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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="FUNCTIONS-WINDOW"
>9.21. Window Functions</A
></H1
><P
>   <I
CLASS="FIRSTTERM"
>Window functions</I
> provide the ability to perform
   calculations across sets of rows that are related to the current query
   row.  See <A
HREF="tutorial-window.html"
>Section 3.5</A
> for an introduction to this
   feature.
  </P
><P
>   The built-in window functions are listed in
   <A
HREF="functions-window.html#FUNCTIONS-WINDOW-TABLE"
>Table 9-49</A
>.  Note that these functions
   <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>must</I
></SPAN
> be invoked using window function syntax; that is an
   <TT
CLASS="LITERAL"
>OVER</TT
> clause is required.
  </P
><P
>   In addition to these functions, any built-in or user-defined aggregate
   function can be used as a window function (see
   <A
HREF="functions-aggregate.html"
>Section 9.20</A
> for a list of the built-in aggregates).
   Aggregate functions act as window functions only when an <TT
CLASS="LITERAL"
>OVER</TT
>
   clause follows the call; otherwise they act as regular aggregates.
  </P
><DIV
CLASS="TABLE"
><A
NAME="FUNCTIONS-WINDOW-TABLE"
></A
><P
><B
>Table 9-49. General-Purpose Window Functions</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><THEAD
><TR
><TH
>Function</TH
><TH
>Return Type</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
>       
       <CODE
CLASS="FUNCTION"
>row_number()</CODE
>
      </TD
><TD
>       <TT
CLASS="TYPE"
>bigint</TT
>
      </TD
><TD
>number of the current row within its partition, counting from 1</TD
></TR
><TR
><TD
>       
       <CODE
CLASS="FUNCTION"
>rank()</CODE
>
      </TD
><TD
>       <TT
CLASS="TYPE"
>bigint</TT
>
      </TD
><TD
>rank of the current row with gaps; same as <CODE
CLASS="FUNCTION"
>row_number</CODE
> of its first peer</TD
></TR
><TR
><TD
>       
       <CODE
CLASS="FUNCTION"
>dense_rank()</CODE
>
      </TD
><TD
>       <TT
CLASS="TYPE"
>bigint</TT
>
      </TD
><TD
>rank of the current row without gaps; this function counts peer groups</TD
></TR
><TR
><TD
>       
       <CODE
CLASS="FUNCTION"
>percent_rank()</CODE
>
      </TD
><TD
>       <TT
CLASS="TYPE"
>double precision</TT
>
      </TD
><TD
>relative rank of the current row: (<CODE
CLASS="FUNCTION"
>rank</CODE
> - 1) / (total rows - 1)</TD
></TR
><TR
><TD
>       
       <CODE
CLASS="FUNCTION"
>cume_dist()</CODE
>
      </TD
><TD
>       <TT
CLASS="TYPE"
>double precision</TT
>
      </TD
><TD
>relative rank of the current row: (number of rows preceding or peer with current row) / (total rows)</TD
></TR
><TR
><TD
>       
       <CODE
CLASS="FUNCTION"
>ntile(<TT
CLASS="REPLACEABLE"
><I
>num_buckets</I
></TT
> <TT
CLASS="TYPE"
>integer</TT
>)</CODE
>
      </TD
><TD
>       <TT
CLASS="TYPE"
>integer</TT
>
      </TD
><TD
>integer ranging from 1 to the argument value, dividing the
       partition as equally as possible</TD
></TR
><TR
><TD
>       
       <CODE
CLASS="FUNCTION"
>         lag(<TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
> <TT
CLASS="TYPE"
>anyelement</TT
>
             [, <TT
CLASS="REPLACEABLE"
><I
>offset</I
></TT
> <TT
CLASS="TYPE"
>integer</TT
>
             [, <TT
CLASS="REPLACEABLE"
><I
>default</I
></TT
> <TT
CLASS="TYPE"
>anyelement</TT
> ]])
       </CODE
>
      </TD
><TD
>       <TT
CLASS="TYPE"
>same type as <TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
></TT
>
      </TD
><TD
>       returns <TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
> evaluated at
       the row that is <TT
CLASS="REPLACEABLE"
><I
>offset</I
></TT
>
       rows before the current row within the partition; if there is no such
       row, instead return <TT
CLASS="REPLACEABLE"
><I
>default</I
></TT
>
       (which must be of the same type as
       <TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
>).
       Both <TT
CLASS="REPLACEABLE"
><I
>offset</I
></TT
> and
       <TT
CLASS="REPLACEABLE"
><I
>default</I
></TT
> are evaluated
       with respect to the current row.  If omitted,
       <TT
CLASS="REPLACEABLE"
><I
>offset</I
></TT
> defaults to 1 and
       <TT
CLASS="REPLACEABLE"
><I
>default</I
></TT
> to null
      </TD
></TR
><TR
><TD
>       
       <CODE
CLASS="FUNCTION"
>         lead(<TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
> <TT
CLASS="TYPE"
>anyelement</TT
>
              [, <TT
CLASS="REPLACEABLE"
><I
>offset</I
></TT
> <TT
CLASS="TYPE"
>integer</TT
>
              [, <TT
CLASS="REPLACEABLE"
><I
>default</I
></TT
> <TT
CLASS="TYPE"
>anyelement</TT
> ]])
       </CODE
>
      </TD
><TD
>       <TT
CLASS="TYPE"
>same type as <TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
></TT
>
      </TD
><TD
>       returns <TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
> evaluated at
       the row that is <TT
CLASS="REPLACEABLE"
><I
>offset</I
></TT
>
       rows after the current row within the partition; if there is no such
       row, instead return <TT
CLASS="REPLACEABLE"
><I
>default</I
></TT
>
       (which must be of the same type as
       <TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
>).
