是否有一个SQL查询将总是返回零结果？ [英] Is there a SQL query that will always return zero results?
问题描述
如果有MySQL / PostgreSQL / Oracle特定的解决方案，我很好奇他们所有。
If there is a MySQL/PostgreSQL/Oracle-specific solution, I'm curious about them all.
推荐答案
在DBMS上，以下中的一个或多个将工作：
Depending on the DBMS, one or more of the following will work:
-
SELECT NULL LIMIT 0（PostgreSQL和MySQL语法）/SELECT TOP 0 1（MS SQL Server语法） -
SELECT NULL WHERE FALSE（DBMS带有布尔类型，例如PostgreSQL）SELECT NULL WHERE 1 = 0（大多数DBMS）
SELECT NULL LIMIT 0(PostgreSQL and MySQL syntax) /SELECT TOP 0 1(MS SQL Server syntax)SELECT NULL WHERE FALSE(DBMS with a boolean type, e.g. PostgreSQL)SELECT NULL WHERE 1=0(most DBMSes)
对于Oracle，这些将需要是 SELECT NULL FROM DUAL 的形式，不能有 SELECT 没有 FROM 子句;不确定 LIMIT / TOP 和 WHERE 它将接受。
For Oracle, these will need to be of the form SELECT NULL FROM DUAL, I believe, as you can't have SELECT without a FROM clause of some sort; not sure which versions of the LIMIT / TOP and WHERE it will accept.
一个更精细的选项是创建一个（临时）表，不插入任何行，它可以给你任意数量的列，有类型关联，即使它们不包含值：
A more elaborate option is to create a (temporary) table and not insert any rows into it, which can give you any number of columns, which will have types associated even though they contain no values:
-- PostgreSQL
CREATE TEMP TABLE dummy ( a Int, b VarChar(42) );
SELECT * FROM dummy;
-- MS SQL Server
CREATE TABLE #DUMMY ( a Int, b VarChar(42) );
SELECT * FROM #DUMMY;
在PostgreSQL中，您甚至可以创建一个没有列的表， 零行零列：
In PostgreSQL, you can even create a table with no columns, allowing you to have a result set with zero rows and zero columns:
CREATE TEMP TABLE empty ();
SELECT * FROM empty;
另一种可能性是，如果DBMS有set-returning函数，它们可能能够返回一个空集。例如，再次在PostgreSQL中，因为它是我最了解的，你可以给一个无效的范围 generate_series（） ：
Another possibility is if the DBMS has set-returning functions, they may be able to return an empty set. For instance, again in PostgreSQL as it's what I know best, you can give an invalid range to generate_series():
SELECT * FROM generate_series(0,-1);
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