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Today, I will talk to you about how to achieve alter operation in mysql, many people may not know much about it. In order to make you understand better, the editor has summarized the following content for you. I hope you can get something according to this article.

-- add a column ALTER TABLE `users`ADD `username` to a table

TEXT NOT NULL;-- such as alter table app_user add starLevel INT (11) NULL default 6;-- when creating a table

Add the default value create table tablename to a column

(columnname datatype default defaultvalue);-- built table modify alter table tablename alter column columnname set default defaultvalue;-- add a unique constraint Alter table user add unique (username) to the username of the user table;-- change the digest field in the app_ activity table to allow empty ALTER TABLE app_activity MODIFY digest VARCHAR (255i) null;-- Delete a field ALTER TABLE mytable DROP field

Name;-- modify the type of column alter table table name

Change field name field name field type [whether non-null is allowed];-- change table name rename table old table name to new table name; / / add utf8 coding library, delete a database CREATE DATABASE `test` DEFAULT CHARACTER SET utf8 COLLATE utf8_general_ci;DROP database test; / / delete the name of an index alter table table name drop index index column; / / View the field information of the table: desc table name; / / view all information of the table: show create table table name / / add primary key constraint: alter table table name add constraint primary key (primary key field); / / add foreign key constraint: alter table from table add constraint foreign key (foreign key field) references (primary key field); / / delete primary key constraint: alter table table name drop primary key;// remove foreign key constraint: alter table table name drop foreign key foreign key (case sensitive) / / Delete a unique constraint (username there is a unique constraint on this column, app_user is the table name) drop index username on app_user

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