Postgresing

Not Null

You usually don't want holes in your data. Generously applying not null constraints can help.

Create a table with the not null modifier used for one of the columns:

create table cocktails (
  id serial primary key,
  name varchar not null,
  description varchar
);

Add a new column with the not null modifier:

alter table cocktails
  add column created_at timestamptz not null;

This statement works cleanly on an empty table, but will fail if there is even one row in the table. To add this column would immediately put each row in the table in violation of the not null constraint. There are techniques for getting around this.

Alter an existing column to have a not null modifier:

alter table cocktails
  alter column description set not null;

This has the same issue as above if there is already data in the table.

Remove a not null modifier from an existing column:

alter table cocktails
  alter column description drop not null;