The document provides an overview of SQL (Structured Query Language) principles, including defining a database, querying, and manipulating data. It discusses key SQL concepts such as the structure of tables, SELECT statements, filtering results, and joining tables, and highlights the importance of ordering and limiting result sets. Various examples throughout illustrate how to write SQL queries for specific data retrieval tasks.

1. SQL
Structured Query Language

2. SQL
● Define the database
● Query data
● Manipulate data
● Add more data

3. Database
applications
applications
applications
reporting
administration
SQL
SQL
SQL

4. reporting
SQL
SQL
SQL

5. SQL 101
● Define the database
● Query data
● Manipulate data
● Add more data

6. Database vs. Spreadsheets
● Each table is like an Excel sheet
● Structure is much more important
● Calculations are not* part of the table

7. Tables
● Divided in rows or records
● And divided in columns or fields
Columns describe the characteristics or
quantities of what is described in each row,
but never* the other way around.

8. Querying

9. vegetables
id name color stock_count
1 Apple Red 10
2 Tomato Red 2
3 Banana Yellow 4
4 Avocado Green 15

10. vegetables
id name color stock_count
1 Apple Red 10
2 Tomato Red 2
3 Banana Yellow 4
4 Avocado Green 15

11. Sentences and clauses
“Return observations on the following
characteristics, from the following tables,
where the following criteria is met,
organized in the following way.”

12. The SELECT statement
“Return observations on the following
characteristics, from the following tables,
where the following criteria is met.”

13. The SELECT statement
“Return the name and color, from the
vegetables table, where the stock
count is more than 4.”

14. The SELECT statement
SELECT the name and color, FROM the
vegetables table, WHERE the stock count is
more than 4.

15. The SELECT statement
SELECT
the name and color,
FROM
the vegetables table
WHERE
the stock count is more than 4.

16. The SELECT statement
SELECT
name, color
FROM
vegetables
WHERE
stock count > 4.

17. The SELECT statement
SELECT
"name", "color"
FROM
"vegetables"
WHERE
"stock_count" > 4;

18. vegetables
id name color stock_count
1 Apple Red 10
2 Tomato Red 2
3 Banana Yellow 4
4 Avocado Green 15

19. Result
name color
Apple Red
Avocado Green

20. Combining expressions
SELECT
"id", "color"
FROM
"vegetables"
WHERE
"stock_count" > 4 AND "color" = 'Red';

21. vegetables
id name color stock_count
1 Apple Red 10
2 Tomato Red 2
3 Banana Yellow 4
4 Avocado Green 15

22. Result
id color
1 Red

23. More combinations
SELECT
"id", "color"
FROM
"vegetables"
WHERE
"stock_count" > 4
AND ("color" = 'Red' OR "name" = 'Pear');

24. SELECT
"id", "color"
FROM
"vegetables"
WHERE
"stock_count" > 4
AND ("color" = 'Red' OR "name" = 'Pear') ;
More combinations

25. SELECT
"id", "color"
FROM
"vegetables"
WHERE
"stock_count" > 4
AND ("color" = 'Red' OR "name" = 'Pear') ;
More combinations

26. More combinations
SELECT
"id", "color"
FROM
"vegetables"
WHERE
"stock_count" > 4
AND ("color" = 'Red' OR "name" = 'Pear') ;

27. Result
id color
1 Red

28. Other comparisons
● Column values can be compared to literals or to other
columns, using:
○ =
○ >
○ <
○ >=
○ <=
○ !=
○ some specialized ones for dates, etc.

29. Sentences and clauses
“Return observations on the following
characteristics, from the following tables,
where the following criteria is met,
organized in the following way.”

30. Ordering results
SELECT
"id", "color"
FROM
"vegetables"
WHERE
"stock_count" > 4
ORDER BY
"id";

31. vegetables
id name color stock_count
1 Apple Red 10
2 Tomato Red 2
3 Banana Yellow 4
4 Avocado Green 15

32. Result
name color
Avocado Green
Apple Red

33. Limiting the result set
SELECT
* --Everything
FROM
"vegetables"
ORDER BY
"stock_count" DESC
LIMIT
2;

34. Result
id name color stock_count
4 Avocado Green 15
1 Apple Red 10

35. Top 10 SKUs by stock level
Chartio example

36. Write a query for a chart (table) that
returns the name of the top 10 customers
by qty. of boxes, whose level is either
gold or bronze.
DIY

37. Recap...
● Identifiers go between double quotes, "color".
● Text goes between simple quotes, like 'Red'.
● Expressions combine with AND, OR, and NOT.
● We can ORDER BY any column, ASC or DESC.
● We can LIMIT the amount of rows we get.

