Mysql: How to alter a column and change the default value?

I got the following error while trying to alter a column's data type and setting a new default 

value:

ALTER TABLE foobar_data ALTER COLUMN col VARCHAR(255) NOT NULL 

SET DEFAULT '{}';

ERROR 1064 (42000): 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 'VARCHAR(255) NOT NULL SET DEFAULT '{}'' at line 1

 Answers

ALTER TABLE foobar_data MODIFY COLUMN col VARCHAR(255) NOT NULL DEFAULT '{}';

A second possibility which does the same (thanks to juergen_d):

ALTER TABLE foobar_data CHANGE COLUMN col col VARCHAR(255) NOT NULL DEFAULT '{}';

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If you want to add default value for already created column, this works for me:

ALTER TABLE Persons
ALTER credit SET DEFAULT 0.0';

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For DEFAULT CURRENT_TIMESTAMP:

ALTER TABLE tablename
 CHANGE COLUMN columnname1 columname1 DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
 CHANGE COLUMN columnname2 columname2 DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP;

Please note double columnname declaration

Removing DEFAULT CURRENT_TIMESTAMP:

ALTER TABLE tablename
 ALTER COLUMN columnname1 DROP DEFAULT,
 ALTER COLUMN columnname2 DROPT DEFAULT;