Not getting expected Output while trying to alter a column default value - mysql

Question

Problem:
I have created a table with a given structure:
CREATE TABLE `X` (
`ID` int(11) NOT NULL AUTO_INCREMENT,
`MSISDN` varchar(25) DEFAULT NULL,
`subservice_id` varchar(25) DEFAULT NULL,
`SUB_START_DATE` datetime DEFAULT NULL,
`START_DATE` datetime DEFAULT NULL,
`END_DATE` datetime DEFAULT NULL,
`scheduler_renewal_status` varchar(25) NOT NULL DEFAULT 'scheduled',
`STATUS` varchar(20) DEFAULT NULL,
`LAST_RENEW_DATE` datetime DEFAULT NULL,
`LAST_CALL_DATE` datetime DEFAULT NULL,
`TRANSACTION_STATUS` varchar(15) DEFAULT NULL,
`CIRCLE` varchar(3) DEFAULT NULL,
`COUNTRY` varchar(3) DEFAULT NULL,
`LANGUAGE` varchar(3) DEFAULT NULL,
`price` float DEFAULT '0',
`CALL_ATTEMPTS` int(3) DEFAULT NULL,
`PRIMARY_ACT_MODE` varchar(25) DEFAULT NULL,
`SECONDARY_ACT_MODE` varchar(25) DEFAULT NULL,
`ERROR_MSG` varchar(25) DEFAULT NULL,
`SUB_TYPE` varchar(10) DEFAULT NULL,
`SUB_TIME_LEFT` int(3) DEFAULT '0',
`SUB_SERVICE_NAME` varchar(25) DEFAULT NULL,
`OPERATOR` varchar(25) DEFAULT NULL,
`GIFT_ID` varchar(25) DEFAULT NULL,
`service_id` varchar(25) DEFAULT NULL,
`RETRY_COUNT` int(2) NOT NULL DEFAULT '0',
`NEXT_RETRY_DATE` datetime DEFAULT NULL,
`CPID` int(11) DEFAULT NULL,
`SCHEDULER_RETRY_STATUS` varchar(25) DEFAULT 'scheduled',
`scheduler_statechanger_status` varchar(25) DEFAULT 'scheduled',
`scheduler_subs_msgs_status` varchar(25) DEFAULT 'scheduled',
`serviceName` varchar(25) DEFAULT NULL,
`transactionId` varchar(25) DEFAULT NULL,
`nextAction` varchar(25) DEFAULT NULL,
`topUpPrice` int(3) DEFAULT NULL,
`topUp_time_left` int(5) NOT NULL DEFAULT '0',
`autoRenew` varchar(10) DEFAULT NULL,
`option1` varchar(25) DEFAULT NULL,
`callBackRecvFlag` varchar(10) DEFAULT NULL,
`last_msg_date` date DEFAULT NULL,
`accountType` varchar(10) DEFAULT 'prepaid',
`optkey` varchar(20) DEFAULT NULL,
`successCallCount` int(3) DEFAULT '0',
PRIMARY KEY (`ID`),
UNIQUE KEY `UK_3` (`MSISDN`,`subservice_id`)
) ENGINE=InnoDB AUTO_INCREMENT=27793 DEFAULT CHARSET=utf8
I want to change the column SUB_START_DATE default type from NULL to TIMESTAMP.
Can anyone guide me the proper syntax for this
Solution:
I am running the following Command:
ALTER TABLE `X` MODIFY COLUMN SUB_START_DATE TIMESTAMP NOT NULL DEFAULT current_timestamp;
It is affecting the rows of the table.
But,if I want to have column datatype datetime only and change the default value to current_timestamp.
I ran the following command:
ALTER TABLE `X` MODIFY COLUMN SUB_START_DATE datetime DEFAULT current_timestamp;
OutputShown:
Error Code : 1067
Invalid default value for 'SUB_START_DATE'

Answer 1

In this case my SQL server does not allow to modify existing default constraint value. So to change the default value we need to delete the existing system generated or user generated default constraint. And after that default value can be set for a particular column.
Follow some steps :
List all existing default value constraints for columns.
Execute this system database procedure, it takes table name as a parameter. It returns list of all constrains for all columns within table.
execute [dbo].[sp_helpconstraint] 'table_name'
Drop existing default constraint for a column.
Syntax:
alter table 'table_name' drop constraint 'constraint_name'
Add new default value constraint for that column:
Syntax:
alter table 'table_name' add default 'current_timestamp' for 'SUB_START_DATE '