Unique "Value" Issue with MySQL database - php

Question

I am having a small and annoying issue with my MySQL database.
I have a field StudentID that is supposed to be unique for each user but is not mandatory.
So when the user registers and doesn't want to add a studentID - he leaves the field blank. However when a 2nd user leaves the field blank - it counts it as a duplicate value.
Is there any way to fix this ?I tried several methods posted below :
Mysql Error : 1062 - Duplicate entry '' for key
'users_studentid_unique'
1st time (works) :
insert into `users` (`username`, `name`, `email`, `studentid`,
`password`, `updated_at`, `created_at`)
values ('aaaaaa263', 'aaaaaa', 'aaaaaa23d#gmail.com','' ,
'yZPeKRV0C0xbr/QCI1w/Y.0Z1qSGnJeivgP71epaHpT139g65hAf2',
'2014-03-20 18:16:54', '2014-03-20 18:16:54')
2nd time (Duplicate value error for studentid) :
insert into `users` (`username`, `name`, `email`, `studentid`,
`password`, `updated_at`, `created_at`)
values ('aaaaaa263', 'aaaaaa', 'aaaaaa23d#gmail.com','' ,
'yZPeKRV0C0xbr/QCI1w/Y.0Z1qSGnJeivgP71epaHpT139g65hAf2',
'2014-03-20 18:16:54', '2014-03-20 18:16:54')
Tried :
Checking the NULL value on the studentid table in the database.
Tried inserting into the db using these values instead of the blank field :
NULL
null
UNDEFINED
Still having the same issue though, - Does anyone have any idea how to fix this problem?

Answer 1

Use a NULL value, that's not considered to be a "duplicate" in a unique index. But the column has to allow NULL values, that is, it cannot be defined as NOT NULL.
To insert a NULL value, use the keyword NULL keyword. Note that this is different than the character string 'NULL'
INSERT INTO mytab (a, b) VALUES (NULL, 'NULL')
Column a is assigned the special NULL value. Column b is assigned a character string.
Also, omitting the column from the INSERT list will cause the default value to be assigned to the column. The default value for a column is NULL, unless you've set the default to be something else.
This statement is equivalent to the statement above, in that column a will be assigned the default value defined for column a, which is
INSERT INTO mytab (b) VALUES ('NULL')
The other option is to use an expression to generate the NULL value for you, in the place of the empty string.
Given a description of your current logic, if the user_id column allows for NULL values, then you could translate the empty string to NULL
insert into `users` (`username`, `name`, `email`, `studentid`,
`password`, `updated_at`, `created_at`)
values ( ?, ?, ?, NULLIF( ? ,''), ?, ?, ?)
The NULLIF(x,y) function is shorthand for IF x = y THEN NULL ELSE x END.

Answer 2

You can set the student id to a timestamp if it's null, then check if its numeric to see if it's really set or not.
<?php
$student_id = is_null($student_id) ? time() : $student_id;
Your request :
insert into `users` (`username`, `name`, `email`, `studentid`,
`password`, `updated_at`, `created_at`)
values ('aaaaaa263', 'aaaaaa', 'aaaaaa23d#gmail.com','$student_id' ,
'yZPeKRV0C0xbr/QCI1w/Y.0Z1qSGnJeivgP71epaHpT139g65hAf2',
'2014-03-20 18:16:54', '2014-03-20 18:16:54')
Then to get the users back :
SELECT *, if(concat('','student_id' * 1) = 'student_id', 0, 1) as is_student_id_valid from ...
Then is_studient_id_valid field will tell you if this id is valid or not.

Answer 3

I have tried to recreate your problem. Alas, it works fine here.
However, i supply all the information so that you can at least check where your setup is different.
Create table statement: Please check all of: the Mysql Version, the table engine, the primary key and the index on the 'studentid' column.
/*
SQLyog Community v10.2
MySQL - 5.5.16 : Database - testmysql
*********************************************************************
*/
/*Table structure for table `users` */
DROP TABLE IF EXISTS `users`;
CREATE TABLE `users` (
`username` varchar(128) COLLATE utf8_unicode_ci NOT NULL,
`name` varchar(128) COLLATE utf8_unicode_ci NOT NULL,
`email` varchar(128) COLLATE utf8_unicode_ci NOT NULL,
`studentid` varchar(128) COLLATE utf8_unicode_ci DEFAULT NULL,
`password` varchar(128) COLLATE utf8_unicode_ci NOT NULL,
`created_at` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
`updated_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
PRIMARY KEY (`username`),
UNIQUE KEY `studentid_uidx` (`studentid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
Typical insert with a null studentid...
INSERT INTO `users` (`username`, `name`, `email`, `studentid`,
`password`, `updated_at`, `created_at`)
VALUES ('aaaaaa004', 'aaaaaa', 'aaaaaa23d#gmail.com', NULL,
'yZPeKRV0C0xbr/QCI1w/Y.0Z1qSGnJeivgP71epaHpT139g65hAf2',
'2014-03-20 18:16:54', '2014-03-20 18:16:54')
query result
username name email studentid password created_at updated_at
aaaaaa001 aaaaaa aaaaaa23d#gmail.com (NULL) yZPeKRV0C0xbr/QCI1w/Y.0Z1qSGnJeivgP71epaHpT139g65hAf2 2014-03-20 18:16:54 2014-03-20 18:16:54
aaaaaa002 aaaaaa aaaaaa23d#gmail.com (NULL) yZPeKRV0C0xbr/QCI1w/Y.0Z1qSGnJeivgP71epaHpT139g65hAf2 2014-03-20 18:16:54 2014-03-20 18:16:54
aaaaaa003 aaaaaa aaaaaa23d#gmail.com stid01 yZPeKRV0C0xbr/QCI1w/Y.0Z1qSGnJeivgP71epaHpT139g65hAf2 2014-03-20 18:16:54 2014-03-20 18:16:54
aaaaaa004 aaaaaa aaaaaa23d#gmail.com (NULL) yZPeKRV0C0xbr/QCI1w/Y.0Z1qSGnJeivgP71epaHpT139g65hAf2 2014-03-20 18:16:54 2014-03-20 18:16:54