Selecting or Projecting Data

Private: Mastering SQL using Postgresql Writing Basic SQL Queries Selecting or Projecting Data

Let us understand different aspects of projecting data. We primarily using SELECT to project the data.

We can project all columns using * or some columns using column names.
We can provide aliases to a column or expression using AS in SELECT clause.
DISTINCT can be used to get the distinct records from selected columns. We can also use DISTINCT * to get unique records using all the columns.
As part of SELECT clause we can have aggregate functions such as count, sum etc.

%load_ext sql

The sql extension is already loaded. To reload it, use:
  %reload_ext sql

%env DATABASE_URL=postgresql://itversity_retail_user:retail_password@localhost:5432/itversity_retail_db

env: DATABASE_URL=postgresql://itversity_retail_user:retail_password@localhost:5432/itversity_retail_db

%sql SELECT * FROM orders LIMIT 10

 * postgresql://itversity_retail_user:***@localhost:5432/itversity_retail_db
10 rows affected.

order_id	order_date	order_customer_id	order_status
1	2013-07-25 00:00:00	11599	CLOSED
2	2013-07-25 00:00:00	256	PENDING_PAYMENT
3	2013-07-25 00:00:00	12111	COMPLETE
4	2013-07-25 00:00:00	8827	CLOSED
5	2013-07-25 00:00:00	11318	COMPLETE
6	2013-07-25 00:00:00	7130	COMPLETE
7	2013-07-25 00:00:00	4530	COMPLETE
8	2013-07-25 00:00:00	2911	PROCESSING
9	2013-07-25 00:00:00	5657	PENDING_PAYMENT
10	2013-07-25 00:00:00	5648	PENDING_PAYMENT

%%sql 

SELECT * FROM information_schema.columns 
WHERE table_catalog = 'itversity_retail_db' 
    AND table_name = 'orders'

 * postgresql://itversity_retail_user:***@localhost:5432/itversity_retail_db
4 rows affected.

table_catalog	table_schema	table_name	column_name	ordinal_position	column_default	is_nullable	data_type	character_maximum_length	character_octet_length	numeric_precision	numeric_precision_radix	numeric_scale	datetime_precision	interval_type	interval_precision	character_set_catalog	character_set_schema	character_set_name	collation_catalog	collation_schema	collation_name	domain_catalog	domain_schema	domain_name	udt_catalog	udt_schema	udt_name	scope_catalog	scope_schema	scope_name	maximum_cardinality	dtd_identifier	is_self_referencing	is_identity	identity_generation	identity_start	identity_increment	identity_maximum	identity_minimum	identity_cycle	is_generated	generation_expression	is_updatable
itversity_retail_db	public	orders	order_id	1	None	NO	integer	None	None	32	2	0	None	None	None	None	None	None	None	None	None	None	None	None	itversity_retail_db	pg_catalog	int4	None	None	None	None	1	NO	NO	None	None	None	None	None	NO	NEVER	None	YES
itversity_retail_db	public	orders	order_date	2	None	NO	timestamp without time zone	None	None	None	None	None	6	None	None	None	None	None	None	None	None	None	None	None	itversity_retail_db	pg_catalog	timestamp	None	None	None	None	2	NO	NO	None	None	None	None	None	NO	NEVER	None	YES
itversity_retail_db	public	orders	order_customer_id	3	None	NO	integer	None	None	32	2	0	None	None	None	None	None	None	None	None	None	None	None	None	itversity_retail_db	pg_catalog	int4	None	None	None	None	3	NO	NO	None	None	None	None	None	NO	NEVER	None	YES
itversity_retail_db	public	orders	order_status	4	None	NO	character varying	45	180	None	None	None	None	None	None	None	None	None	None	None	None	None	None	None	itversity_retail_db	pg_catalog	varchar	None	None	None	None	4	NO	NO	None	None	None	None	None	NO	NEVER	None	YES

%%sql 

SELECT order_customer_id, order_date, order_status 
FROM orders 
LIMIT 10

 * postgresql://itversity_retail_user:***@localhost:5432/itversity_retail_db
10 rows affected.

order_customer_id	order_date	order_status
11599	2013-07-25 00:00:00	CLOSED
256	2013-07-25 00:00:00	PENDING_PAYMENT
12111	2013-07-25 00:00:00	COMPLETE
8827	2013-07-25 00:00:00	CLOSED
11318	2013-07-25 00:00:00	COMPLETE
7130	2013-07-25 00:00:00	COMPLETE
4530	2013-07-25 00:00:00	COMPLETE
2911	2013-07-25 00:00:00	PROCESSING
5657	2013-07-25 00:00:00	PENDING_PAYMENT
5648	2013-07-25 00:00:00	PENDING_PAYMENT

%%sql 

SELECT order_customer_id, 
    to_char(order_date, 'yyyy-MM'), 
    order_status 
FROM orders 
LIMIT 10

 * postgresql://itversity_retail_user:***@localhost:5432/itversity_retail_db
10 rows affected.

order_customer_id	to_char	order_status
11599	2013-07	CLOSED
256	2013-07	PENDING_PAYMENT
12111	2013-07	COMPLETE
8827	2013-07	CLOSED
11318	2013-07	COMPLETE
7130	2013-07	COMPLETE
4530	2013-07	COMPLETE
2911	2013-07	PROCESSING
5657	2013-07	PENDING_PAYMENT
5648	2013-07	PENDING_PAYMENT

%%sql 

SELECT order_customer_id, 
    to_char(order_date, 'yyyy-MM') AS order_month, 
    order_status 
FROM orders 
LIMIT 10

 * postgresql://itversity_retail_user:***@localhost:5432/itversity_retail_db
10 rows affected.

order_customer_id	order_month	order_status
11599	2013-07	CLOSED
256	2013-07	PENDING_PAYMENT
12111	2013-07	COMPLETE
8827	2013-07	CLOSED
11318	2013-07	COMPLETE
7130	2013-07	COMPLETE
4530	2013-07	COMPLETE
2911	2013-07	PROCESSING
5657	2013-07	PENDING_PAYMENT
5648	2013-07	PENDING_PAYMENT

%%sql 

SELECT DISTINCT to_char(order_date, 'yyyy-MM') AS order_month 
FROM orders

 * postgresql://itversity_retail_user:***@localhost:5432/itversity_retail_db
13 rows affected.

order_month
2014-01
2014-05
2013-12
2013-11
2014-04
2014-07
2014-03
2013-08
2013-10
2013-07
2014-02
2013-09
2014-06

%sql SELECT count(1) FROM orders

 * postgresql://itversity_retail_user:***@localhost:5432/itversity_retail_db
1 rows affected.

count
68883

%%sql 

SELECT count(DISTINCT to_char(order_date, 'yyyy-MM')) AS distinct_month_count 
FROM orders

 * postgresql://itversity_retail_user:***@localhost:5432/itversity_retail_db
1 rows affected.

distinct_month_count
13

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