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Databases - Practical PostgreSQL
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Specifying Target Columns

While SELECT * is a good example of a basic query, and is sometimes very useful, you will probably be interested in retrieving only a few columns worth of information at a time. To stay efficient, and to keep your queries clear, it is a good idea to explicitly specify the intended target columns rather than to use the asterisk. This is especially true when using the JOIN clause, as will be discussed in the Section called Joining Data Sets with JOIN ."

To specify the target columns for a query, list the names of the columns following the SELECT keyword. The query will return data for only those columns that you list. The order of these columns need not match their literal order in the table, and columns may be listed more than once, or not at all, as shown in Example 4-24.

Example 4-24. Re-Ordering columns

booktown=# 
SELECT id, author_id, title, id

booktown-# 
       FROM books;

  id   | author_id |            title            |  id
-------+-----------+-----------------------------+-------
  7808 |      4156 | The Shining                 |  7808
  4513 |      1866 | Dune                        |  4513
  4267 |      2001 | 2001: A Space Odyssey       |  4267
  1608 |      1809 | The Cat in the Hat          |  1608
  1590 |      1809 | Bartholomew and the Oobleck |  1590
 25908 |     15990 | Franklin in the Dark        | 25908
  1501 |      2031 | Goodnight Moon              |  1501
   190 |        16 | Little Women                |   190
  1234 |     25041 | The Velveteen Rabbit        |  1234
  2038 |      1644 | Dynamic Anatomy             |  2038
   156 |       115 | The Tell-Tale Heart         |   156
 41472 |      1212 | Practical PostgreSQL        | 41472
 41473 |      7805 | Programming Python          | 41473
 41477 |      7805 | Learning Python             | 41477
 41478 |      7806 | Perl Cookbook               | 41478
(15 rows)

As you can see, the data sets returned in both Example 4-24 and Example 4-23 are nearly identical. The second set is returned in a different column arrangement, (omitting the subject_id column, and repeating the id column twice) as a result of the target list.

Databases - Practical PostgreSQL
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