MYSQL：如何创建MySQL表，字段的默认值是函数的输出

QUESTION How can I create a MySQL table so, that a field's default value is the output of a function.

问题如何创建MySQL表，以便字段的默认值是函数的输出。

Can it be done without an insert trigger?

可以在没有插入触发器的情况下完成吗？

BACKGROUND: Some entities and the groups they belong to need to be observed on a daily basis.

背景：需要每天观察一些实体及其所属的群体。

To do that I created the following MySQL table:

为此，我创建了以下MySQL表：

CREATE TABLE entity (
    entity_id  VARCHAR(10) NOT NULL,
    group_id   VARCHAR(10) NOT NULL,
    first_seen DATE        NOT NULL,
    last_seen  DATE        NOT NULL,
    PRIMARY KEY (entity_id,group_id)
)

A script parses the entity logs on a daily basis and writes to to entity_raw, I then want to update the tables using

脚本每天解析实体日志并写入entity_raw，然后我想使用更新表

REPLACE INTO entity (entity_id,group_id,last_seen) 
SELECT entity_id,group_id,record_date 
  FROM entity_raw 
 WHERE (record_date = DATE(DATE_SUB(NOW(),INTERVAL 1 DAY))

Naturally I get an error when inserting new (entity_id,group_id) pairs, since first_seen may not be NULL...

当然，插入新的（entity_id，group_id）对时会出错，因为first_seen可能不是NULL ...

Can this be done in one statement, short of using a trigger (I dislike them, too surprising for maintenance)

这可以在一个声明中完成，没有使用触发器（我不喜欢它们，太难以维护）

or do I need to select for existance first, then insert/update accordingly?

或者我是否需要先选择存在，然后相应地插入/更新？

4 个解决方案

#1

No, what you're asking isn't possible without triggers. The only example in MySQL where a field can be assigned a default value from a function is the TIMESTAMP type that can have CURRENT_TIMESTAMP as default.

不，没有触发器，你所要求的是不可能的。 MySQL中唯一可以为函数分配默认值的示例是TIMESTAMP类型，它可以将CURRENT_TIMESTAMP作为默认值。

#2

To be honest it's pretty much impossible to combine an update or insert into one statement.

说实话，将更新或插入组合成一个语句几乎是不可能的。

If you are using a script, I would do as you suggested. First check for existence, then update or insert accordingly.

如果您使用的是脚本，我会按照您的建议进行操作。首先检查是否存在，然后相应地更新或插入。

Otherwise a trigger or stored procedure would do the job.

否则，触发器或存储过程将完成这项工作。

#3

You could use the TIMESTAMP data type, which allows a default value of CURRENT_TIMESTAMP, but that would set it to NOW() and not to a day ago.

您可以使用TIMESTAMP数据类型，它允许默认值CURRENT_TIMESTAMP，但这会将其设置为NOW（）而不是前一天。

So, I would rather use use an INSERT... ON DUPLICATE KEY UPDATE statement:

所以，我宁愿使用INSERT ... ON DUPLICATE KEY UPDATE语句：

INSERT INTO entity (entity_id,group_id,first_seen,last_seen) 
SELECT entity_id,group_id,record_date, record_date 
    FROM entity_raw 
    WHERE (record_date = DATE(DATE_SUB(NOW(),INTERVAL 1 DAY))
ON DUPLICATE KEY UPDATE last_seen = VALUES(last_seen)

with a unique index en entity_id and/or group_id. This will overwrite only last_seen when an existing row is found.

具有唯一索引en entity_id和/或group_id。当找到现有行时，这将仅覆盖last_seen。

#4

After some searching I came up with this .. it solves my "problem", but not the question I asked :-(

经过一番搜索，我想出了这个..它解决了我的“问题”，但不是我问的问题:-(

REPLACE INTO entity (entity_id,group_id,first_seen,last_seen) 
SELECT r.entity_id              AS entity_id
     , r.group_id               AS group_id
     , ISNULL( e.first_seen
              ,r.record_date
             )                  AS first_seen
     , r.record_date            AS last_seen
  FROM entity_raw r
  LEFT OUTER JOIN entity e ON (    r.entity_id=e.entity_id 
                               AND r.group_id=e.group_id
                              )
 WHERE (record_date = DATE(DATE_SUB(NOW(),INTERVAL 1 DAY))

The initial insert needs to be done with the where clause disabled (there were already 5weeks of data in the entity_raw table)

初始插入需要在禁用where子句的情况下完成（entity_raw表中已有5周的数据）

#1