insert-php+mysql中插入语句，时间前面为什么要有空

数据表如下：

#
Name
Type
Collation
Attributes
Null
Default
Extra
Action

1
id
int(10)

UNSIGNED
No
None
AUTO_INCREMENT
Change
Drop
More

2
author
char(20)
utf8_general_ci

No
None

Change
Drop
More

3
addtime
datetime

No
None

Change
Drop
More

4
content
varchar(100)
utf8_general_ci

No
None

Change
Drop
More

5
reply
varchar(100)
utf8_general_ci

No
None

Change
Drop
More
Check AllWith selected: Browse Change Drop Primary Unique Index

Print view Relation view Propose table structure Move columns

Add column(s) At End of Table At Beginning of Table Afteridauthor addtimecontentreply

• IndexesInformationSpace usageData16KiBIndex0BTotal16KiBRow statisticsFormatCompactCollationutf8_general_ciNext autoindex2CreationApr 18, 2015 at 10:51 AM

1.只写入时间。
$sql = "INSERT INTO message(addtime,) VALUES ('2015-04-22 08:33:46')";
可以写入，结果如下：

id
主键，自动增长
author
作者
addtime
留言时间
content
留言内容
reply
回复

Edit
Copy
Delete
1

2015-04-22 08:33:46

2.按照表单填写留言。
$sql="INSERT INTO message(author, addtime, content) VALUES ('$name','$addtime','$content')";
可以正常写入，让我困惑的是addtime中的addtime前面为什么必须要有空格？