vault/Career/Database - SQL/Using Joins, Constraints, Normalization, and Subqueries.md

note type, date, done, link

note type	date	done	link
sql database theory	2026-06-03		https://app.pluralsight.com/ilx/video-courses/sql-joins-constraints-normalization-subqueries/course-overview

Common Aggregate Functions

title	cost	duration
Gone with the wind	390000	220
Frankenstein	3000000	50
Creature from the black lagoon	500000	79
NULL	100	10

1. COUNT()

Count all rows

SELECT COUNT(*) FROM Movies;

Output:

4

• Counts all rows, including rows where columns are NULL.

Count non-NULL values in a column

SELECT COUNT(title) FROM Movies;

Output:

3

• NULL title is not counted.

2. MIN()

SELECT MIN(cost) FROM Movies;

Output:

100

• Returns the smallest value in the column.

3. MAX()

SELECT MAX(cost) FROM Movies;

Output:

3000000

• Returns the largest value.

4. SUM()

SELECT SUM(cost) FROM Movies;

Output:

3890100

Calculation:

390000 + 3000000 + 500000 + 100 = 3890100

• Adds all values in the column.
• Ignores NULL values (none in cost here).

5. AVG()

SELECT AVG(cost) FROM Movies;

Output:

972525

Calculation:

3890100 / 4 = 972525

• Computes the average of all values.

---

Using Multiple Aggregates

SELECT

    COUNT(*) AS total_movies,

    MIN(cost) AS cheapest,

    MAX(cost) AS most_expensive,

    SUM(cost) AS total_cost,

    AVG(cost) AS average_cost

FROM Movies;

Output:

total_movies | cheapest | most_expensive | total_cost | average_cost
4            | 100      | 3000000        | 3890100    | 972525

Filtering Aggregates

title	cost	duration	genre
Gone with the wind	390000	220	Drama
Frankenstein	3000000	50	Horror
Creature from the black lagoon	500000	79	Horror
Casablanca	1000000	102	Drama
Toy Story	2000000	81	Family
NULL	100	10	Horror

GROUP BY

GROUP BY groups rows so aggregates are calculated per group rather than across the whole table.

SELECT genre, SUM(cost) AS total_cost
FROM Movies
GROUP BY genre;

Output:

genre   | total_cost
Drama   | 1390000
Horror  | 3500100
Family  | 2000000

SELECT genre, AVG(duration) AS avg_duration
FROM Movies
GROUP BY genre;

Output:

genre   | avg_duration
Drama   | 161
Horror  | 46.33
Family  | 81

---

WHERE (filtering rows before aggregation)

WHERE filters rows before grouping and aggregation.

SELECT genre, COUNT(*) AS num_movies
FROM Movies
WHERE duration > 60
GROUP BY genre;

Output:

genre   | num_movies
Drama   | 2
Horror  | 1
Family  | 1

Explanation:

• The row with duration = 10 is excluded before grouping.

HAVING (filtering groups after aggregation)

HAVING filters results after aggregation.

SELECT genre, SUM(cost) AS total_cost
FROM Movies
GROUP BY genre
HAVING SUM(cost) >= 2000000;

Output:

genre   | total_cost
Horror  | 3500100
Family  | 2000000   -- note: included only if >= was used

If strictly > 2000000, only:

Horror  | 3500100

SELECT genre, COUNT(*) AS num_movies
FROM Movies
GROUP BY genre
HAVING COUNT(*) > 2;

Output:

genre   | num_movies
Horror  | 3

Combined Example

SELECT genre, AVG(cost) AS avg_cost
FROM Movies
WHERE duration > 60
GROUP BY genre
HAVING AVG(cost) > 1000000;

Steps:

1. WHERE filters rows (duration > 60)
2. GROUP BY groups remaining data
3. HAVING filters grouped results

Output:

genre   | avg_cost
Family  | 2000000