MySQL临时表创建及旧表建新表

1、创建临时表

　　临时表是一张表，用来临时保存一些数据

特点：

　　只对创建该临时表的用户可见；

　　当会话结束时，MySQL自动删除临时表。

临时表的核心：建表和删表消耗资源极其少

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创建临时表的基本格式：

　　CREATE TEMPORARY TABLE  tbl_name(……);

①创建的临时表在当前会话，正常使用

②断开连接，再重新连接后执行查询，抛出异常：

　　错误代码： 1146

　　Table ‘db_name.temtbl_name’ doesn‘t exist。//该临时表在会话结束的时候被系统删除。

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注意：用户可以创建一个和已有的普通表名字相同的临时表。

在这种情况下，

　　该用户只能看到临时表而看不见同名的普通表；

　　当临时表被删除后，才可以看到普通表。

示例：使用相同的名字创建一个普通表和临时表

mysql> create table test_table(num int);
Query OK,  rows affected (0.12 sec)
 
mysql> insert into test_table values();
Query OK,  row affected (0.04 sec)
 
mysql> create temporary table test_table(num int,name varchar());
Query OK,  rows affected (0.04 sec)
 
mysql> insert into test_table values(,'临时表测试');
Query OK,  row affected (0.00 sec)
 
mysql> select * from test_table;
+------+-----------------+
| num  | name            |
+------+-----------------+
|     | 临时表测试       |
+------+-----------------+
 row in set (0.00 sec)
 
mysql> drop table test_table;
Query OK,  rows affected (0.00 sec)
 
mysql> select * from test_table;
+------+
| num  |
+------+
|     |
+------+
 row in set (0.00 sec)

Q：当创建表时，如果表已经存在了，则MySQL会返回出错消息，我们不希望看到报错的信息，该如何处理？

A：添加IF NOT EXISTS选项，则强制不显示这个出错消息；但是，语句执行失败---建表失败。

mysql> create table if not exists PLAYERS(id int(),name varchar());
Query OK,  rows affected,  warning (0.00 sec)
 
mysql> show warnings;
+-------+------+--------------------------------+
| Level | Code | Message                        |
+-------+------+--------------------------------+
| Note  |  | Table 'PLAYERS' already exists |
+-------+------+--------------------------------+
 row in set (0.00 sec)
 
mysql> show tables;
+-------------------+
| Tables_in_TENNIS  |
+-------------------+
| COMMITTEE_MEMBERS |
| MATCHES           |
| PENALTIES         |
| PLAYERS           |
| TEAMS             |
| test_table        |
+-------------------+
6 rows in set (0.00 sec)

2、根据已有的表来创建新表

语法1：只想拷贝表结构

　　CREATE TABLE new_tbl LIKE orig_tbl;

将从源表复制列名、数据类型、大小、非空约束以及索引；而表的内容以及其它约束不会复制，新表是一张空表。

mysql> desc TEAMS;
+----------+---------+------+-----+---------+-------+
| Field    | Type    | Null | Key | Default | Extra |
+----------+---------+------+-----+---------+-------+
| TEAMNO   | int() | NO   | PRI | NULL    |       |
| PLAYERNO | int() | NO   |     | NULL    |       |
| DIVISION | char() | NO   |     | NULL    |       |
+----------+---------+------+-----+---------+-------+
 rows in set (0.00 sec)
 
mysql> create table copy_TEAMS like TEAMS;
Query OK,  rows affected (0.11 sec)
 
mysql> select * from copy_TEAMS;
Empty set (0.00 sec)
 
mysql> desc copy_TEAMS;
+----------+---------+------+-----+---------+-------+
| Field    | Type    | Null | Key | Default | Extra |
+----------+---------+------+-----+---------+-------+
| TEAMNO   | int() | NO   | PRI | NULL    |       |
| PLAYERNO | int() | NO   |     | NULL    |       |
| DIVISION | char() | NO   |     | NULL    |       |
+----------+---------+------+-----+---------+-------+
 rows in set (0.00 sec)

语法2：根据SELECT子查询的拷贝

　　CREATE TABLE new_tbl [AS] SELECT {*|column,...} FROM orig_tbl;

新表的结构由select列表决定；同时把查询返回的结果集中的行插入到目标表中；只能把非空约束带入到新表中(在有的时候就会显得很鸡肋了)，也不会复制索引

mysql> create table p_m
    -> as
    -> select a.NAME,a.SEX,b.MATCHNO,b.WON,b.LOST
    -> from PLAYERS a,MATCHES b
    -> where a.PLAYERNO=b.PLAYERNO;
Query OK,  rows affected (0.15 sec)
Records:   Duplicates:   Warnings: 
 
mysql> select * from p_m;
+-----------+-----+---------+-----+------+
| NAME      | SEX | MATCHNO | WON | LOST |
+-----------+-----+---------+-----+------+
| Parmenter | M   |        |    |     |
| Parmenter | M   |        |    |     |
| Parmenter | M   |        |    |     |
| Baker     | M   |        |    |     |
| Hope      | M   |        |    |     |
| Everett   | M   |        |    |     |
| Brown     | M   |        |    |     |
| Newcastle | F   |        |    |     |
| Collins   | F   |        |    |     |
| Moorman   | F   |       |    |     |
| Bailey    | F   |       |    |     |
| Bailey    | F   |       |    |     |
| Newcastle | F   |       |    |     |
+-----------+-----+---------+-----+------+
 rows in set (0.00 sec)

通过根据已有表来创建新表，添加选项temporary，创建临时副本用来做练习最合适：表内容可一样，会话结束，临时表自动删除，原始表中的内容不受任何影响。

注意：

　　如果在表名后面指定的列名和原始表中的列名相同，则可以改变列的大小和非空约束；

　　如果在表名后面指定的列名和原始表中的列名不同，则它作为一个新的列。

mysql> select * from TEAMS;
+--------+----------+----------+
| TEAMNO | PLAYERNO | DIVISION |
+--------+----------+----------+
|       |         | first    |
|       |        | second   |
+--------+----------+----------+
 
mysql> CREATE TABLE teams_copy
    -> (
    ->    teamno INTEGER NOT NULL PRIMARY KEY,
    ->    playerno INTEGER NULL,
    ->    division char() NOT NULL,
    ->    coach varchar()
    -> )
    -> as
    -> select * from TEAMS;
Query OK,  rows affected (0.11 sec)
Records:   Duplicates:   Warnings: 
 
mysql> select * from teams_copy;
+-------+--------+----------+----------+
| coach | TEAMNO | PLAYERNO | DIVISION |
+-------+--------+----------+----------+
| NULL  |       |         | first    |
| NULL  |       |        | second   |
+-------+--------+----------+----------+