MYSQL向数据表插入默认字段值的方法
数据表设计过程中很多时候会设定字段的默认值,那么向该表插入数据时会出现以下情况: 情况1:如果该字段为NULL类型,那么直接将NULL插入到该字段,字段值就自动变成了设定的默认值; 情况2:如果该字段为NOT NULL类型,那么直接将NULL插入到该字段的话,在
数据表设计过程中很多时候会设定字段的默认值,那么向该表插入数据时会出现以下情况:
情况1:如果该字段为NULL类型,那么直接将NULL插入到该字段,字段值就自动变成了设定的默认值;
情况2:如果该字段为NOT NULL类型,那么直接将NULL插入到该字段的话,在普通模式下该字段的值会变成默认值并发出警告,但是strict模式下会直接报错;
对于情况2的解决方法是:使用DEFAULT关键词。
官方文档(地址)中对此有了说明:
说明1:insert的语法说明(注意加红色的关键字)
INSERT [LOW_PRIORITY | DELAYED | HIGH_PRIORITY] [IGNORE]
[INTO] tbl_name
[PARTITION (partition_name,...)]
[(col_name,...)]
{VALUES | VALUE} ({expr | DEFAULT},…),(…),…
[ ON DUPLICATE KEY UPDATE
col_name=expr
[, col_name=expr] … ]
说明2:使用DEFAULT关键字的说明
Use the keyword DEFAULT to set a column explicitly to its default value. This makes it easier to write INSERTstatements that assign values to all but a few columns, because it enables you to avoid writing an incompleteVALUES list that does not include a value for each column in the table. Otherwise, you would have to write out the list of column names corresponding to each value in the VALUES list.
翻译过来大致就是说,用关键字“DEFAULT”可以显式的在insert的过程中将一个字段设定为它的默认值。
比如一条SQL可以是这样的:insert into student(sno,sname,sage,class) values(1, ‘crazyant’, ’25′, DEFAULT),这时候就直接将班级class的默认值插入进去了。
然而使用load data infile to db,即直接将文件导入到数据表时,却无法使用默认的DEFAULT值,因为将该字段设定为”DEFAULT”会被认为是个字符串,改成NULL(用\N)表示则又会出现将NULL插入到NOT NULL的错误。
?还有一种方法是程序自动取出数据表的默认值,然后在插入数据之前,自动将数据中的NULL值替换成数据表该字段的默认值,如下步骤所示:
插入数据前替换NULL为数据表默认值的方法
步骤1: 使用SHOW COLUMNS取出数据表字段的默认值
mysql> SHOW COLUMNS FROM City; +------------+----------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +------------+----------+------+-----+---------+----------------+ | Id | int(11) | NO | PRI | NULL | auto_increment | | Name | char(35) | NO | | | | | Country | char(3) | NO | UNI | -1 | | | District | char(20) | YES | MUL | | | | Population | int(11) | NO | | 0 | | +------------+----------+------+-----+---------+----------------+ 5 rows in set (0.00 sec)
如上所示,第5列”Default”下的这一列就是数据表字段的默认值,我们可以取出第一列和第五列,放在一个数组或字典里面:
array[id]=NULL
array[Name]=”"
array[Country]=-1
array[District]=”"
array[Population]=0
步骤2,插入数据前替换其中的NULL为字典存放的默认值
在插入数据表之前,如果某个字段的值为NULL,就根据字段名从上述字典取出默认值,替换掉NULL,然后再插入数据。
这种方法在load data from file中可用,因为load data的效率比insert的效率高太多,所以值得这么做。
原文地址:MYSQL向数据表插入默认字段值的方法, 感谢原作者分享。

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