sql - LEFT JOIN 形式之间的差异

Question

这些查询有什么区别

SELECT
    A.AnyField
FROM
    A
    LEFT JOIN B ON B.AId = A.Id
    LEFT JOIN C ON B.CId = C.Id 

这是另一个查询

SELECT
    A.AnyField
FROM
    A
    LEFT JOIN
    (
        B
        JOIN C ON B.CId = C.Id
    ) ON B.AId = A.Id

Answer 1

原答案:

它们不一样。

例如，a left join b left join c 将返回 a 行，加上 b 行，即使没有 c 行也是如此。

a left join (b join c) 如果没有 c 行，则永远不会返回 b 行。

稍后添加:

SQL>create table a (id int); 
SQL>create table b (id int);
SQL>create table c (id int);
SQL>insert into a values (1);
SQL>insert into a values (2);
SQL>insert into b values (1);
SQL>insert into b values (1);
SQL>insert into c values (2);
SQL>insert into c values (2);
SQL>insert into c values (2);
SQL>insert into c values (2);
SQL>select a.id from a left join b on a.id = b.id left join c on b.id = c.id;
         id
===========
          1
          1
          2

                  3 rows found

SQL>select a.id from a left join (b join c on b.id = c.id) on a.id = b.id;
         id
===========
          1
          2

                  2 rows found