Difference between revisions of "Howto create mysql table"

Revision as of 15:27, 17 May 2013

Basic CREATE TABLE statement

A very basic CREATE TABLE statement which should work in any SQL database:

 mysql> CREATE TABLE example (
        id INT,
        data VARCHAR(100)
      );
 Query OK, 0 rows affected (0.03 sec)

Creating a table with a particular storage engine

MySQL provides a variety of different table types with differing levels of functionality. The usual default, and most widely used, is MyISAM. Other storage types must be explicitly defined:

 mysql>  CREATE TABLE example_innodb (
         id INT,
         data VARCHAR(100)
       ) TYPE=innodb;
 Query OK, 0 rows affected (0.03 sec)

Note that beginning with MySQL 4.1 ENGINE=innodb is the preferred method of defining the storage type.

Use SHOW CREATE TABLE (see below) to check that MySQL has created the table as you defined it. Creating a table with auto_increment

Often you'll want to be able to automatically assign a sequential value to a column:

 mysql> CREATE TABLE example_autoincrement (
        id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
        data VARCHAR(100)
      );
 Query OK, 0 rows affected (0.01 sec)

 mysql> INSERT INTO example_autoincrement (data)
   ->      VALUES ('Hello world');
 Query OK, 1 row affected (0.01 sec)

 mysql> SELECT * FROM  example_autoincrement;
 +----+-------------+
 | id | data        |
 +----+-------------+
 |  1 | Hello world |
 +----+-------------+
 1 row in set (0.01 sec)

Creating a table with the current timestamp

Often it's useful to have an automatic timestamp on each record. The MySQL special datatype TIMESTAMP enables you to keep track of changes to a record:

 mysql> CREATE TABLE example_timestamp (
        id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
        data VARCHAR(100),
        cur_timestamp TIMESTAMP(8)
      );
 Query OK, 0 rows affected (0.00 sec)

 mysql> INSERT INTO example_timestamp (data)
           VALUES ('The time of creation is:');
 Query OK, 1 row affected (0.00 sec)

 mysql> SELECT * FROM example_timestamp;
 +----+--------------------------+---------------------+
 | id | data                     | cur_timestamp       |
 +----+--------------------------+---------------------+
 |  1 | The time of creation is: | 2004-12-01 20:37:22 |
 +----+--------------------------+---------------------+
 1 row in set (0.00 sec)

 mysql> UPDATE example_timestamp 
         SET data='The current timestamp is: ' 
       WHERE id=1;
 Query OK, 1 row affected (0.03 sec)
 Rows matched: 1  Changed: 1  Warnings: 0

 mysql> SELECT * FROM example_timestamp;
 +----+---------------------------+---------------------+
 | id | data                      | cur_timestamp       |
 +----+---------------------------+---------------------+
 |  1 | The current timestamp is: | 2004-12-01 20:38:55 |
 +----+---------------------------+---------------------+
 1 row in set (0.01 sec)

The column cur_timestamp is automagically updated every time the record is changed. Creating a table with TIMESTAMP DEFAULT NOW()

MySQL supports the construct TIMESTAMP DEFAULT NOW() only from verson 4.1:

 CREATE TABLE example_default_now (
   id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
   data VARCHAR(100),
   created TIMESTAMP DEFAULT NOW()
 );

In this case the column created retains its initial value and is not changed during subsequent updates.

For versions prior to 4.1, the only workaround is to create two timestamp columns in a table, and explicitly set the second one when inserting the record. Remember: the first TIMESTAMP will be automagically updated on each record update. Viewing a table definition

For basic information on table columns, use DESC tablename:

 mysql> DESC example;
 +-------+--------------+------+-----+---------+-------+
 | Field | Type         | Null | Key | Default | Extra |
 +-------+--------------+------+-----+---------+-------+
 | id    | int(11)      | YES  |     | NULL    |       |
 | data  | varchar(100) | YES  |     | NULL    |       |
 +-------+--------------+------+-----+---------+-------+
 2 rows in set (0.00 sec)

Exact definition of the table:

 mysql> SHOW CREATE TABLE example;
 +---------+------------------------------------------------+
 | Table   | Create Table                                   |
 +---------+------------------------------------------------+
 | example | CREATE TABLE `example` (
   `id` int(11) default NULL,
   `data` varchar(100) default NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1 |
 +---------+------------------------------------------------+
 1 row in set (0.00 sec)

(this example from MySQL 4.1)