Postgresql 中文操作指南
7.3. Select Lists #
如前一节所示，@{20} 命令中的表表达式通过组合表、视图、消除行、分组等可能构造一个中间虚拟表。该表最终将传递给 @{21} 处理。选择列表确定实际输出的中间表的 @{22}。
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.
7.3.1. Select-List Items #
最简单的选择列表是 *，它会发出表表达式生成的全部列。否则，选择列表是不定值表达式的逗号分隔列表（如下 Section 4.2 中定义）。例如，它可以是列名称列表：
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 ...列名称 a、b 和 c 要么是 FROM 子句中引用的表的列的实际名称，要么是按照 Section 7.2.1.2 中解释的那样赋予它们的别名。选择列表中可用的名称空间与 WHERE 子句中的相同，除非使用了分组，在这种情况下，它与 HAVING 子句中的名称空间相同。
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.
如果多张表具有同名列，则还必须给出表名，如：
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 ...在使用多张表时，要求所有列都是特定表时也很有用：
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 ...有关 table_name_.*_ 表示法的更多信息，请参见 Section 8.16.5。
See Section 8.16.5 for more about the table_name_.*_ notation.
如果在选择列表中使用了任意值表达式，它在概念上会向返回的表格中添加一个新的虚拟列。对于每一行结果，都会使用这行的值替换所有列引用后对此值表达式进行一次求值。但选择列表中的表达式不必引用 FROM 条款的表格表达式中的任何列；它们可以是常数算术表达式，例如。
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.
7.3.2. Column Labels #
可以选择列表中的条目以供后续处理，例如用于 ORDER BY 子句或供客户端应用程序显示。例如：
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 ...如果没有使用 AS 指定输出列名称，则系统将分配一个默认列名称。对于简单的列引用，这是引用的列的名称。对于函数调用，这是函数的名称。对于复杂表达式，系统会生成一个通用名称。
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.
AS 关键字通常是可选的，但在某些情况下，当所需的列名称与 PostgreSQL 关键字相同时，您必须编写 AS 或对列名称加双引号以避免歧义。（ Appendix C 显示了哪些关键字需要使用 AS 作为列标签。）例如，FROM 是这样的一个关键字，因此这不适用于：
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 ...但以下两种方法都可以：
but either of these do:
SELECT a AS from, b + c AS sum FROM ...
SELECT a "from", b + c AS sum FROM ...为了最安全地防止将来可能添加关键字，建议您始终编写 AS 或为输出列名称加上双引号。
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.
Note
此处输出列的命名方式不同于_FROM_子句中的命名方式（请参阅 Section 7.2.1.2）。可以将同一列重命名两次，但是选择列表中指定的名称是将被传递的名称。
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.
7.3.3. DISTINCT #
在处理完选择列表之后，结果表可以选择进行重复行的消除。为此，在 SELECT 后直接写入 DISTINCT 关键字：
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 ...（可以将关键字 ALL 用作 DISTINCT，而不是将其用于指定保留所有行的默认行为。）
(Instead of DISTINCT the key word ALL can be used to specify the default behavior of retaining all rows.)
显然，如果两个行的至少一个列值不同，则它们被视为是不同的。在该比较中空值被视为相等的。
Obviously, two rows are considered distinct if they differ in at least one column value. Null values are considered equal in this comparison.
或者，可以用任意表达式来确定哪些行被视为不同的：
Alternatively, an arbitrary expression can determine what rows are to be considered distinct:
SELECT DISTINCT ON (expression [, expression ...]) select_list ...此处 expression 是对所有行求值的自变量表达式。其中所有表达式均相等的那些行都被视为重复行，并且只保留该组的第一行作为输出。请注意，除非对足够多的列对查询进行排序以保证到达 DISTINCT 过滤器的行具有唯一的排序顺序，否则一组的“第一行”是不可预测的。（DISTINCT ON 处理发生在 ORDER BY 排序之后。）
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.)
DISTINCT ON 子句不属于 SQL 标准，有时被认为是不佳风格，因为其结果本质上是不可确定的。通过明智地使用 FROM 中的 GROUP BY 和子查询，可以避免这种结构，但它通常是最方便的备选方案。
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.