## Monday, April 21, 2014

### TSQL Tutorial : SUM Function

The TSQL SUM function is used to return the sum of an expression in a SELECT statement. The syntax for the TSQL SUM function is:

`SELECT SUM(expression)`
`FROM tables`
`WHERE conditions;`

## With Single Expression

For example, you might wish to know how the combined total salary of all employees whose salary is above \$25,000 / year.

`SELECT SUM(salary) AS "Total Salary"`
`FROM employees`
`WHERE salary > 25000;`

In this TSQL SUM Function example, we've aliased the SUM(salary) expression as "Total Salary". As a result, "Total Salary" will display as the field name when the result set is returned.

## Using TSQL DISTINCT with SUM

You can use the TSQL DISTINCT clause within the TSQL SUM function. For example, the TSQL SELECT statement below returns the combined total salary of unique salary values where the salary is above \$25,000 / year.

`SELECT SUM(DISTINCT salary) AS "Total Salary"`
`FROM employees`
`WHERE salary > 25000;`

If there were two salaries of \$30,000/year, only one of these values would be used in the TSQL SUM function.

## SUM in Formula

The expression contained within the TSQL SUM function does not need to be a single field. You could also use a formula. For example, you might want the net income for a business. Net Income is calculated as total income less total expenses.

`SELECT SUM(income - expenses) AS "Net Income"`
`FROM gl_transactions;`

You might also want to perform a mathematical operation within the TSQL SUM function. For example, you might determine total commission as 10% of total sales.

`SELECT SUM(sales * 0.10) AS "Commission"`
`FROM order_details;`

## Using TSQL GROUP BY with SUM

In some cases, you will be required to use the TSQL GROUP BY clause with the TSQL SUM function.

For example, you could also use the TSQL SUM function to return the name of the department and the total sales (in the associated department).

`SELECT department, SUM(sales) AS "Total sales"`
`FROM order_details`
`GROUP BY department;`

Because you have listed one column in your TSQL SELECT statement that is not encapsulated in the TSQL SUM function, you must use the TSQL GROUP BY clause. The department field must, therefore, be listed in the TSQL GROUP BY section.