# SQL: SUM() reference

19 March 2023 (Updated 19 March 2023)

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`SUM()`

is an aggregate function that lets you calculate the sum of values in a set.

### Syntax

`SUM(DISTINCT expression)`

### Example & execution plan

`select sum(salary) from salaries;`

The above query will be handled by MySQL as follows:

- Read through the
`salaries`

table and extract the value from the`salary`

column for each row. - For each value in the
`salary`

column, add it to a running total. - Return the running total as the value of the
`SUM()`

function.

By default `SUM()`

will calculate the sum of all values in the set, including duplicate values. You can prefix your expression with `DISTINCT`

to tell MySQL only to sum the values that are distinct.

Tagged:
MySQL