Php 如何左键联接另一个嵌套表？

如何左键联接另一个嵌套表

第一个问题,

SELECT *
FROM category AS a
WHERE a.type = 'content'

结果,

category_id  type       code
1            content    content_1
2            content    content_2
3            content    content_3

category_id  value       article_id
1            xxa         4
2            xxb         4

第二个问题

SELECT*
FROM content AS c

LEFT JOIN article_has_content AS x
ON x.content_id = c.content_id

WHERE x.article_id = '4'

结果,

category_id  type       code
1            content    content_1
2            content    content_2
3            content    content_3

category_id  value       article_id
1            xxa         4
2            xxb         4

我追求的结果

category_id  type       code          value
1            content    content_1     xxa
2            content    content_2     xxb
3            content    content_3     NULL

第三个问题

SELECT*
FROM 
(
    SELECT *
    FROM category AS a
    WHERE a.type = 'content'
) a
LEFT JOIN b
(
    SELECT*
    FROM content AS c

    LEFT JOIN article_has_content AS x
    ON x.content_id = c.content_id

    WHERE x.article_id = '4'
) b
ON b.category_id = a.category_id

错误

1064-您的SQL语法有错误；检查与您的MySQL服务器版本对应的手册，以了解要使用的正确语法 近' 挑选* 从内容转换为c

LEFT JOIN article_has_content AS x
ON x.content' at line 9

你几乎是对的：

SELECT *
FROM 
(
    SELECT *
    FROM category AS a
    WHERE a.type = 'content'
) a
LEFT JOIN                              -- the alias name here was wrong
(
    SELECT*
    FROM content AS c

    LEFT JOIN article_has_content AS x
    ON x.content_id = c.content_id

    WHERE x.article_id = '4'
) b                                    -- and you had the alias already here at the right position
ON b.category_id = a.category_id

---

您将别名放在子查询之后。但总的来说，您的查询过于复杂。我想这正是你想要的：

SELECT c.category_id, c.type, c.code, con.value
FROM category c left join
     content con
     on c.category_id = con.category_id left join
     article_has_content ahc
     on con.content_id = ahc.content_id and
        ahc.article_id = '4';

---

如何组合这两个查询，如下所示：

SELECT*
FROM content AS c

LEFT JOIN article_has_content AS x
ON x.content_id = c.content_id

LEFT JOIN category a ON a.category_id =  c.category_id AND a.type = 'content'

WHERE x.article_id = '4'

---

该错误是MySQL错误，因此我删除了sql server标记。如果您将表定义放在SQL FIDLE上也会有所帮助。我在“on子句”@tealou中得到一个错误-1054-未知列“ahc.category_id”。我不得不猜测哪个表有category_id，因为您的问题在这一点上不清楚。它必须在内容上。谢谢您的编辑，但它不会输出我需要的空行。@tealou。关于左连接，我也错了。这些都是需要的。关键是对on子句中除第一个表外的所有表进行筛选。完成此操作。它不输出我需要的空值。