULL |  9997.98 |
|  1 | tom  |   40 |  8888.98 |
+----+------+------+----------+
3 rows in set (0.00 sec)

删除

DELETE FROM tbl_name [WHERE condition]

mysql> DELETE FROM user WHERE id=3;
Query OK, 1 row affected (0.07 sec)

自动编号

AUTO_INCREMENT; 默认起始值为1，递增量为1；

• 必须同主键一起使用