As shown in the previous section,
   the table expression in the SELECT command
   constructs an intermediate virtual table by possibly combining
   tables, views, eliminating rows, grouping, etc.  This table is
   finally passed on to processing by the select list.  The select
   list determines which columns of the
   intermediate table are actually output.
  
    The simplest kind of select list is * which
    emits all columns that the table expression produces.  Otherwise,
    a select list is a comma-separated list of value expressions (as
    defined in Section 4.2).  For instance, it
    could be a list of column names:
SELECT a, b, c FROM ...
     The columns names a, b, and c
     are either the actual names of the columns of tables referenced
     in the FROM clause, or the aliases given to them as
     explained in Section 7.2.1.2.  The name
     space available in the select list is the same as in the
     WHERE clause, unless grouping is used, in which case
     it is the same as in the HAVING clause.
   
If more than one table has a column of the same name, the table name must also be given, as in:
SELECT tbl1.a, tbl2.a, tbl1.b FROM ...
When working with multiple tables, it can also be useful to ask for all the columns of a particular table:
SELECT tbl1.*, tbl2.a FROM ...
    See Section 8.16.5 for more about
    the table_name.* notation.
   
    If an arbitrary value expression is used in the select list, it
    conceptually adds a new virtual column to the returned table.  The
    value expression is evaluated once for each result row, with
    the row's values substituted for any column references.  But the
    expressions in the select list do not have to reference any
    columns in the table expression of the FROM clause;
    they can be constant arithmetic expressions, for instance.
   
    The entries in the select list can be assigned names for subsequent
    processing, such as for use in an ORDER BY clause
    or for display by the client application.  For example:
SELECT a AS value, b + c AS sum FROM ...
    If no output column name is specified using AS,
    the system assigns a default column name.  For simple column references,
    this is the name of the referenced column.  For function
    calls, this is the name of the function.  For complex expressions,
    the system will generate a generic name.
   
    The AS key word is usually optional, but in some
    cases where the desired column name matches a
    PostgreSQL key word, you must write
    AS or double-quote the column name in order to
    avoid ambiguity.
    (Appendix C shows which key words
    require AS to be used as a column label.)
    For example, FROM is one such key word, so this
    does not work:
SELECT a from, b + c AS sum FROM ...
but either of these do:
SELECT a AS from, b + c AS sum FROM ... SELECT a "from", b + c AS sum FROM ...
    For greatest safety against possible
    future key word additions, it is recommended that you always either
    write AS or double-quote the output column name.
   
     The naming of output columns here is different from that done in
     the FROM clause (see Section 7.2.1.2).  It is possible
     to rename the same column twice, but the name assigned in
     the select list is the one that will be passed on.
    
DISTINCT #
    After the select list has been processed, the result table can
    optionally be subject to the elimination of duplicate rows.  The
    DISTINCT key word is written directly after
    SELECT to specify this:
SELECT DISTINCT select_list ...
    (Instead of DISTINCT the key word ALL
    can be used to specify the default behavior of retaining all rows.)
   
Obviously, two rows are considered distinct if they differ in at least one column value. Null values are considered equal in this comparison.
Alternatively, an arbitrary expression can determine what rows are to be considered distinct:
SELECT DISTINCT ON (expression[,expression...])select_list...
    Here expression is an arbitrary value
    expression that is evaluated for all rows.  A set of rows for
    which all the expressions are equal are considered duplicates, and
    only the first row of the set is kept in the output.  Note that
    the “first row” of a set is unpredictable unless the
    query is sorted on enough columns to guarantee a unique ordering
    of the rows arriving at the DISTINCT filter.
    (DISTINCT ON processing occurs after ORDER
    BY sorting.)
   
    The DISTINCT ON clause is not part of the SQL standard
    and is sometimes considered bad style because of the potentially
    indeterminate nature of its results.  With judicious use of
    GROUP BY and subqueries in FROM, this
    construct can be avoided, but it is often the most convenient
    alternative.