跨表 SQL 查询的正确语法是什么? [英] What is the proper syntax for a cross-table SQL query?

问题描述

现在，我有

SELECT gp_id FROM gp.keywords 
WHERE keyword_id = 15 
AND (SELECT practice_link FROM gp.practices 
     WHERE practice_link IS NOT NULL 
     AND id = gp_id)

这不会提供语法错误，但是对于应该返回行的值，它只返回 0 行.

This does not provide a syntax error, however for values where it should return row(s), it just returns 0 rows.

我想要做的是从 gp.keywords 获取 gp_id，其中关键字表 keyword_id 列是特定值，而 practice_link 是实践表对应于我拥有的 gp_id，它存储在 id 中该表的列.

What I'm trying to do is get the gp_id from gp.keywords where the the keywords table keyword_id column is a specific value and the practice_link is the practices table corresponds to the gp_id that I have, which is stored in the id column of that table.

推荐答案

我什至不确定这是有效的 SQL，所以我很惊讶它竟然能正常工作:

I'm not even sure that is valid SQL, so I'm surprised it is working at all:

SELECT gp_id
FROM gp.keywords
WHERE keyword_id = 15
    AND (SELECT practice_link FROM gp.practices WHERE practice_link IS NOT NULL AND id = gp_id)

这个怎么样:

SELECT kw.gp_id, p.practice_link
FROM gp.keywords AS kw
INNER JOIN gp.practices AS p
    ON p.id = kw.gp_id
WHERE kw.keyword_id = 15

我会像在其他示例中一样避开隐式连接.以后只会流泪.

I would steer clear of implicit joins as in the other examples. It only leads to tears later.

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