MySQL-SQL基础

mysql> use test;
Database changed
mysql> create table emp(ename varchar(10),hirdate date,sal decimal(10,2),deptno int(2));
Query OK, 0 rows affected (0.01 sec)

mysql> show tables;
+----------------+
| Tables_in_test |
+----------------+
| emp            |
+----------------+
1 row in set (0.00 sec)

mysql> desc emp;
+---------+---------------+------+-----+---------+-------+
| Field   | Type          | Null | Key | Default | Extra |
+---------+---------------+------+-----+---------+-------+
| ename   | varchar(10)   | YES  |     | NULL    |       |
| hirdate | date          | YES  |     | NULL    |       |
| sal     | decimal(10,2) | YES  |     | NULL    |       |
| deptno  | int(2)        | YES  |     | NULL    |       |
+---------+---------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

mysql> show create table emp\G
*************************** 1. row ***************************
       Table: emp
Create Table: CREATE TABLE `emp` (
  `ename` varchar(10) DEFAULT NULL,
  `hirdate` date DEFAULT NULL,
  `sal` decimal(10,2) DEFAULT NULL,
  `deptno` int(2) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

mysql> alter table emp modify ename varchar(20);
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc emp;
+---------+---------------+------+-----+---------+-------+
| Field   | Type          | Null | Key | Default | Extra |
+---------+---------------+------+-----+---------+-------+
| ename   | varchar(20)   | YES  |     | NULL    |       |
| hirdate | date          | YES  |     | NULL    |       |
| sal     | decimal(10,2) | YES  |     | NULL    |       |
| deptno  | int(2)        | YES  |     | NULL    |       |
+---------+---------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

mysql> alter table emp add column age int(3);
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc emp;
+---------+---------------+------+-----+---------+-------+
| Field   | Type          | Null | Key | Default | Extra |
+---------+---------------+------+-----+---------+-------+
| ename   | varchar(20)   | YES  |     | NULL    |       |
| hirdate | date          | YES  |     | NULL    |       |
| sal     | decimal(10,2) | YES  |     | NULL    |       |
| deptno  | int(2)        | YES  |     | NULL    |       |
| age     | int(3)        | YES  |     | NULL    |       |
+---------+---------------+------+-----+---------+-------+
5 rows in set (0.00 sec)

mysql> alter table emp drop column age;
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc emp;
+---------+---------------+------+-----+---------+-------+
| Field   | Type          | Null | Key | Default | Extra |
+---------+---------------+------+-----+---------+-------+
| ename   | varchar(20)   | YES  |     | NULL    |       |
| hirdate | date          | YES  |     | NULL    |       |
| sal     | decimal(10,2) | YES  |     | NULL    |       |
| deptno  | int(2)        | YES  |     | NULL    |       |
+---------+---------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

mysql> alter table emp add column age int(3);
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table emp change column age age1 int(4);
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc emp;
+---------+---------------+------+-----+---------+-------+
| Field   | Type          | Null | Key | Default | Extra |
+---------+---------------+------+-----+---------+-------+
| ename   | varchar(20)   | YES  |     | NULL    |       |
| hirdate | date          | YES  |     | NULL    |       |
| sal     | decimal(10,2) | YES  |     | NULL    |       |
| deptno  | int(2)        | YES  |     | NULL    |       |
| age1    | int(4)        | YES  |     | NULL    |       |
+---------+---------------+------+-----+---------+-------+
5 rows in set (0.00 sec)

mysql> alter table emp add birth date after ename;
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql>
mysql> desc emp;
+---------+---------------+------+-----+---------+-------+
| Field   | Type          | Null | Key | Default | Extra |
+---------+---------------+------+-----+---------+-------+
| ename   | varchar(20)   | YES  |     | NULL    |       |
| birth   | date          | YES  |     | NULL    |       |
| hirdate | date          | YES  |     | NULL    |       |
| sal     | decimal(10,2) | YES  |     | NULL    |       |
| deptno  | int(2)        | YES  |     | NULL    |       |
| age1    | int(4)        | YES  |     | NULL    |       |
+---------+---------------+------+-----+---------+-------+
6 rows in set (0.00 sec)

mysql> alter table emp rename emp1;
Query OK, 0 rows affected (0.01 sec)

mysql> desc emp1;
+---------+---------------+------+-----+---------+-------+
| Field   | Type          | Null | Key | Default | Extra |
+---------+---------------+------+-----+---------+-------+
| ename   | varchar(20)   | YES  |     | NULL    |       |
| birth   | date          | YES  |     | NULL    |       |
| hirdate | date          | YES  |     | NULL    |       |
| sal     | decimal(10,2) | YES  |     | NULL    |       |
| deptno  | int(2)        | YES  |     | NULL    |       |
| age1    | int(4)        | YES  |     | NULL    |       |
+---------+---------------+------+-----+---------+-------+
6 rows in set (0.00 sec)

mysql>