Constrain with conditional value check in MySQL

Question

I need to add a combinational and value based Constrain check

My Table Structure is

CREATE TABLE IF NOT EXISTS `ContactPhone` (
  `ContactPhoneId` int(10) unsigned NOT NULL,
  `ContactId` int(11) NOT NULL,
  `PhoneId` smallint(5) unsigned NOT NULL,
  `Status` tinyint(1) NOT NULL DEFAULT '1'
) ENGINE=MyISAM AUTO_INCREMENT=2 DEFAULT CHARSET=latin1;

--
-- Indexes for dumped tables
--

--
-- Indexes for table `ContactPhone`
--
ALTER TABLE `ContactPhone`
  ADD PRIMARY KEY (`ContactPhoneId`);

--
-- AUTO_INCREMENT for dumped tables
--

--
-- AUTO_INCREMENT for table `ContactPhone`
--
ALTER TABLE `ContactPhone`
  MODIFY `ContactPhoneId` int(10) unsigned NOT NULL
  AUTO_INCREMENT,AUTO_INCREMENT=1;

I need to ensure the record based on the following combinational conditional constrain

• ContactId
• PhoneId
• Status == 1 (value should be 1 - Active Record)

Note: No need of the In-active Records (i.e., Status == 0). I need this constrain only for the Active Records.

Kindly assist me how to add this constrain for the above motioned MySQL Table.

Answer 1

According to the documentation,

A UNIQUE index permits multiple NULL values for columns that can contain NULL.

So removing the not null-constraint from Status and adding a unique index on (ContactId,PhoneId,Status) will work as you want it to, if you use null instead of 0 for inactive records.

If you don't want to or cannot use null for your Status column, want to make sure both Status=0 and Status=null behave identically, or e.g. want to treat Status=2 as active (and enforcing uniqueness) too, you can add a dummy column that will be calculated from Status.

If you are using MySQL 5.7+, you can do this with a generated column:

CREATE TABLE IF NOT EXISTS `ContactPhone` (
  `ContactPhoneId` int(10) unsigned NOT NULL auto_increment primary key,
  `ContactId` int(11) NOT NULL,
  `PhoneId` smallint(5) unsigned NOT NULL,
  `Status` tinyint(1) NOT NULL DEFAULT '1',
  `StatusUnq` tinyint(1) as (if(Status <> 0, 1, null)) stored null,
  constraint unique (ContactId, PhoneId, StatusUnq)
) ENGINE=MyISAM AUTO_INCREMENT=1 DEFAULT CHARSET=latin1;

insert into ContactPhone (ContactPhoneId, ContactId, PhoneId, Status)
values (1, 1, 1, 1);
insert into ContactPhone (ContactPhoneId, ContactId, PhoneId, Status)
values (2, 1, 1, 1);
-- Duplicate key error 
insert into ContactPhone (ContactPhoneId, ContactId, PhoneId, Status)
values (3, 1, 1, 0);
insert into ContactPhone (ContactPhoneId, ContactId, PhoneId, Status)
values (4, 1, 1, 0);
update ContactPhone set Status = 1 where ContactPhoneId = 4;
-- Duplicate key error 

Otherwise, you can use a normal column and use triggers to calculate the value of the column, e.g.:

create trigger trbi_contactPhoneUnique before insert on ContactPhone 
for each row
  set new.StatusUnq = if(new.Status <> 0, 1, null);

create trigger trbu_contactPhoneUnique before update on ContactPhone 
for each row
  set new.StatusUnq = if(new.Status <> 0, 1, null);

You can of course switch the formula to e.g. if(new.Status <> 0, new.Status, null); if you want to allow different values of Status too.