SQL: SUM Function
The SQL SUM function returns the summed value of an expression.
The syntax for the SQL SUM function is:
SELECT SUM(expression ) FROM tables WHERE predicates;
expression can be a numeric field or formula.
SQL SUM Function - Single Field example
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 SQL SUM Function example, we've aliased the sum(salary) field as "Total Salary". As a result, "Total Salary" will display as the field name when the result set is returned.
SQL SUM Function - Using SQL DISTINCT Clause example
You can use the SQL DISTINCT clause within the SQL SUM function. For example, the SQL 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 SQL SUM function.
SQL SUM Function - Using a Formula example
The expression contained within the SQL 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 SQL SUM function. For example, you might determine total commission as 10% of total sales.
SELECT SUM(sales * 0.10) as "Commission" FROM order_details;
SQL SUM Function - Using SQL GROUP BY Clause example
In some cases, you will be required to use the SQL GROUP BY clause with the SQL SUM function.
For example, you could also use the QSL 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 SQL SELECT statement that is not encapsulated in the SQL SUM function, you must use the SQL GROUP BY clause. The department field must, therefore, be listed in the SQL GROUP BY section.