[<< wikibooks] Structured Query Language/SELECT: Predefined Functions
There are two groups of predefined functions:

aggregate functions. They work on a set of rows, which means they receive one value for each row of a set of rows and returns one value for the whole set. If they are called in the context of a GROUP BY clause, they are called once per group, else once for all rows.
scalar functions. They work on single rows, which means they receive one value of a single row and returns one value for each of them.

== Aggregate functions ==
They work on a set of rows and return one single value like the number of rows, the highest or lowest value, the standard deviation, etc. The most important aggregate functions are:

As an example we retrieve the maximum weight of all persons:

A Word of Caution
Aggregate functions result in one value for a set of rows. Therefore it is not possible to use them together with 'normal' columns in the projection (the part behind SELECT keyword). If we specify, for example,

we try to instruct the DBMS to show a lot of rows containing the lastname simultaneously with one value. This is a contradiction and the system will throw an exception. We can use a lot of aggregate functions within one projection but we are not allowed to use them together with 'normal' columns.

If we use aggregate functions in the context of commands containing a GROUP BY, the aggregate functions are called once per group.

In such cases the GROUP BY column(s) may be displayed as it is impossible that they change within the group.

=== The NULL special marker ===
If a row contains no value (it holds the NULL special marker) in the named column, the row is not part of the computation.

=== ALL vs. DISTINCT ===
The complete signatures of the functions are a little more detailed. We can prepend the column name with one of the two key words ALL or DISTINCT. If we specify ALL, which is the default, every value is part of the computation, else only those, which are distinct from each other.

function_name ([ALL|DISTINCT])
COUNT (DISTINCT weight) -- as an example

=== Hint ===
The standard defines some more aggregate functions to compute statistical measures. Also the keywords ANY, EVERY and SOME formally are defined as aggregate functions. We will discuss them on a separate page.

== Scalar functions ==
Scalar functions act on a 'per row basis'. They are called once per row and they return one value per call. Often they are grouped according to the data types they act on:

String functionsSUBSTRING( FROM  FOR ) returns a string starting at position  (first character counts '1') in the length of .
UPPER() returns the uppercase equivalent of the column value.
LOWER() returns the lowercase equivalent of the column value.
CHARACTER_LENGTH() returns the length of the column value.
TRIM() returns the column value without leading and trailing spaces.
TRIM(LEADING  FROM ) returns the column value without leading spaces.
TRIM(TRAILING FROM ) returns the column value without trailing spaces.Numeric functionsSQRT() returns the square root of the column value.
ABS() returns the absolute value of the column value.
MOD(, ) returns the remaining of column value divided by divisor.
others: FLOOR, CEIL, POWER, EXP, LN.Date, Time & Interval functionsEXTRACT(month FROM date_of_birth) returns the month of column date_of_birth.build-in functions. They do not have any input parameter.CURRENT_DATE() returns the current date.
CURRENT_TIME() returns the current time.There is another wikibook where those functions are shown in detail. The data type of the return value is not always identical to the type of the input, e.g. 'character_length()' receives a string and returns a number.
Here is an example with some scalar functions: 

== Exercises ==