38. More than one table
Joining

39. Sets, the cartesian product
Car
Truck
Van
Red
Green
Blue
color vehicle

40. Sets, the cartesian product
Car
Truck
Van
Red
Green
Blue
Red Car
Red Van
Red Truck
Blue Car
Blue Van
Blue Truck
...
color vehiclePRODUCT

41. Two tables
id name color
1 Jacket 2
2 Sweater 1
3 Shirt 1
id name
1 Red
2 Green
3 Blue
color item

42. Two tables
id name color
1 Jacket 2
2 Sweater 1
3 Shirt 1
id name
1 Red
2 Green
3 Blue
color item

43. Two tables
id name color
1 Jacket 2
2 Sweater 1
3 Shirt 1
id name
1 Red
2 Green
3 Blue
color item

44. SELECT
*
FROM
"item"
JOIN
"color";
The JOIN clause

45. Two tables product
id name id name color
1 Red 1 Jacket 2
1 Red 2 Sweater 1
1 Red 3 Shirt 1
2 Green 1 Jacket 2
2 Green 2 Sweater 1
2 Green 3 Shirt 1
3 Blue 1 Jacket 2
3 Blue 2 Sweater 1
3 Blue 3 Shirt 1
item X color

46. Two tables, also quite useless
id name color
1 Jacket 2
2 Sweater 1
3 Shirt 1
id name
1 Red
2 Green
3 Blue
color item

47. Two tables, product
id name id name color
1 Red 1 Jacket 2
1 Red 2 Sweater 1
1 Red 3 Shirt 1
2 Green 1 Jacket 2
2 Green 2 Sweater 1
2 Green 3 Shirt 1
3 Blue 1 Jacket 2
3 Blue 2 Sweater 1
3 Blue 3 Shirt 1
item X color

48. Two tables, restricted product
id name id name color
1 Red 1 Jacket 2
1 Red 2 Sweater 1
1 Red 3 Shirt 1
2 Green 1 Jacket 2
2 Green 2 Sweater 1
2 Green 3 Shirt 1
3 Blue 1 Jacket 2
3 Blue 2 Sweater 1
3 Blue 3 Shirt 1
item X color, only WHERE the color "id" equals "color"

49. Two tables, restricted product
item X color, only WHERE the color "id" equals "color"
id name id name color
1 Red 2 Sweater 1
1 Red 3 Shirt 1
2 Green 1 Jacket 2

50. Only some columns
item X color product, only WHERE the color "id" equals the item "color",
SELECTing only some columns.
name name
Sweater Red
Shirt Red
Jacket Green

51. SELECT
"item"."name",
"color"."name"
FROM
"item"
JOIN
"color" ON "item"."color" = "color"."id";
The JOIN clause, ON

52. Name name what?
name name
Sweater Red
Shirt Red
Jacket Green

53. SELECT
"item"."name" AS "Item Name",
"color"."name" AS "Color"
FROM
"item"
JOIN
"color" ON "item"."color" = "color"."id";
Aliases, AS

54. Better
Item name Color
Sweater Red
Shirt Red
Jacket Green

55. SELECT
i."name" AS "Item Name",
c."name" AS "Color"
FROM
"item" AS i
JOIN
"color" AS c ON i."color" = c."id";
Aliases, AS also for tables

56. The personal shopper names of the
last 20 signups, without repeating.
Chartio example

57. Write a query for a chart (table) that
returns the name of the top 10 customers
by qty. of boxes, whose level is either
gold or bronze. Also include the name of
their PS.
DIY

59. Summing, counting, averaging multiple
rows into one.
Next: Aggregation

60. Iván Stepaniuk
@istepaniuk