Sql ONLY 关键字

Question

在 SQL 语法中，我发现奇怪的规则表明 select * from ONLY (t1)是有效的 SQL。

我的问题是:什么是 ONLY在这种情况下是什么意思？

它在规范的“7.6 table reference”部分:

<table primary> ::=
           <table or query name> [ [ AS ] <correlation name> [ <left paren> <derived column list> <right paren> ] ]
     |     <derived table> [ AS ] <correlation name> [ <left paren> <derived column list> <right paren> ]
     |     <lateral derived table> [ AS ] <correlation name> [ <left paren> <derived column list> <right paren> ]
     |     <collection derived table> [ AS ] <correlation name> [ <left paren> <derived column list> <right paren> ]
     |     <table function derived table> [ AS ] <correlation name> [ <left paren> <derived column list> <right paren> ]
     |     <only spec> [ [ AS ] <correlation name> [ <left paren> <derived column list> <right paren> ] ]
     |     <left paren> <joined table> <right paren> 

<only spec> ::= ONLY <left paren> <table or query name> <right paren> 

Answer 1

如果表参与表继承，则 ONLY 关键字用于限制查询中使用的表。

在规范中进一步解释为:

If ONLY is specified, then the result of TP is a table that consists of every row in T, except those rows that have a subrow in a proper subtable of T

据我所知，目前仅支持 Postgres

效果可以通过下面的例子看到:

create table base (id integer, some_data varchar(100));
create table child () inherits (base);

insert into base values (1, 'base');
insert into child values (2, 'child');

下列:

select *
from base;

返回:

id | some_data
---+----------
 1 | base     
 2 | child    

而以下内容:

select *
from only (base);

返回:

id | some_data
---+----------
 1 | base     

在线示例: http://rextester.com/JVUM87016