# SQL COUNT(), AVG(), and SUM() Functions

SQL provides several aggregate functions that allow you to perform calculations on a set of records. The COUNT(), AVG(), and SUM() functions are some of the most commonly used aggregate functions in SQL.

## SQL COUNT() Function

The COUNT() function is used to count the number of rows in a table. The syntax of the COUNT() function is as follows:

```SELECT COUNT(column_name)
FROM table_name;
```

Where “column_name” is the name of the column that you want to count the number of rows for, and “table_name” is the name of the table that contains the column.

For example, consider the following “employees” table:

```+----+----------+--------+
| ID | Name     | Salary |
+----+----------+--------+
|  1 | John Doe |  50000 |
|  2 | Jane Doe |  55000 |
|  3 | Bob Smith|  45000 |
+----+----------+--------+
```

To count the number of employees in the “employees” table, you would run the following query:

```SELECT COUNT(ID)
FROM employees;
```

This would return the following result:

```+----------+
| COUNT(ID)|
+----------+
|         3|
+----------+
```

## SQL AVG() Function

The AVG() function is used to find the average of a set of values in a column. The syntax of the AVG() function is as follows:

```SELECT AVG(column_name)
FROM table_name;
```

Where “column_name” is the name of the column for which you want to find the average value, and “table_name” is the name of the table that contains the column.

For example, consider the following “employees” table:

```+----+----------+--------+
| ID | Name     | Salary |
+----+----------+--------+
|  1 | John Doe |  50000 |
|  2 | Jane Doe |  55000 |
|  3 | Bob Smith|  45000 |
+----+----------+--------+
```

To find the average salary of employees in the “employees” table, you would run the following query:

```SELECT AVG(Salary)
FROM employees;
```

This would return the following result:

```+-----------+
| AVG(Salary)|
+-----------+
|     50000.0|
+-----------+
```

## SQL SUM() Function

The SUM() function is used to find the sum of a set of values in a column. The syntax of the SUM() function is as follows:

```SELECT SUM(column_name)
FROM table_name;
```

Where “column_name” is the name of the column for which you want to find the sum, and “table_name” is the name of the table that contains the column.

For example, consider the following “employees” table:

```+----+----------+--------+
| ID | Name     | Salary |
+----+----------+--------+
|  1 | John Doe |  50000 |
|  2 | Jane Doe |  55000 |
|  3 | Bob Smith|  45000 |
+----+----------+--------+
```

To find the sum of salaries of employees in the “employees” table, you would run the following query:

```SELECT SUM(Salary)
FROM employees;
```

This would return the following result:

```+-----------+
| SUM(Salary)|
+-----------+
|     150000 |
+-----------+
```

In conclusion, the COUNT(), AVG(), and SUM() functions in SQL provide an easy and efficient way to perform aggregate calculations on data stored in a database. These functions are commonly used in data analysis and reporting, and are a fundamental part of SQL.