Question

我有SQL查询，如下：

SELECT *r1.val, r2.prevVal
FROM (Select statement) AS r1,
(Select statement) AS r2
WHERE 
r1.object_id = r2.object_id

它工作，问题是，当r2 select语句没有返回任何内容时，整个语句不返回任何内容，因为它无法执行r1.object_id = r2.object_id。

如果select语句r1中有值，并且select语句r2中没有值，我怎样才能重现输出值的相同语句？所以我可以访问值r1.val。

Answer 1

使用LEFT加入，如：

SELECT *r1.val, r2.prevVal
FROM (Select statement) AS r1
LEFT JOIN (Select statement) AS r2 ON r1.object_id = r2.object_id    ---------------------Formatted as code

SQL查询结合了select语句

1 个答案: