How to add unique constraint in already made table in sqlite ios?

You can't add a constraint to existing table in SQLite,(In SQL there is a option for do that). You need to re-create the table with necessary contraints.

There is only a few option available for alter table command in sqlite. Please check the image:

Also check Sqlite org reference.

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EDIT

However you can add unique index for your table to achieve the same effect. So you can use the following query to achieve that:

CREATE UNIQUE INDEX your_unique_index ON your_table(column_name);

In most cases, UNIQUE and PRIMARY KEY constraints are implemented by creating a unique index in the database.

Reference : SQLite Constraints

You can add constraint by creating unique index:

CREATE UNIQUE INDEX ux_friend_name ON friend(name);

where ux_friend_name is unique index name, friend(name) - table and its columns that will be affected by this constraint.

You can read more here.