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
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。
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 |+-------+--------+----------+----------+
还没有人抢沙发呢~