Question

我有两个类型兼容的表； A和B。我想要一个SQL查询，该查询将告诉我B的所有元素是否也是A的元素。

示例1：

     Table A
c1      | c2
--------+--------
"Hello" | "World"
"Hello" | "Kitty"
"Hello" | "pretty"

     Table B
c1      | c2
--------+--------
"Hello" | "World"
"Hello" | "Kitty"

expected: TRUE

示例2：

     Table A
c1      | c2
--------+--------
"Cat"   | 1
"Gato"  | 2
"Neko"  | 3

     Table B
c1      | c2
--------+--------
"Cat"   | 1
"Chat"  | 4

expected: FALSE

Answer 1

一种方法可以是联合和左联接

select case when t2.col is not null then 'all elements of B are also elements of A.' else null end  from
(
select distinct c1 as col from A 
union
select distinct c2 from A
) t1
left join
(
select distinct c1 as col from B
union
select distinct c2 from B
) t2 on t1.col=t2.col

Answer 2

此查询将尝试同时使用列b和a将表c1中的所有值与表c2进行匹配。

如果它们匹配，则查询的equals将为true，否则为false。

工作示例：https://www.db-fiddle.com/f/d1N7ZaUSdogoTVCYefpdc1/1

SELECT COUNT(*) = 0 AS equal
FROM b
LEFT JOIN a USING (c1, c2)
WHERE a.c1 IS NULL;

SQL：如果一个表是另一个表的子集，将返回什么查询？

2 个答案: