sql - 叉积（交叉连接，笛卡尔积）和自然连接的区别

Question

在用 SQL 编写时，我怎么知道我应该使用交叉积（交叉联接、笛卡尔积）还是自然联接？

Answer 1

CROSS JOIN creates all possible pairings of rows from two tables, whether they match or not. You don't use any join condition for a cross product, because the condition would always be true for any pairing.

An example of using CROSS JOIN: you have tables of ShoeColors and ShoeSizes, and you want to know how many possible combinations there are. SELECT COUNT(*) FROM ShoeColors CROSS JOIN ShoeSizes;

NATURAL JOIN is just like an INNER JOIN, but it assumes the condition is equality and applies for all columns names that appear in both tables. I never use NATURAL JOIN, because I can't assume that just because columns have the same name, that they should be related. That would require a very strict column naming convention, and practically no real-world project has such discipline.

Answer 2

简而言之，交叉连接在两个不同表的行之间执行了笛卡尔积..列名可能匹配也可能不匹配......但在自然连接中，为了执行连接操作，列名是强制性的必须匹配两个表

Answer 3

自然联接是交叉联接，其中条件位于两个表中具有相同名称的列上。

假设您有两个表：A 和 B

>>select * from A;
        
            col1 col2 col3
            ---------------
            1     2    1
            1     4    0 
>>select * from B;
            col1 col2
            ---------
            1     2   
    
            1     3
    
>>select * from A cross join B;
    
    col1 col2 col3 col1_2 col2_2
    -----------------------------
    1     2    1    1      2
    1     2    1    1      3
    1     4    0    1      2
    1     4    0    1      3
    
>>select * from A natural join B;
    
    col1 col2 col3
    ---------------
    1     2    1
    1     2    1

更多详情请访问我的帖子。https://eduit.nuxaavi.com.mx/2021/10/16/producto-cartesiano-sql/