SQL sublanguage: DQL $($Data Query Language$)$

$*$

$*$ Aggregate functions in SQL are functions which preform operations on multiple rows to produce a single output.

$*$ For instance, COUNT$()$ is an aggregate function. Count will return the number of rows of data in the result set.

$*$ So if we query a table with $12$ rows using a statement like this:

$*$ SELECT COUNT$(*)$ FROM table$\_$name;

$*$ We should get the value $12$

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$*$ Aggregate functions "aggregate" data, combining and changing it into some output based on the input. Regardless of

$*$ how large that input is$,$ the output will always be a single value. If we repeated the above query but the table had

$*10,000$ rows, the result would still be a single value: $10,000.$

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$*$ There are many aggregate functions built into SQL$,$ some commonly used ones include:

$*-$ SUM$()-$ outputs the sum of the values in a single column from the result set

$*-$ AVG$()-$ outputs the average $($mean$)$ value of the values in a single column from the result set

$*-$ MIN$()-$ outputs the least value among the values in a single column from the result set

$*-$ MAX$()-$ similar to MIN but outputs the greatest value

$*-$ FIRST$()-$ outputs the first value found in a column of the result set

$*-$ LAST$()-$ outputs the last value found in a column of the result set

$*$

$*$ Example: You might use the SUM$()$ function to find the total of all employee salaries:

$*$ SELECT SUM$($salary$)$ FROM employee;

$*/$