mysql表默认公式

Question

In mysql (Ubuntu 13.10, MySql 5.5) I'm trying to create a table that will automatically create a random alphanumeric ID with this code:

create table YGraph (
    YGraphEdgeId CHAR(8) NOT NULL PRIMARY KEY DEFAULT SUBSTRING(MD5(RAND()) FROM 1 FOR 8),
    YGraphStartVertex CHAR(6) NOT NULL,
    YGraphEndVertex CHAR(6) NOT NULL
);

but phpmyadmin is complaining:

#1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'SUBSTRING(MD5(RAND()) FROM 1 FOR 8), YGraphStartVertex CHAR(6) NOT NULL, ' at line 2

. . . and putting quotes around the formula only give a generic

#1067 - Invalid default value for 'YGraphEdgeId'

I simply want a new record creation to run the default formula and stick an 8-place random alphanum in the YGraphEdgeId field. I seem to remember this formula working in an INSERT. What am I doing wrong?

Answer 1

目前您无法使用函数为列提供默认值，请参阅https://dev.mysql.com/doc/refman/5.5/en/data-type-defaults.html

Answer 2

As per MySQL Documentation on Data Type Default Values

...the default value must be a constant; it cannot be a function or an expression....

Hence you can't use functions or expressions there.

Secondly, you can assign a default string value but it should abide by the length of the column data-type used. String used to define default value was more that 8 characters. And hence the error thrown.