3.3.1. Columns

v1.2.0 Summary Chapter 3: Database objects 3. Introduction 3.3. Tables 3.3.1. Columns

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Columns are the small unity of a table and they define how data must be stored through their data types and additional attributes as shown in the image below.

Attribute	Description
`Data Type`	This attribute handles the data type assigned to the column.
`Default Value`	This field defines the default value for the column in case none is specified in an `INSERT` command. If the user wants to set up a simple expression use the attribute Expression. The Generated option indicates that the column's default value is generated from other columns' values. For example, say we have a column called `height_in` that represents the height in inches and another generated column called `height_cm` which represents the height in centimeters. The default value for `height_cm` can be easily computed from `height_in` by performing a simple conversion like `height_cm / 2.54`. The attribute Sequence is used to configure a default value for serial data types based upon a sequence object. Once selected the sequence this object will be converted into an expression like: `nextval('seq'::regclass)`. The Identity attribute is used to configure an identity column (introduced in PostgreSQL 10). The possible values of this field are `ALWAYS` and `BY DEFAULT`. In newer versions, it's possible to tweak the sequence parameters associated with the identity column.
`NOT NULL`	Creates a NOT NULL constraint in the column.