SELECT 1 FROM r HAVING 1=1

手段

SELECT 1 FROM r GROUP BY () HAVING 1=1

引用ISO / IEC 9075-2：2011 7.10语法规则1(HAVING子句定义的一部分)：

Let HC be the <having clause>. Let TE be the <table expression> that
immediately contains HC. If TE does not immediately contain a
<group by clause>,then “GROUP BY ()” is implicit. Let T be the
descriptor of the table defined by the <group by clause> GBC immediately
contained in TE and let R be the result of GBC.

好的,非常清楚.

断言：1 = 1是真正的搜索条件.我不会为此提供任何引用.

现在

SELECT 1 FROM r GROUP BY () HAVING 1=1

是等于

SELECT 1 FROM r GROUP BY ()

引用ISO / IEC 9075-2：2011 7.10一般规则1：

The <search condition> is evaluated for each group
of R. The result of the <having clause> is a grouped
table of those groups of R for which the result of the
<search condition> is True.

逻辑：由于搜索条件始终为真,因此结果为R,这是group by expression的结果.

以下摘录自7.9的一般规则(GROUP BY CLAUSE的定义)

1) If no <where clause> is specified,then let T be the result of the preceding <from clause>; otherwise,let T be the result of the preceding <where clause>.

2) Case:

a) If there are no grouping columns,then the result of
the <group by clause> is the grouped table consisting of T as its only group.

因此,我们可以得出结论

FROM r GROUP BY ()

得到一个分组表,由一个组组成,零行(因为R为空).

7.12的一般规则的摘录,它定义了一个查询规范(a.k.a一个SELECT语句)：

1) Case:

a) If T is not a grouped table,then […]

b) If T is a grouped table,then

Case:

i) If T has 0 (zero) groups,then let TEMP be an empty table.

ii) If T has one or more groups,then each <value expression> is applied
to each group of T yielding a table TEMP of M rows,where M is the
number of groups in T. The i-th column of TEMP contains the values
derived by the evaluation of the i-th <value expression>. […]

2) Case:

a) If the <set quantifier> DISTINCT is not specified,then the
result of the <query specification> is TEMP.

因此,由于表有一个组,因此它必须有一个结果行.

从而

SELECT 1 FROM r HAVING 1=1

应返回1行结果集.

证明完毕