在同一查询中使用查询值

Question

如果我有一个有两个选择的查询，我可以使用第二个选择中第一个选择的值吗？

即：

(SELECT post_id AS post_1 FROM posts WHERE user_id = 100) UNION (SELECT post_id AS post_2 FROM posts WHERE user_id = post_1)

上述方法不起作用，我该怎么办呢？

Answer 1

这取决于您尝试做什么，您可以使用子查询来获取WHERE子句的结果

例如

SELECT post_id AS post_2 FROM posts
WHERE user_id = (SELECT post_id AS post_1 FROM posts WHERE user_id = 100)

或者您可以按顺序运行两个查询，将第一个结果分配给变量并使用

例如

SET myVar = (SELECT post_id AS post_1 FROM posts WHERE user_id = 100);

SELECT post_id AS post_2 FROM posts WHERE user_id = myVar  

Answer 2

您好像正在寻找加入？

SELECT p1.post_id as post_id1
     , p2.post_id as post_id2
  FROM posts p1
  JOIN posts p2
    ON p2.user_id = p1.post_id
 WHERE p1.user_id = 100;