Postgres - AS and Union in PostgreSQL

SQL 'AS' is used to assign a new name temporarily to a table column or even a table.
It makes an easy presentation of query results and allows the developer to label results more accurately without permanently renaming table columns or even the table itself.
SELECT Column_Name1 AS New_Column_Name, Column_Name2 As New_Column_Name FROM Table
Here, the Column_Name is the name of a column in the original table, and the New_Column_Name is the name assigned to a particular column only for that specific query. This means that New_Column_Name is a temporary name that will be assigned to a query.

Union:
In SQL, the UNION operator selects rows from two or more tables. If rows of tables are the same, those rows are only included once in the result set. For example,
SELECT age
FROM Teachers
UNION
SELECT age
FROM Students;

To use UNION in SQL, we must always remember,

Column count in all tables must be the same. For example, Teachers and Students both tables have three columns.
The data types of columns must be the same. For example, the age column in Teachers is integer, so is the age in Students table.
The columns must be in the same order in each table. For example, the order of columns is id-name-age in Teachers, so in the Students table.

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