## Key Concepts

Review core concepts you need to learn to master this subject

### Aggregate Functions in SQL

Aggregate functions perform a calculation on a set of values and return a single value:

• `COUNT()`
• `SUM()`
• `MAX()`
• `MIN()`
• `AVG()`

### `COUNT()` Aggregate Function

The `COUNT()` aggregate function in SQL returns the total number of rows that match the specified criteria. For instance, to find the total number of employees who have more than 5 years of experience, the given query can be used.

Note: A column name of the table can also be used instead of `*`. Unlike `COUNT(*)`, this variation `COUNT(column)` will not count `NULL` values in that column.

### `SUM()` Aggregate Function

The `SUM()` aggregate function takes the name of a column as an argument and returns the sum of all the value in that column.

### `AVG()` Aggregate Function

The `AVG()` aggregate function returns the average value in a column. For instance, to find the average `salary` for the employees who have less than 5 years of experience, the given query can be used.

### `ROUND()` Function

The `ROUND()` function will round a number value to a specified number of places. It takes two arguments: a number, and a number of decimal places. It can be combined with other aggregate functions, as shown in the given query. This query will calculate the average rating of movies from 2015, rounding to 2 decimal places.

### `GROUP BY` Clause

The `GROUP BY` clause will group records in a result set by identical values in one or more columns. It is often used in combination with aggregate functions to query information of similar records. The `GROUP BY` clause can come after `FROM` or `WHERE` but must come before any `ORDER BY` or `LIMIT` clause.

The given query will count the number of movies per rating.

### Column References

The `GROUP BY` and `ORDER BY` clauses can reference the selected columns by number in which they appear in the `SELECT` statement. The example query will count the number of movies per rating, and will:

• `GROUP BY` column `2` (`rating`)
• `ORDER BY` column `1` (`total_movies`)

### `HAVING` Clause

The `HAVING` clause is used to further filter the result set groups provided by the `GROUP BY` clause. `HAVING` is often used with aggregate functions to filter the result set groups based on an aggregate property. The given query will select only the records (rows) from only years where more than 5 movies were released per year.

### `MAX()` Aggregate Function

The `MAX()` aggregate function in SQL takes the name of a column as an argument and returns the largest value in a column. The given query will return the largest value from the `amount` column.

### `MIN()` Aggregate Function

The `MIN()` aggregate function in SQL returns the smallest value in a column. For instance, to find the smallest value of the `amount` column from the table named `transactions`, the given query can be used.

Aggregate Functions
Lesson 1 of 1
1. 1
We’ve learned how to write queries to retrieve information from the database. Now, we are going to learn how to perform calculations using SQL. Calculations performed on multiple rows of a table a…
2. 2
The fastest way to calculate how many rows are in a table is to use the COUNT() function. COUNT() is a function that takes the name of a column as an argument and counts the number of non-empty va…
3. 3
SQL makes it easy to add all values in a particular column using SUM(). SUM() is a function that takes the name of a column as an argument and returns the sum of all the values in that column. Wh…
4. 4
The MAX() and MIN() functions return the highest and lowest values in a column, respectively. How many downloads does the most popular app have? SELECT MAX(downloads) FROM fake_apps; The most p…
5. 5
SQL uses the AVG() function to quickly calculate the average value of a particular column. The statement below returns the average number of downloads for an app in our database: SELECT AVG(downl…
6. 6
By default, SQL tries to be as precise as possible without rounding. We can make the result table easier to read using the ROUND() function. ROUND() function takes two arguments inside the parenth…
7. 7
Oftentimes, we will want to calculate an aggregate for data with certain characteristics. For instance, we might want to know the mean IMDb ratings for all movies each year. We could calculate ea…
8. 8
Sometimes, we want to GROUP BY a calculation done on a column. For instance, we might want to know how many movies have IMDb ratings that round to 1, 2, 3, 4, 5. We could do this using the follow…
9. 9
In addition to being able to group data using GROUP BY, SQL also allows you to filter which groups to include and which to exclude. For instance, imagine that we want to see how many movies of di…
10. 10
Congratulations! You just learned how to use aggregate functions to perform calculations on your data. What can we generalize so far? - COUNT(): count the number of rows - SUM(): the sum of the v…

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