This document contains examples of SQL queries with explanations and formatting guidelines. The queries select data from tables to return item details, revenues, and ratios. Comments provide best practices for formatting, commenting code, and modeling data. The document also discusses data structures, common table expressions, views, functions and technologies.

1. SQL
Adilson Mendonca

3. select i_ITEM_id,i_item_desc,i_category,i_class,i_CURRENT_price, sum(cs_EXT_sales_price) as
itemrevenue, sum(cs_ext_sales_price) *100/sum(sum(cs_ext_sales_price)) over (partition by
I_class) as revenueratio
From catalog_sales,item,date_dim
where cs_item_sk = i_item_sk AND i_category in ('Jewelry', 'Sports', 'Books')
and cs_sold_date_sk = d_date_sk and cast(d_date as timestamp) between cast('2001-01-12' as
timestamp)
and (cast('2001-01-12' as timestamp) + interval 30 days)
group by i_item_id,i_item_desc,i_category,i_class,i_current_price
order by i_category,i_class,i_item_id,i_item_desc,revenueratio
limit 100;

4. Code Visibility

6. select
i_ITEM_id,
I_item_desc,
I_category,
I_class,
i_CURRENT_price,
sum(cs_EXT_sales_price) as itemrevenue,
sum(cs_ext_sales_price) *100/sum(sum(cs_ext_sales_price))
over (partition by I_class) as revenueratio
FROM catalog_sales,item,date_dim
where cs_item_sk = i_item_sk
AND i_category in ('Jewelry', 'Sports', 'Books')
and cs_sold_date_sk = d_date_sk
and cast(d_date as timestamp) between cast('2001-01-12' as timestamp)
and (cast('2001-01-12' as timestamp) + interval 30 days)
group by i_item_id,i_item_desc,i_category,i_class,i_current_price
order by i_category,i_class,i_item_id,i_item_desc,revenueratio
limit 100;

7. select i_ITEM_id,
I_item_desc,
I_category,
I_class,
i_CURRENT_price,
sum(cs_EXT_sales_price) as itemrevenue,
sum(cs_ext_sales_price) *100/sum(sum(cs_ext_sales_price))
over (partition by I_class) as revenueratio
FROM catalog_sales,item,date_dim
where cs_item_sk = i_item_sk
AND i_category in ('Jewelry', 'Sports', 'Books')
and cs_sold_date_sk = d_date_sk
and cast(d_date as timestamp)
between cast('2001-01-12' as timestamp)
and (cast('2001-01-12' as timestamp) + interval 30 days)
group by i_item_id,
I_item_desc,
I_category,
I_class,
i_current_price
order by i_category,
I_class,
I_item_id,
I_item_desc,
revenueratio
limit 100;

8. SELECT i_item_id,
,i_item_desc
,i_category
,i_class
,i_current_price
,SUM(cs_ext_sales_price) AS item_revenue
,SUM(cs_ext_sales_price) * 100
/ SUM(SUM(cs_ext_sales_price))
OVER (PARTITION BY i_class) AS revenue_ratio
FROM catalog_sales
JOIN item ON cs_item_sk = i_item_sk
JOIN date_dim ON cs_sold_date_sk = d_date_sk
WHERE i_category IN ('Jewelry', 'Sports', 'Books')
AND CAST(d_date AS TIMESTAMP)
BETWEEN CAST('2001-01-12' AS TIMESTAMP)
AND CAST('2001-01-12' AS TIMESTAMP) + INTERVAL 30 DAYS
GROUP BY 1, 2, 3, 4, 5
ORDER BY 3, 4, 1
LIMIT 100

9. SELECT item.i_item_id,
item.i_item_desc,
item.i_category,
item.i_class,
item.i_current_price,
SUM(catalog_sales.cs_ext_sales_price) AS item_revenue,
SUM(catalog_sales.cs_ext_sales_price) * 100
/ SUM(SUM(catalog_sales.cs_ext_sales_price))
OVER (PARTITION BY item.i_class) AS revenue_ratio
FROM catalog_sales
JOIN item
ON catalog_sales.cs_item_sk = item.i_item_sk
JOIN date_dim
ON catalog_sales.cs_sold_date_sk = date_dim.d_date_sk
WHERE item.i_category IN ('Jewelry', 'Sports', 'Books')
AND CAST(date_dim.d_date AS TIMESTAMP)
BETWEEN CAST('2001-01-12' AS TIMESTAMP)
AND CAST('2001-01-12' AS TIMESTAMP) + INTERVAL 30 DAYS
GROUP BY 1, 2, 3, 4, 5
ORDER BY 3, 4, 1
LIMIT 100

10. Over commenting
-- get id, name and open only once
SELECT DISTINCT t1.id , t1.name , t3.open
-- select names from table 1, order them by date and return first 3
FROM (SELECT id , name FROM table1 ORDER BY date DESC LIMIT 3) AS t1
-- get open from table 2 IF id is there, open = 1 and type =2
LEFT JOIN (
( SELECT open , name_id FROM table2 WHERE open=1 AND type=2 ) AS t3 )
ON t1.id = t3.name_id
-- order by name from A-Z
ORDER BY t1.name ASC

13. Follow Patterns
Indentation
Don’t over comment - just clear code
Remove commented lines of code
Use alias on all column when joining
Know your PKs and Unique keys
Make easy future maintenance
Execution time
JUST select the tables/columns which be in USE

14. Code block

16. WITH
patient_data AS
(SELECT patient_id,
patient_name,
hospital,
drug_dosage
FROM hospital_registry
WHERE (COALESCE(last_visit,NOW()) > NOW() - INTERVAL '14 days')
AND city = "Los Angeles"
),
average_dosage AS
(SELECT hospital,
AVG(drug_dosage) AS Average
FROM patient_data
GROUP BY hospital
)
SELECT count(hospital)
FROM average_dosage
WHERE drug_dosage > 1000

17. Master the use of:
Functions
Window Functions (OLAP functions)
CTE - Common Table Expression
Views
UDF - User Defined Function

18. Lost

20. Data Structure
Build an ERD if you don’t have
Primary Keys
Unique Keys
Table Size
Number of columns (columnar databases)
KNOW YOUR DATA

21. Data Modelling

22. Modelling techniques
Transactional
3NF
Star Schema - Data Marts
Integration - Data Vault
Data Lake - Big Data
Flat tables

23. Data Lake or Data swamp

24. AVOID the journey to a SWAMP
Organize your data and contents
Use Name conventions - rules
Be aware of object creation
Document them on same way

25. Verbose

27. Customer
id
name
date
Sales
id
date
Cust_id
amount
Customer
id_customer
name
date_of_birth
Sales
id_sales
sales_date
fk_customer
amount_inc_tax
SELECT c.name AS customer_name,
c.date AS customer.dob,
s.date AS sales_date,
COUNT(1) AS no_of_sales,
SUM(amount) AS amount
FROM customer c
JOIN sales s
ON c.id = s.id
SELECT c.name AS customer_name,
c.date_of_birth,
s.sales_date,
COUNT(1) no_of_sales,
SUM(amount_inc_tax) AS amount_inc_tax
FROM customer c
JOIN sales s
ON c.id_customer = s.fk_customer

28. Minimise usage of non standard abbreviations
Don’t use too long names - you will need to type them one day
PK & FK should be a pattern
ID & table name, FK and link table
Maybe:
Data types definition on names like: price_amt, tax_pct

29. Know technologies

30. Know you databases & tools
Differences, limitations, strength and
weakness
Columnar databases
Functionalities
Access & Security

31. Help make a better
world with
beautiful code!!!