Documentation Source Text

<tcl>hd_keywords {window functions}</tcl>

<title>SQLite SQL Window Function Support</title>
<table_of_contents>

<h2 style="margin-left:1.0em" notoc id=overview> Overview</h2>

<p>This page describes the support for SQL window functions added to SQLite
[version 3.25.0] ([dateof:3.25.0]).

<h1>Introduction to Window Functions</h1>

<p>A window function is a special type of SQL function for which the results
depend on the contents of a "window" of one or more contiguous rows returned
by the SELECT statement containing the window function. A window function may
only be used in the select-list or ORDER BY clause of a SELECT or sub-select

<p>The example above uses the special built-in window function row_number().
This function returns a monotonically increasing integer assigned to each
row in order of the the "ORDER BY" clause within the
<i>&lt;window-definition&gt;</i> (in this case "ORDER BY y"). Note that 
this does not affect the order in which results are returned to the user -
that is still governed by the ORDER BY clause attached to the SELECT
statement (in this case "ORDER BY x").

<p>Named <i>&lt;window-definitions&gt;</i> may also be added to a SELECT 
statement using a WINDOW clause and then refered to by name within window
function invocations. For example, the following SELECT statement contains
two named <i>&lt;window-definitions&gt;</i>, "win1" and "win2":

<codeblock>
  SELECT x, y, row_number() OVER <b>win1</b>, rank OVER <b>win2</b> FROM t0 
  WINDOW <b>win1</b> AS (ORDER BY y RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW),
         <b>win2</b> AS (PARTITION BY y ORDER BY x)
  ORDER BY x;
</codeblock>

<p>The WINDOW clause, when one is present, comes after any HAVING clause and
before any ORDER BY.

<h1>Aggregate Window Functions</h1>

<p> The examples in this section all assume that the database is populated as
follows:

<codeblock>

  <i>--     5|E|two|A.D.G.C.F.B.E</i>
  <i>--     6|F|three|A.D.G.C.F</i>
  <i>--     7|G|one|A.D.G</i>
  <i>-- </i>
  SELECT a, b, c, group_concat(b, '.') OVER (ORDER BY c) FROM t1 ORDER BY a;
</codeblock>

<p> All of SQLite's [Aggregate Functions|aggregate functions] may be used as aggregate window
functions. It is also possible to 
[user-defined window functions|create user-defined aggregate window functions].

<h2>Frame Specifications</h2>

<p> A window frame specification consists of three things:

<ul>
  <li> A frame type - either RANGE or ROWS.
  <li> A starting frame boundary, and

        FOLLOWING" is the same as "CURRENT ROW". This frame boundary type may
        only be used with ROWS frames.
<tr><td>UNBOUNDED&nbsp;FOLLOWING <td> This may only be used as an ending
        frame boundary. In which case the end of the frame is always the last
        row in the set.
</table>

<p> The ending frame boundary must not take a form that appears higher in
the above list than the starting frame boundary.

<p> In the following example, the window frame for each row consists of all
rows from the current row to the end of the set, where rows are sorted
according to "ORDER BY a".

<codeblock>
  <i>-- The following SELECT statement returns:</i>
  <i>-- </i>
  <i>-- one  |1|A|A.D.G.C.F.B.E</i>
  <i>-- one  |4|D|D.G.C.F.B.E</i>
  <i>-- one  |7|G|G.C.F.B.E</i>
  <i>-- three|3|C|C.F.B.E</i>
  <i>-- three|6|F|F.B.E</i>
  <i>-- two  |2|B|B.E</i>
  <i>-- two  |5|E|E</i>
  <i>-- </i>
  SELECT c, a, b, group_concat(b, '.') OVER (
    ORDER BY c, a ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
  ) 
  FROM t1 ORDER BY c, a;
</codeblock>


<h2>The PARTITION BY Clause</h2>

<p> A <i>&lt;window-definition&gt;</i> may also include a PARTITION BY clause.
If so, the rows returned by the SELECT statement are divided into groups -
partitions - with the same values for each PARTITION BY expression, and then
window-function processing performed as described above separately for each
partition. This is similar to the way the rows traversed by an aggregate query
are divided into groups before any aggregate processing is performed.

<p> For example:

<codeblock>
  <i>-- The following SELECT statement returns:</i>
  <i>-- </i>
  <i>-- one  |1|A|A</i>
  <i>-- one  |4|D|A.D</i>
  <i>-- one  |7|G|A.D.G</i>
  <i>-- three|3|C|C</i>
  <i>-- three|6|F|C.F</i>
  <i>-- two  |2|B|B</i>
  <i>-- two  |5|E|B.E</i>
  <i>-- </i>
  SELECT c, a, b, group_concat(b, '.') OVER (
    PARTITION BY c ORDER BY a RANGE BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
  ) 
  FROM t1 ORDER BY c, a;
</codeblock>

<h2>The FILTER Clause</h2>

<p>If a FILTER clause is provided, then only rows for which the <i>expr</i> is
true are included in the window frame. The aggregate window still returns a
value for every row, but those for which the FILTER expression evaluates to
other than true are not included in the window frame for any row. For example:

<codeblock>
  <i>-- The following SELECT statement returns:</i>
  <i>--</i>
  <i>-- one  |1|A|A.C.D.F.G</i>
  <i>-- two  |2|B|C.D.F.G</i>
  <i>-- three|3|C|C.D.F.G</i>
  <i>-- one  |4|D|D.F.G</i>
  <i>-- two  |5|E|F.G</i>
  <i>-- three|6|F|F.G</i>
  <i>-- one  |7|G|G</i>
  <i>-- </i>
  SELECT c, a, b, group_concat(b, '.') FILTER (WHERE c!='two') OVER (ORDER BY a)
  FROM t1 ORDER BY a;
</codeblock>

<h2 tags="user-defined window functions">User-Defined Aggregate Window Functions</h2>

<p>TODO: Link to C API docs (sqlite3_create_window_function()).

