sql - 多个 LEFT JOIN - "left"表是什么？

Question

我已经使用它年了，所以是时候全面了解它了。假设这样的查询:

SELECT 
  *
FROM a
LEFT JOIN b ON foo...
LEFT JOIN c ON bar...

documentation告诉我们

T1 { [INNER] | { LEFT | RIGHT | FULL } [OUTER] } JOIN T2 ON boolean_expression

LEFT OUTER JOIN

First, an inner join is performed. Then, for each row in T1 that does not satisfy the join condition with any row in T2, a joined row is added with null values in columns of T2. Thus, the joined table always has at least one row for each row in T1.

问题很简单:在这种情况下，T1 是什么？它是 a 吗？还是 a LEFT JOIN b ON foo？ (或者，是一样的吗？)

Answer 1

FROM 子句从左到右解析条件(除非被括号覆盖)。所以:

FROM a 
LEFT JOIN b
     ON foo... 
LEFT JOIN c
     ON bar...

解析为:

FROM (
        a 
        LEFT JOIN b
           ON foo...
     ) 
LEFT JOIN c
     ON bar...

这在 documentation 中有解释在 FROM 子句的 join-type 部分下:

Use parentheses if necessary to determine the order of nesting. In the absence of parentheses, JOINs nest left-to-right. In any case JOIN binds more tightly than the commas separating FROM-list items.

因此，一系列 LEFT JOIN 将所有记录保存在第一个提到的表中。这是一个方便。

请注意，无论连接类型如何，FROM 子句的解析都是相同的。