Dear Wiki user,

You have subscribed to a wiki page or wiki category on "Pig Wiki" for change 
notification.

The following page has been changed by OlgaN:
http://wiki.apache.org/pig/PigLatin

------------------------------------------------------------------------------
  
  The following table lists the various methods of referring to data.
  
- || Method of Referring to Data || Example || Value for example tuple =t= || 
Notes ||
+ || Method of Referring to Data || Example || Value for example tuple '''t''' 
|| Notes ||
- || '''Constant''' || ''''1.0'''', or ''''apache.org'''', or ''''blah'''' || 
Value constant irrespective of =t= || ||
+ || '''Constant''' || ''''1.0'''', or ''''apache.org'''', or ''''blah'''' || 
Value constant irrespective of '''t''' || ||
  || '''Field referred to by position''' || '''$0''' || Data Atom '1' || '''In 
Pig, positions start at 0 and not 1''' ||
- || '''Field referred to by name''' || *f2*|| Bag {<2,3>,<4,6>,<5,7>} || ||
+ || '''Field referred to by name''' || '''f2'''|| Bag {<2,3>,<4,6>,<5,7>} || ||
  || '''Projection''' of another data item || '''f2.$0''' || Bag {<2>,<4>,<5>} 
- the bag f2 projected to the first field || ||
  || '''Map Lookup''' against another data item || '''f3#'apache'''' || Data 
Atom 'search' ||* User's responsibility to ensure that a lookup is written only 
against a  data map, otherwise a runtime error is thrown <br>   * If the key 
being looked up does not exist, a Data Atom with an empty string is returned||
  || '''Function''' applied to another data item || '''SUM(f2.$0)''' || 2+4+5 = 
11 || SUM is a builtin Pig function. See PigFunctions for how to write your own 
functions ||
@@ -74, +74 @@

  
  Suppose we want to refer to the 3 fields as f1, f2, and f3. We can load this 
relation using the following command:
  
- <blockquote><verbatim>
+ {{{
  A = LOAD 'myfile.txt' USING PigStorage('\t') AS (f1,f2,f3);
- </verbatim></blockquote>
+ }}}
  
- <noautolink>
  Here, PigStorage is the name of a "storage function" that takes care of 
parsing the file into a Pig relation. This storage function expects simple 
newline-separated records with delimiter-separated fields; it has one 
parameter, namely the field delimiter(s).  
- </noautolink>
  
  Future Pig Latin commands can refer to the alias "A" and will receive data 
that has been loaded from "myfile.txt". A will contain this data:
  
@@ -107, +105 @@

  ==== FILTER: Getting rid of data you are not interested in  ====
  Very often, the first thing that you want to do with data is to get rid of 
tuples that you are not interested in. This can be done by the filter 
statement. For example,
  
- <blockquote><verbatim>
+ {{{
  Y = FILTER A BY f1 == '8';
- </verbatim></blockquote>
+ }}}
  
  The result is Y =
  
@@ -129, +127 @@

        * '''matches''' for regular expression matching, e.g., $0 matches 
"*apache*". The 
http://java.sun.com/j2se/1.5.0/docs/api/java/util/regex/Pattern.html][format of 
regular expressions is that supported by Java.
  
  Thus, a somewhat more complicated condition can be
- <blockquote><verbatim>
+ {{{
  Y = FILTER A BY (f1 == '8') OR (NOT (f2+f3 > f1));
- </verbatim></blockquote>
+ }}}
  
  Note:
     * If you want to get rid of specifc columns or fields, rather than whole 
tuples, you should use the [[#ForeachS][FOREACH]] statement and not the filter 
statement.
@@ -142, +140 @@

  
  We can group the tuples in A according to some specification. A simple 
specification is to group according to the value of one of the fields, e.g. the 
first field. This is done as follows:
  
- <blockquote><verbatim>
+ {{{
  X = GROUP A BY f1;
  X = GROUP A BY (f1, f2 ..);
- </verbatim></blockquote>
+ }}}
  
  The result of the group statement consists of one tuple for each group. The 
first field of the tuple has name `group` and has the value on which the 
grouping has been performed, and the second field has name A and is a bag 
containing the tuples belonging to that group. Thus, X = :
  
@@ -170, +168 @@

  
  We can ''co-group'' A and B, which means that we jointly group the tuples 
from A and B, using this command:
  
- <blockquote><verbatim>
+ {{{
  COGROUP A BY f1, B BY $0;
- </verbatim></blockquote>
+ }}}
  
  You can co-group by multiple columns the same way as for group.
  
@@ -190, +188 @@

  
  Note that some of the bags are empty, which indicates that no tuples from the 
corresponding input belong to that group. If we only wish to see groups for 
which <i>both</i> inputs have at least one tuple, we can write:
  
- <blockquote><verbatim>
+ {{{
  C = COGROUP A BY $0 INNER, B BY $0 INNER;
- </verbatim></blockquote>
+ }}}
  
  The result is C = 
  
@@ -206, +204 @@

  
  In addition to using columns to group the data, an arbitrary expression can 
be used:
  
- <blockquote><verbatim>
+ {{{
  grunt> cat a      
  r1    1       2
  r2    2       1
@@ -234, +232 @@

  (2.0, {(r1, 1, 2), (r2, 2, 1)})
  (16.0, {(r3, 2, 8), (r4, 4, 4)})
  grunt> 
- </verbatim></blockquote>
+ }}}
  
  Note: 
     * If we want all tuples to go to a single group, e.g., when doing 
aggregates across entire relations, we can write `GROUP A ALL`.
@@ -256, +254 @@

  
  To select a subset of columns from a relation, use this command:
  
- <blockquote><verbatim>
+ {{{
  X = FOREACH A GENERATE f1, f2;
- </verbatim></blockquote>
+ }}}
  
  X contains tuples from A, but with only the first and second fields present 
in each tuple. For the value of A given above, X =
  
@@ -280, +278 @@

  
  As with SQL, asterisk (*) is shorthand for all columns. For example, with:
  
- <blockquote><verbatim>
+ {{{
  X = FOREACH A GENERATE *;
- </verbatim></blockquote>
+ }}}
  
  X is identical to A.
  
@@ -291, +289 @@

  
  If one of the fields in the input relation, is a non-atomic field, we can 
perform projection on that field. For example, 
  
- <blockquote><verbatim>
+ {{{
  FOREACH C GENERATE group, B.$1;
- </verbatim></blockquote>
+ }}}
  
  The result is:
  
@@ -305, +303 @@

  
  Here is another example, in which multiple nested columns are retained:
  
- <blockquote><verbatim>
+ {{{
  FOREACH C GENERATE group, A.(f1, f2);
- </verbatim></blockquote>
+ }}}
  
  The result is:
  
@@ -322, +320 @@

  
  Pig has a number of built-in functions. An example is the SUM() function, 
which takes the sum of a set of numbers in a bag. For example:
  
- <blockquote><verbatim>
+ {{{
  FOREACH C GENERATE group, SUM(A.f1);
- </verbatim></blockquote>
+ }}}
  
  gives:
  
@@ -341, +339 @@

  
  Sometimes we want to eliminate nesting. This can be accomplished via the 
FLATTEN keyword which can be attached before any valid data item. For example:
  
- <blockquote><verbatim>
+ {{{
  FOREACH C GENERATE group, FLATTEN(A);
- </verbatim></blockquote>
+ }}}
  
  yields:
  
@@ -357, +355 @@

  
  As another example,
  
- <blockquote><verbatim>
+ {{{
  FOREACH C GENERATE group, FLATTEN(A.f3);
- </verbatim></blockquote>
+ }}}
  
  yields:
  
@@ -373, +371 @@

  
  As a final example,
  
- <blockquote><verbatim>
+ {{{
  FOREACH C GENERATE flatten(A.(f1, f2)), flatten(B.$1);
- </verbatim></blockquote>
+ }}}
  
  yields:
  
@@ -396, +394 @@

  
  The equi-join of A and B on column 0 can be expressed as follows:
  
- <blockquote><verbatim>
+ {{{
  JOIN A BY $0, B BY $0;
- </verbatim></blockquote>
+ }}}
  
  which is equivalent to:
  
- <blockquote><verbatim>
+ {{{
  X = COGROUP A BY $0 INNER, B BY $0 INNER;
  FOREACH X GENERATE FLATTEN(A), FLATTEN(B);
- </verbatim></blockquote>
+ }}}
  
  The result is:
  
@@ -491, +489 @@

  
  To compute the cross product (also known as "cartesian product") of two or 
more relations, use:
  
- <blockquote><verbatim>
+ {{{
  X = CROSS A, B;
- </verbatim></blockquote>
+ }}}
  
  Based on the values of A and B given earlier in the document, the result is X 
=
  
@@ -518, +516 @@

  
  We can vertically glue together contents of multiple aliases into a single 
alias by the UNION command. For example,
  
- <blockquote><verbatim>
+ {{{
  X = UNION A, B;
- </verbatim></blockquote>
+ }}}
  
  The result is X =
  
@@ -554, +552 @@

  
  An example of a SPLIT statement is the following,
  
- <blockquote><verbatim>
+ {{{
  SPLIT A INTO X IF $0 < 7, Y IF ($0 > 2 AND $0<> 7);
- </verbatim></blockquote>
+ }}}
  
  The output is 
  
@@ -590, +588 @@

  
  The syntax for doing the nested operations is very similar to the regular 
syntax and is demonstrated by the following example:
  
- <blockquote><verbatim>
+ {{{
  W = LOAD '...' AS (url, outlink);
  G = GROUP W by url;
  R = FOREACH G {
@@ -599, +597 @@

        DW = DISTINCT PW;
        GENERATE group, COUNT(DW);
  }
- </verbatim></blockquote>
+ }}}
  
  Notes:
     * Note the nested block within the FOREACH...GENERATE statement. The 
syntax is the same as regular Pig Latin syntax.

Reply via email to