MySQL臨時表創建及舊表建新表

1、創建臨時表

　　臨時表是一張表，用來臨時保存一些數據

特點：

　　只對創建該臨時表的用戶可見；

　　當會話結束時，MySQL自動刪除臨時表。

臨時表的核心：建表和刪表消耗資源極其少

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 創建臨時表的基本格式：

　　CREATE TEMPORARY TABLE  tbl_name(……); 

①創建的臨時表在當前會話，正常使用

②斷開連接，再重新連接后執行查詢，拋出異常：

　　錯誤代碼： 1146

　　Table ‘db_name.temtbl_name’ doesn‘t exist。//該臨時表在會話結束的時候被系統刪除。

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 注意：用戶可以創建一個和已有的普通表名字相同的臨時表。

在這種情況下，

　　該用戶只能看到臨時表而看不見同名的普通表；

　　當臨時表被刪除后，才可以看到普通表。

示例：使用相同的名字創建一個普通表和臨時表

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,'臨時表測試'); Query OK, 1 row affected (0.00 sec) mysql> select * from test_table; +------+-----------------+
| num  | name            |
+------+-----------------+
|    2 | 臨時表測試       |
+------+-----------------+
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：當創建表時，如果表已經存在了，則MySQL會返回出錯消息，我們不希望看到報錯的信息，該如何處理？

A：添加IF NOT EXISTS選項，則強制不顯示這個出錯消息；但是，語句執行失敗---建表失敗。

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、根據已有的表來創建新表

語法1：只想拷貝表結構

　　CREATE TABLE new_tbl LIKE orig_tbl;

將從源表復制列名、數據類型、大小、非空約束以及索引；而表的內容以及其它約束不會復制，新表是一張空表。

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)

 

語法2：根據SELECT子查詢的拷貝

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

新表的結構由select列表決定；同時把查詢返回的結果集中的行插入到目標表中；只能把非空約束帶入到新表中(在有的時候就會顯得很雞肋了)，也不會復制索引

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: 0 mysql> 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)

 

通過根據已有表來創建新表，添加選項temporary，創建臨時副本用來做練習最合適：表內容可一樣，會話結束，臨時表自動刪除，原始表中的內容不受任何影響。

注意：

　　如果在表名后面指定的列名和原始表中的列名相同，則可以改變列的大小和非空約束；

　　如果在表名后面指定的列名和原始表中的列名不同，則它作為一個新的列。

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: 0 mysql> select * from teams_copy; +-------+--------+----------+----------+
| coach | TEAMNO | PLAYERNO | DIVISION |
+-------+--------+----------+----------+
| NULL  |      1 |        6 | first    |
| NULL  |      2 |       27 | second   |
+-------+--------+----------+----------+