Select

Select statements is used to query data from tables and views.

Example:

SELECT column1, column2, column3 FROM table_name;

# Query all columns in a table.
SELECT * FROM rental ;

# Queries specific columns.
SELECT country_name FROM address ;
--or
SELECT first_name, email, username FROM staff ;

# If you do not specify a specific column and use the '*' expression, all row and column in the table you specified will be displayed.
Select * from rental ;

# When you want to extract values from more than one column in the table, if you specify the column name one by one with ',' , the query result will appear in the same order.

SELECT address_id FROM address ;
SELECT country_id FROM city ;
SELECT first_name, email, username FROM staff ;

Column Concatenation

The SQL statement returns the values of the first_name and last_name lines in one column, and the email column in the other line.

SELECT first_name || ' ' || last_name, email FROM customer;

Alias

An alias is given to the column during the query.

SELECT column_name AS alias_name FROM table_name;

# The following query is used to combine the first_name and last_name into a single column and give the column an alias 'name'.

SELECT first_name || ' ' || last_name as name , email FROM
customer;

SELECT 20/5 as result ;
SELECT last_name as surname , first_name from customer ;
SELECT first_name, last_name surname FROM customer;

# If the alias of the column contains spaces, the alias is written in double quotes.
SELECT first_name || ' ' || last_name “full name” FROM customer;