How can data types be converted in PostgreSQL database?.

In PostgreSQL, data type conversions can be performed in several ways:

1. Explicit conversion (CAST) — using the CAST operator or the :: syntax:

SELECT CAST(column_name AS new_data_type) FROM table_name;
-- or
SELECT column_name::new_data_type FROM table_name;

2. Automatic conversion — PostgreSQL attempts to automatically convert types during operations when possible.

3. Changing column type — using the ALTER TABLE command:

ALTER TABLE table_name ALTER COLUMN column_name TYPE new_data_type USING expression;

For example, to convert text to an integer:

ALTER TABLE users ALTER COLUMN age TYPE integer USING age::integer;

It's important to use USING when the conversion is not trivial, to specify how to convert old data.