Howto define a constraint in a existing column - Mailing list pgsql-novice

I am tying to define a primary key constraint to an existing table,
this is what I have tried so far

ALTER TABLE mytable ALTER COLUMN field13 CONSTRAINT name PRIMARY KEY;

I get the following error: ERROR: syntax error at or near "CONSTRAINT"
SQL state: 42601
Character: 61

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Sindile Bidla