MySQL中怎么让一个列的默认值等于另一个列的值

1. 判断是否是NULL应使用is，而不是=

2. 修改字段值应使用UPDATE语句

故你的代码修改成下面这样，试试看

create trigger tigger1 on UserInfo

for insert

as

declare @i1 as int --这里类型实际应修改为a的类型

select @i1 = UserInfo.a

from UserInfo join inserted on UserInfo.主键 = inserted.主键

if ( @i1 is null)

update UserInfo

set a = UserInfo.b

from UserInfo join inserted on UserInfo.主键 = inserted.主键

SELECT TOP number|percent column_name(s)

FROM table_name

MySQL 和 Oracle 中的 SQL SELECT TOP 是等价的

MySQL 语法

SELECT column_name(s)

FROM table_name

LIMIT number

例子

SELECT *

FROM Persons

LIMIT 5

Oracle 语法

SELECT column_name(s)

FROM table_name

WHERE ROWNUM <= number

例子

SELECT *

FROM Persons

WHERE ROWNUM <= 5