如何在 MySQL LEFT JOIN 中使用别名

Question

我的原始查询是使用 WHERE 子句而不是 JOIN 进行连接。我意识到这不是返回没有任何明星或流派的电影没有出现，所以我认为我必须做一个 LEFT JOIN 才能展示每部电影。这是我的原始 SQL：

SELECT *
FROM movies m, stars s, stars_in_movies sm, genres g, genres_in_movies gm
WHERE m.id = sm.movie_id
AND sm.star_id = s.id
AND gm.genre_id = g.id
AND gm.movie_id = m.id
AND m.title LIKE '%the%'
AND s.first_name LIKE '%Ben%'
ORDER BY m.title ASC
LIMIT 5;

我尝试对电影进行 LEFT JOIN，但我肯定做错了什么。

SELECT *
FROM movies m, stars s, stars_in_movies sm, genres g, genres_in_movies gm
LEFT JOIN movies m1 ON m1.id = sm.movie_id
LEFT JOIN movies m2 ON m2.id = gm.movie_id
AND sm.star_id = s.id
AND gm.genre_id = g.id
ORDER BY m.title ASC
LIMIT 5;

我得到ERROR 1054 (42S22): Unknown column 'sm.movie_id' in 'on clause' 很明显我做错了加入，我只是不明白它是什么。

Answer 1

不要将逗号运算符与JOIN 混用——它们有不同的优先级！ manual 中甚至对此有警告：

但是，逗号运算符的优先级低于 INNER JOIN、CROSS JOIN、LEFT JOIN 等。如果在存在连接条件时将逗号连接与其他连接类型混合使用，则可能会出现Unknown column 'col_name' in 'on clause' 形式的错误。本节稍后会提供有关处理此问题的信息。

试试这个：

SELECT *
FROM movies m
LEFT JOIN (
   stars s
   JOIN stars_in_movies sm
       ON sm.star_id = s.id
) ON m.id = sm.movie_id AND s.first_name LIKE '%Ben%'
LEFT JOIN (
    genres g
    JOIN genres_in_movies gm
        ON gm.genre_id = g.id
) ON gm.movie_id = m.id
WHERE m.title LIKE '%the%'
ORDER BY m.title ASC
LIMIT 5;

Answer 2

您应该将与您的 JOIN 相关的条件放在同一个 ON 子句中。但是，对于您的上述问题，您应该使用以下查询：

SELECT *
FROM movies m 
LEFT JOIN stars_in_movies sm ON sm.movie_id = m.id
JOIN stars s ON sm.star_id = s.id
LEFT JOIN genres_in_movies gm ON gm.movie_id = m.id
JOIN genres g ON gm.genre_id = g.id
ORDER BY m.title ASC
LIMIT 5;

Answer 3

也许丑陋，但它的工作方式就在这里。当心这很丑陋，很多人都在警告这种黑客行为

SELECT *
FROM movies m, stars_in_movies sm LEFT JOIN movies m1 ON m1.id = sm.movie_id, stars s
ORDER BY m.title ASC
LIMIT 5;

当使用连接时，您必须使用包含您要比较的列的正确表进行连接。

Answer 4

SQL Join（MySQL 中的内连接）

select emp1.id,emp1.name,emp1.job  from (select id, type as name, description as job from component_type as emp1)emp1
inner join 
emp
on emp1.id=emp.id;

左连接

select emp1.id,emp1.name,emp1.job  from (select id, type as name, description as job from component_type as emp1 where id between '1' AND '5')emp1
left join 
emp
on emp1.id=emp.id;

右连接

select emp1.id,emp1.name,emp1.job  from (select id, type as name, description as job from component_type as emp1)emp1
Right join 
(select * from emp where id between '1' and '5')exe
on emp1.id=exe.id;

Answer 5

使用别名连接多个表而不使用连接..

select sum(s.salary_amount) as total_expenses_paid_to_all_department
from salary_mas_tbl s,dept_mas_tbl d
where s.salary_dept=d.dept_id;