<h1>Built-in Window Functions</h1>

<p> As well as aggregate window functions, SQLite features a set of built-in
window functions based on 
<a href=https://www.postgresql.org/docs/10/static/functions-window.html>
those supported by PostgreSQL</a>.

<p> Built-in window functions honor any PARTITION BY clause in the same way
as aggregate window functions - each selected row is assigned to a partition 
and each partition is processed separately. The ways in which any ORDER BY
clause affects each built-in window function is described below. Some of
the window functions (rank(), dense_rank(), percent_rank() and ntile()) use
the concept of "peer groups" (rows within the same partition that have the
same values for all ORDER BY expressions). In these cases, it does not matter
whether the <i>&lt;frame-specification&gt;</i> specifies ROWS or RANGE - for
the purposes of built-in window function processing, rows with the same values
for all ORDER BY expressions are considered peers regardless of the frame type.

<p> Most built-in window functions ignore the
<i>&lt;frame-specification&gt;</i>, the exceptions being first_value(),
last_value() and nth_value(). It is a syntax error to specify a FILTER 
clause as part of a built-in window function invocation.

<p> SQLite supports the following 11 built-in window functions:

<dl>
  <dt><p><b>row_number()</b>
  <dd><p> The number of the row within the current partition. Rows are
       numbered starting from 1 in the order defined by the ORDER BY clause in
       the window definition, or in arbitrary order otherwise.
  <dt><p><b>rank()</b>
  <dd><p> The row_number() of the first peer in each group - the rank of the
       current row with gaps. If there is no ORDER BY clause, then all rows
       are considered peers and this function always returns 1.
  <dt><p><b>dense_rank()</b>
  <dd><p> The number of the current row's peer group within its partition - the
       rank of the current row without gaps. Partitions are numbered starting
       from 1 in the order defined by the ORDER BY clause in the window
       definition. If there is no ORDER BY clause, then all rows are
       considered peers and this function always returns 1.
  <dt><p><b>percent_rank()</b>
  <dd><p> Despite the name, this function always returns a value between 0.0
       and 1.0 equal to (<i>rank</i> - 1)/(<i>partition-rows</i> - 1), where
       <i>rank</i> is the value returned by built-in window function rank()
       and <i>partition-rows</i> is the total number of rows in the
       partition. If the partition contains only one row, this function
       returns 0.0.
  <dt><p><b>cume_dist()</b>
  <dd><p> The cumulative distribution. Calculated as
       <i>row-number</i>/<i>partition-rows</i>, where <i>row-number</i> is
       the value returned by row_number() for the last peer in the group
       and <i>partition-rows</i> the number of rows in the partition.
  <dt><p><b>ntile(N)</b>
  <dd><p> Argument <i>N</i> is handled as an integer. This function divides the
       partition into N groups as evenly as possible and assigns an integer
       between 1 and <i>N</i> to each group, in the order defined by the ORDER
       BY clause, or in arbitrary order otherwise. If necessary, larger groups
       occur first. This function returns the integer value assigned to the
       group that the current row is a part of.

  <dt><p><b>lag(expr)<br>lag(expr, offset)<br>lag(expr, offset, default)</b>
  <dd><p> The first form of the lag() function returns the result of evaluating
       expression <i>expr</i> against the previous row in the partition. Or, if
       there is no previous row (because the current row is the first), NULL.

      <p> If the <i>offset</i> argument is provided, then it must be a
       non-negative integer. In this case the value returned is the result
       of evaluating <i>expr</i> against the row <i>offset</i> rows before the
       current row within the partition. If <i>offset</i> is 0, then
       <i>expr</i> is evaluated against the current row. If there is no row
       <i>offset</i> rows before the current row, NULL is returned.

      <p> If <i>default</i> is also provided, then it is returned instead of
      NULL if row identified by <i>offset</i> does not exist.
       
  <dt><p><b>lead(expr)<br>lead(expr, offset)<br>lead(expr, offset, default)</b>
  <dd><p> The first form of the lead() function returns the result of evaluating
       expression <i>expr</i> against the next row in the partition. Or, if
       there is no next row (because the current row is the last), NULL.

      <p> If the <i>offset</i> argument is provided, then it must be a
       non-negative integer. In this case the value returned is the result
       of evaluating <i>expr</i> against the row <i>offset</i> rows after the
       current row within the partition. If <i>offset</i> is 0, then
       <i>expr</i> is evaluated against the current row. If there is no row
       <i>offset</i> rows after the current row, NULL is returned.

      <p> If <i>default</i> is also provided, then it is returned instead of
      NULL if row identified by <i>offset</i> does not exist.
  <dt><p><b>first_value(expr)</b>
  <dd><p> This built-in window function calculates the window frame for each
       row in the same way as an aggregate window function. It returns the
       value of <i>expr</i> evaluated against the first row in the window frame
       for each row.
  <dt><p><b>last_value(expr)</b>
  <dd><p> This built-in window function calculates the window frame for each
       row in the same way as an aggregate window function. It returns the
       value of <i>expr</i> evaluated against the last row in the window frame
       for each row.
  <dt><p><b>nth_value(expr, N)</b>
  <dd><p> This built-in window function calculates the window frame for each
       row in the same way as an aggregate window function. It returns the
       value of <i>expr</i> evaluated against the row <i>N</i> of the window
       frame. Rows are numbered within the window frame starting from 1 in
       the order defined by the ORDER BY clause if one is present, or in
       arbitrary order otherwise.
  <dd>
</dl>