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How do you set a default value for a MySQL Datetime column?

分类 : 互动问答 | 发布时间 : 2008-10-04 04:24:30 | 评论 : 24 | 浏览 : 1077336 | 喜欢 : 790

How do you set a default value for a MySQL Datetime column?

In SQL Server it's getdate(). What is the equivalant for MySQL? I'm using MySQL 5.x if that is a factor.

回答(24)

  • 1楼
  • IMPORTANT EDIT: It is now possible to achieve this with DATETIME fields since MySQL 5.6.5, take a look at the other post below...

    Previous versions can't do that with DATETIME...

    But you can do it with TIMESTAMP:

    mysql> create table test (str varchar(32), ts TIMESTAMP DEFAULT CURRENT_TIMESTAMP);
    Query OK, 0 rows affected (0.00 sec)
    
    mysql> desc test;
    +-------+-------------+------+-----+-------------------+-------+
    | Field | Type        | Null | Key | Default           | Extra |
    +-------+-------------+------+-----+-------------------+-------+
    | str   | varchar(32) | YES  |     | NULL              |       | 
    | ts    | timestamp   | NO   |     | CURRENT_TIMESTAMP |       | 
    +-------+-------------+------+-----+-------------------+-------+
    2 rows in set (0.00 sec)
    
    mysql> insert into test (str) values ("demo");
    Query OK, 1 row affected (0.00 sec)
    
    mysql> select * from test;
    +------+---------------------+
    | str  | ts                  |
    +------+---------------------+
    | demo | 2008-10-03 22:59:52 | 
    +------+---------------------+
    1 row in set (0.00 sec)
    
    mysql>
    

    **CAVEAT: IF you define a column with CURRENT_TIMESTAMP ON as default, you will need to ALWAYS specify a value for this column or the value will automatically reset itself to "now()" on update. This means that if you do not want the value to change, your UPDATE statement must contain "[your column name] = [your column name]" (or some other value) or the value will become "now()". Weird, but true. I hope this helps. I am using 5.5.56-MariaDB **

  • 3楼
  • MySQL (before version 5.6.5) does not allow functions to be used for default DateTime values. TIMESTAMP is not suitable due to its odd behavior and is not recommended for use as input data. (See MySQL Data Type Defaults.)

    That said, you can accomplish this by creating a Trigger.

    I have a table with a DateCreated field of type DateTime. I created a trigger on that table "Before Insert" and "SET NEW.DateCreated=NOW()" and it works great.

    I hope this helps somebody.

  • 4楼
  • For me the trigger approach has worked the best, but I found a snag with the approach. Consider the basic trigger to set a date field to the current time on insert:

    CREATE TRIGGER myTable_OnInsert BEFORE INSERT ON `tblMyTable`
        FOR EACH ROW SET NEW.dateAdded = NOW();
    

    This is usually great, but say you want to set the field manually via INSERT statement, like so:

    INSERT INTO tblMyTable(name, dateAdded) VALUES('Alice', '2010-01-03 04:30:43');
    

    What happens is that the trigger immediately overwrites your provided value for the field, and so the only way to set a non-current time is a follow up UPDATE statement--yuck! To override this behavior when a value is provided, try this slightly modified trigger with the IFNULL operator:

    CREATE TRIGGER myTable_OnInsert BEFORE INSERT ON `tblMyTable`
        FOR EACH ROW SET NEW.dateAdded = IFNULL(NEW.dateAdded, NOW());
    

    This gives the best of both worlds: you can provide a value for your date column and it will take, and otherwise it'll default to the current time. It's still ghetto relative to something clean like DEFAULT GETDATE() in the table definition, but we're getting closer!

  • 5楼
  • I was able to solve this using this alter statement on my table that had two datetime fields.

    ALTER TABLE `test_table`
      CHANGE COLUMN `created_dt` `created_dt` TIMESTAMP NOT NULL DEFAULT '0000-00-00 00:00:00',
      CHANGE COLUMN `updated_dt` `updated_dt` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP;
    

    This works as you would expect the now() function to work. Inserting nulls or ignoring the created_dt and updated_dt fields results in a perfect timestamp value in both fields. Any update to the row changes the updated_dt. If you insert records via the MySQL query browser you needed one more step, a trigger to handle the created_dt with a new timestamp.

    CREATE TRIGGER trig_test_table_insert BEFORE INSERT ON `test_table`
        FOR EACH ROW SET NEW.created_dt = NOW();
    

    The trigger can be whatever you want I just like the naming convention [trig]_[my_table_name]_[insert]

  • 6楼
  • You can use triggers to do this type of stuff.

    CREATE TABLE `MyTable` (
    `MyTable_ID`  int UNSIGNED NOT NULL AUTO_INCREMENT ,
    `MyData`  varchar(10) NOT NULL ,
    `CreationDate`  datetime NULL ,
    `UpdateDate`  datetime NULL ,
    PRIMARY KEY (`MyTable_ID`)
    )
    ;
    
    CREATE TRIGGER `MyTable_INSERT` BEFORE INSERT ON `MyTable`
    FOR EACH ROW BEGIN
            -- Set the creation date
        SET new.CreationDate = now();
    
            -- Set the udpate date
        Set new.UpdateDate = now();
    END;
    
    CREATE TRIGGER `MyTable_UPDATE` BEFORE UPDATE ON `MyTable`
    FOR EACH ROW BEGIN
            -- Set the udpate date
        Set new.UpdateDate = now();
    END;
    
  • 7楼
  • For all those who lost heart trying to set a default DATETIME value in MySQL, I know exactly how you feel/felt. So here is is:

    ALTER TABLE  `table_name` CHANGE `column_name` DATETIME NOT NULL DEFAULT 0
    

    Carefully observe that I haven't added single quotes/double quotes around the 0

    I'm literally jumping after solving this one :D

  • 10楼
  • If you have already created the table then you can use

    To change default value to current date time

    ALTER TABLE <TABLE_NAME> 
    CHANGE COLUMN <COLUMN_NAME> <COLUMN_NAME> DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP;
    

    To change default value to '2015-05-11 13:01:01'

    ALTER TABLE <TABLE_NAME> 
    CHANGE COLUMN <COLUMN_NAME> <COLUMN_NAME> DATETIME NOT NULL DEFAULT '2015-05-11 13:01:01';
    
  • 11楼
  • For all who use the TIMESTAMP column as a solution i want to second the following limitation from the manual:

    http://dev.mysql.com/doc/refman/5.0/en/datetime.html

    "The TIMESTAMP data type has a range of '1970-01-01 00:00:01' UTC to '2038-01-19 03:14:07' UTC. It has varying properties, depending on the MySQL version and the SQL mode the server is running in. These properties are described later in this section. "

    So this will obviously break your software in about 28 years.

    I believe the only solution on the database side is to use triggers like mentioned in other answers.

  • 12楼
  • While defining multi-line triggers one has to change the delimiter as semicolon will be taken by MySQL compiler as end of trigger and generate error. e.g.

    DELIMITER //
    CREATE TRIGGER `MyTable_UPDATE` BEFORE UPDATE ON `MyTable`
    FOR EACH ROW BEGIN
            -- Set the udpate date
        Set new.UpdateDate = now();
    END//
    DELIMITER ;
    
  • 13楼
  • You can use now() to set the value of a datetime column, but keep in mind that you can't use that as a default value.

  • 14楼
  • I'm running MySql Server 5.7.11 and this sentence:

    ALTER TABLE table_name CHANGE date_column datetime NOT NULL DEFAULT '0000-00-00 00:00:00'
    

    is not working. But the following:

    ALTER TABLE table_name CHANGE date_column datetime NOT NULL DEFAULT '1000-01-01 00:00:00'
    

    just works.

