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

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+---
+note type:
+  - sql
+  - database
+  - theory
+date: 2026-06-03
+done:
+link: 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
+```
+
+
+