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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


 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

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。


 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, 0 rows affected (0.12 sec)mysql> insert into test_table values(1);Query OK, 1 row affected (0.04 sec)mysql> create temporary table test_table(num int,name varchar(20));Query OK, 0 rows affected (0.04 sec)mysql> insert into test_table values(2,'临时表测试');Query OK, 1 row affected (0.00 sec)mysql> select * from test_table;+------+-----------------+| num  | name            |+------+-----------------+|    2 | 临时表测试       |+------+-----------------+1 row in set (0.00 sec)mysql> drop table test_table;Query OK, 0 rows affected (0.00 sec)mysql> select * from test_table;+------+| num  |+------+|    1 |+------+1 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(5),name varchar(20));Query OK, 0 rows affected, 1 warning (0.00 sec)mysql> show warnings;+-------+------+--------------------------------+| Level | Code | Message                        |+-------+------+--------------------------------+| Note  | 1050 | Table 'PLAYERS' already exists |+-------+------+--------------------------------+1 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(11) | NO   | PRI | NULL    |       || PLAYERNO | int(11) | NO   |     | NULL    |       || DIVISION | char(6) | NO   |     | NULL    |       |+----------+---------+------+-----+---------+-------+3 rows in set (0.00 sec)mysql> create table copy_TEAMS like TEAMS;Query OK, 0 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(11) | NO   | PRI | NULL    |       || PLAYERNO | int(11) | NO   |     | NULL    |       || DIVISION | char(6) | NO   |     | NULL    |       |+----------+---------+------+-----+---------+-------+3 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, 13 rows affected (0.15 sec)Records: 13  Duplicates: 0  Warnings: 0mysql> select * from p_m;+-----------+-----+---------+-----+------+| NAME      | SEX | MATCHNO | WON | LOST |+-----------+-----+---------+-----+------+| Parmenter | M   |       1 |   3 |    1 || Parmenter | M   |       2 |   2 |    3 || Parmenter | M   |       3 |   3 |    0 || Baker     | M   |       4 |   3 |    2 || Hope      | M   |       5 |   0 |    3 || Everett   | M   |       6 |   1 |    3 || Brown     | M   |       7 |   3 |    0 || Newcastle | F   |       8 |   0 |    3 || Collins   | F   |       9 |   3 |    2 || Moorman   | F   |      10 |   3 |    2 || Bailey    | F   |      11 |   2 |    3 || Bailey    | F   |      12 |   1 |    3 || Newcastle | F   |      13 |   0 |    3 |+-----------+-----+---------+-----+------+13 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 |+--------+----------+----------+|      1 |        6 | first    ||      2 |       27 | second   |+--------+----------+----------+mysql> CREATE TABLE teams_copy    -> (    ->    teamno INTEGER NOT NULL PRIMARY KEY,    ->    playerno INTEGER NULL,    ->    division char(10) NOT NULL,    ->    coach varchar(20)    -> )    -> as    -> select * from TEAMS;Query OK, 2 rows affected (0.11 sec)Records: 2  Duplicates: 0  Warnings: 0mysql> select * from teams_copy;+-------+--------+----------+----------+| coach | TEAMNO | PLAYERNO | DIVISION |+-------+--------+----------+----------+| NULL  |      1 |        6 | first    || NULL  |      2 |       27 | second   |+-------+--------+----------+----------+

文章转载于:https://www.cnblogs.com/geaozhang/p/6782285.html

原著是一个有趣的人,若有侵权,请通知删除

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