SQL Queries II
Objetives
Filter Data
GROUP BY
-- Structure
SELECT fields
FROM table_name
GROUP BY fieldSELECT COUNT(week_id)
FROM days
GROUP BY week_id;
GROUP BY
Example using SQLBOLT
SELECT
director,
count(title) AS titles
FROM movies
GROUP BY director;HAVING
-- Structure
SELECT fields
FROM table_name
GROUP BY field
HAVING conditionSELECT count(week_id)
FROM days
GROUP BY week_id
HAVING count(week_id) > 2HAVING
Example using SQLBOLT
SELECT
director,
count(title) AS titles
FROM movies
GROUP BY director
HAVING movies > 1HAVING & WHERE
SELECT
director,
count(title) AS titles
FROM movies
WHERE NOT director = "Pete Docter"
GROUP BY director
HAVING movies > 1Normalization
Normalization is the process of organizing data in a database.
First Normal Form (1FN)
-
Remove repeated groups from individual tables.
-
Create a separate table for each set of related data.
-
Identify each set of related data with a primary key
Second Normal Form (2FN)
-
Create separate tables for sets of values that apply to multiple records.
-
Relate these tables with a foreign key.
Third Normal Form (3FN)
-
Delete the fields that do not depend on the key.
Clauses to query data from multiple tables
JOINS