    As a sidenote, it is mentioned in the mysql docs:

    The DATE type is used for values with a date part but no time part. MySQL retrieves and displays DATE values in 'YYYY-MM-DD' format. The supported range is '1000-01-01' to '9999-12-31'.

    even if they also say:

    Invalid DATE, DATETIME, or TIMESTAMP values are converted to the “zero” value of the appropriate type ('0000-00-00' or '0000-00-00 00:00:00').

  • 15楼
  • While you can't do this with DATETIME in the default definition, you can simply incorporate a select statement in your insert statement like this:

    INSERT INTO Yourtable (Field1, YourDateField) VALUES('val1', (select now()))
    

    Note the lack of quotes around the table.

    For MySQL 5.5

  • 16楼
  • Here is how to do it on MySQL 5.1:

    ALTER TABLE `table_name` CHANGE `column_name` `column_name` 
    TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP;
    

    I have no clue why you have to enter the column name twice.

  • 17楼
  • If you are trying to set default value as NOW(), I don't think MySQL supports that. In MySQL, you cannot use a function or an expression as the default value for any type of column, except for the TIMESTAMP data type column, for which you can specify the CURRENT_TIMESTAMP as the default.

  • 18楼
  • CREATE TABLE `testtable` (
        `id` INT(10) NULL DEFAULT NULL,
        `colname` DATETIME NULL DEFAULT '1999-12-12 12:12:12'
    )
    

    In the above query to create 'testtable', i used '1999-12-12 12:12:12' as default value for DATETIME column colname

  • 19楼
  • Use the following code

    DELIMITER $$
    
        CREATE TRIGGER bu_table1_each BEFORE UPDATE ON table1 FOR EACH ROW
        BEGIN
          SET new.datefield = NOW();
        END $$
    
        DELIMITER ;
    
  • 20楼
  • I think it simple in mysql since mysql the inbuilt function called now() which gives current time(time of that insert).

    So your query should look like similarly

    CREATE TABLE defaultforTime(
        `creation_time`     DATETIME DEFAULT CURRENT_TIMESTAMP,
        `modification_time` DATETIME default now()
    );
    

    Thank you.

  • 21楼
  • If you are trying to set default value as NOW(),MySQL supports that you have to change the type of that column TIMESTAMP instead of DATETIME. TIMESTAMP have current date and time as default..i think it will resolved your problem..

  • 22楼
  • Take for instance If I had a table named 'site' with a created_at and an update_at column that were both DATETIME and need the default value of now, I could execute the following sql to achieve this.

    ALTER TABLE `site` CHANGE `created_at` `created_at` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP;
    
    ALTER TABLE `site` CHANGE `created_at` `created_at` DATETIME  NULL DEFAULT NULL;
    
    ALTER TABLE `site` CHANGE `updated_at` `updated_at` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP;
    
    ALTER TABLE `site` CHANGE `updated_at` `updated_at` DATETIME NULL DEFAULT  NULL;
    

    The sequence of statements is important because a table can not have two columns of type TIMESTAMP with default values of CUREENT TIMESTAMP

  • 23楼
  • This is my trigger example:

    /************ ROLE ************/
    drop table if exists `role`;
    create table `role` (
        `id_role` bigint(20) unsigned not null auto_increment,
        `date_created` datetime,
        `date_deleted` datetime,
        `name` varchar(35) not null,
        `description` text,
        primary key (`id_role`)
    ) comment='';
    
    drop trigger if exists `role_date_created`;
    create trigger `role_date_created` before insert
        on `role`
        for each row 
        set new.`date_created` = now();

  • 24楼
  • You can resolve the default timestamp. First consider which character set you are using for example if u taken utf8 this character set support all languages and if u taken laten1 this character set support only for English. Next setp if you are working under any project you should know client time zone and select you are client zone. This step are mandatory.

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