       Both <TT
CLASS="REPLACEABLE"
><I
>offset</I
></TT
> and
       <TT
CLASS="REPLACEABLE"
><I
>default</I
></TT
> are evaluated
       with respect to the current row.  If omitted,
       <TT
CLASS="REPLACEABLE"
><I
>offset</I
></TT
> defaults to 1 and
       <TT
CLASS="REPLACEABLE"
><I
>default</I
></TT
> to null
      </TD
></TR
><TR
><TD
>       
       <CODE
CLASS="FUNCTION"
>first_value(<TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
> <TT
CLASS="TYPE"
>any</TT
>)</CODE
>
      </TD
><TD
>       <TT
CLASS="TYPE"
>same type as <TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
></TT
>
      </TD
><TD
>       returns <TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
> evaluated
       at the row that is the first row of the window frame
      </TD
></TR
><TR
><TD
>       
       <CODE
CLASS="FUNCTION"
>last_value(<TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
> <TT
CLASS="TYPE"
>any</TT
>)</CODE
>
      </TD
><TD
>       <TT
CLASS="TYPE"
>same type as <TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
></TT
>
      </TD
><TD
>       returns <TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
> evaluated
       at the row that is the last row of the window frame
      </TD
></TR
><TR
><TD
>       
       <CODE
CLASS="FUNCTION"
>         nth_value(<TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
> <TT
CLASS="TYPE"
>any</TT
>, <TT
CLASS="REPLACEABLE"
><I
>nth</I
></TT
> <TT
CLASS="TYPE"
>integer</TT
>)
       </CODE
>
      </TD
><TD
>       <TT
CLASS="TYPE"
>same type as <TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
></TT
>
      </TD
><TD
>       returns <TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
> evaluated
       at the row that is the <TT
CLASS="REPLACEABLE"
><I
>nth</I
></TT
>
       row of the window frame (counting from 1); null if no such row
      </TD
></TR
></TBODY
></TABLE
></DIV
><P
>   All of the functions listed in
   <A
HREF="functions-window.html#FUNCTIONS-WINDOW-TABLE"
>Table 9-49</A
> depend on the sort ordering
   specified by the <TT
CLASS="LITERAL"
>ORDER BY</TT
> clause of the associated window
   definition.  Rows that are not distinct in the <TT
CLASS="LITERAL"
>ORDER BY</TT
>
   ordering are said to be <I
CLASS="FIRSTTERM"
>peers</I
>; the four ranking functions
   are defined so that they give the same answer for any two peer rows.
  </P
><P
>   Note that <CODE
CLASS="FUNCTION"
>first_value</CODE
>, <CODE
CLASS="FUNCTION"
>last_value</CODE
>, and
   <CODE
CLASS="FUNCTION"
>nth_value</CODE
> consider only the rows within the <SPAN
CLASS="QUOTE"
>"window
   frame"</SPAN
>, which by default contains the rows from the start of the
   partition through the last peer of the current row.  This is
   likely to give unhelpful results for <CODE
CLASS="FUNCTION"
>last_value</CODE
> and
   sometimes also <CODE
CLASS="FUNCTION"
>nth_value</CODE
>.  You can redefine the frame by
   adding a suitable frame specification (<TT
CLASS="LITERAL"
>RANGE</TT
> or
   <TT
CLASS="LITERAL"
>ROWS</TT
>) to the <TT
CLASS="LITERAL"
>OVER</TT
> clause.
   See <A
HREF="sql-expressions.html#SYNTAX-WINDOW-FUNCTIONS"
>Section 4.2.8</A
> for more information
   about frame specifications.
  </P
><P
>   When an aggregate function is used as a window function, it aggregates
   over the rows within the current row's window frame.
   An aggregate used with <TT
CLASS="LITERAL"
>ORDER BY</TT
> and the default window frame
   definition produces a <SPAN
CLASS="QUOTE"
>"running sum"</SPAN
> type of behavior, which may or
   may not be what's wanted.  To obtain
   aggregation over the whole partition, omit <TT
CLASS="LITERAL"
>ORDER BY</TT
> or use
   <TT
CLASS="LITERAL"
>ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING</TT
>.
   Other frame specifications can be used to obtain other effects.
  </P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>    The SQL standard defines a <TT
CLASS="LITERAL"
>RESPECT NULLS</TT
> or
    <TT
CLASS="LITERAL"
>IGNORE NULLS</TT
> option for <CODE
CLASS="FUNCTION"
>lead</CODE
>, <CODE
CLASS="FUNCTION"
>lag</CODE
>,
    <CODE
CLASS="FUNCTION"
>first_value</CODE
>, <CODE
CLASS="FUNCTION"
>last_value</CODE
>, and
    <CODE
CLASS="FUNCTION"
>nth_value</CODE
>.  This is not implemented in
    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>: the behavior is always the
    same as the standard's default, namely <TT
CLASS="LITERAL"
>RESPECT NULLS</TT
>.
    Likewise, the standard's <TT
CLASS="LITERAL"
>FROM FIRST</TT
> or <TT
CLASS="LITERAL"
>FROM LAST</TT
>
    option for <CODE
CLASS="FUNCTION"
>nth_value</CODE
> is not implemented: only the
    default <TT
CLASS="LITERAL"
>FROM FIRST</TT
> behavior is supported.  (You can achieve
    the result of <TT
CLASS="LITERAL"
>FROM LAST</TT
> by reversing the <TT
CLASS="LITERAL"
>ORDER BY</TT
>
    ordering.)
   </P
></BLOCKQUOTE
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