[sql] How do I ALTER a PostgreSQL table and make a column unique?

I have a table in PostgreSQL where the schema looks like this:

CREATE TABLE "foo_table" (
    "id" serial NOT NULL PRIMARY KEY,
    "permalink" varchar(200) NOT NULL,
    "text" varchar(512) NOT NULL,
    "timestamp" timestamp with time zone NOT NULL
)

Now I want to make the permalink unique across the table by ALTER-ing the table. Can anybody help me with this?

TIA

it's also possible to create a unique constraint of more than 1 column:

ALTER TABLE the_table 
    ADD CONSTRAINT constraint_name UNIQUE (column1, column2);

Or, have the DB automatically assign a constraint name using:

ALTER TABLE foo ADD UNIQUE (thecolumn);

it's also possible to create a unique constraint of more than 1 column:

ALTER TABLE the_table 
    ADD CONSTRAINT constraint_name UNIQUE (column1, column2);

Or, have the DB automatically assign a constraint name using:

ALTER TABLE foo ADD UNIQUE (thecolumn);