postgis-workshop/simple_sql — Ukrainian @ OSGeo Weblate: Function List

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	A select query is generally of the form::
	For a synopsis of all ``SELECT`` parameters, see the `PostgresSQL documentation <http://www.postgresql.org/docs/current/interactive/sql-select.html>`_.
	The ``some_columns`` are either column names or functions of column values. The ``some_data_source`` is either a single table, or a composite table created by joining two tables on a key or condition. The ``some_condition`` is a filter that restricts the number of rows to be returned.
	"What are the names of all the neighborhoods in Brooklyn?"
	We return to our ``nyc_neighborhoods`` table with a filter in hand. The table contains all the neighborhoods in New York, but we only want the ones in Brooklyn.
	The query will run for even fewer (milli)seconds and return the 23 results.
	Sometimes we will need to apply a function to the results of our query. For example,
	"What is the number of letters in the names of all the neighborhoods in Brooklyn?"
	Fortunately, PostgreSQL has a string length function, :command:`char_length(string)`.
	Often, we are less interested in the individual rows than in a statistic that applies to all of them. So knowing the lengths of the neighborhood names might be less interesting than knowing the average length of the names. Functions that take in multiple rows and return a single result are called "aggregate" functions.
	PostgreSQL has a series of built-in aggregate functions, including the general purpose :command:`avg()` for average values and :command:`stddev()` for standard deviations.
	"What is the average number of letters and standard deviation of number of letters in the names of all the neighborhoods in Brooklyn?"
	The aggregate functions in our last example were applied to every row in the result set. What if we want the summaries to be carried out over smaller groups within the overall result set? For that we add a ``GROUP BY`` clause. Aggregate functions often need an added ``GROUP BY`` statement to group the result-set by one or more columns.
	"What is the average number of letters in the names of all the neighborhoods in New York City, reported by borough?"
	We include the ``boroname`` column in the output result so we can determine which statistic applies to which borough. In an aggregate query, you can only output columns that are either (a) members of the grouping clause or (b) aggregate functions.
	Function List
	`avg(expression) <http://www.postgresql.org/docs/current/static/functions-aggregate.html#FUNCTIONS-AGGREGATE-TABLE>`_: PostgreSQL aggregate function that returns the average value of a numeric column.
	`char_length(string) <http://www.postgresql.org/docs/current/static/functions-string.html>`_: PostgreSQL string function that returns the number of character in a string.
	`stddev(expression) <http://www.postgresql.org/docs/current/static/functions-aggregate.html#FUNCTIONS-AGGREGATE-STATISTICS-TABLE>`_: PostgreSQL aggregate function that returns the standard deviation of input values.