Getting Started with PostgreSQL - PostgreSQL Developer's Guide - page 15

warehouse_db=# CREATE TABLE tools
(
tool_id INTEGER UNIQUE,
tool_name TEXT,
tool_class NUMERIC
);
Alternatively, the same constraint can be declared at the end of all columns.
For instance, this can look like the following:
warehouse_db=# CREATE TABLE tools
(
tool_id INTEGER,
tool_name TEXT,
tool_class NUMERIC,
UNIQUE (tool_id)
);
When deining the unique constraints for a group of columns, all columns must be
listed separately using commas. Consider the following example:
warehouse_db=# CREATE TABLE cards
(
card_id INTEGER,
owner_number INTEGER,
owner_name TEXT,
UNIQUE (card_id, owner_number)
);
The preceding query will create the cards table with a unique constraint
implemented on the card_id and owner_number columns. Note that the unique
constraint is not applicable on null values. This means that in the cards table, two
records can have the same record if they have card_id and owner_number as null.
Not-null constraints
A not-null constraint makes sure that a column must have some values and a value
is not left as null. Drop the previously created tools table and create the tools table
again using this constraint using the following example:
warehouse_db=# CREATE TABLE tools
(
tool_id INTEGER NOT NULL,
tool_name TEXT,
tool_class NUMERIC
);