无法找出mysql查询中的错误

Question

我现在尝试了一个小时，但仍然无法弄清楚此查询中的问题。 ：/

SELECT * FROM question ORDER BY question_id DESC LIMIT 3 WHERE topic_name = (SELECT * FROM topic WHERE subject_name = 'Maths')

这是错误：

1064-您的SQL语法有误； 在第1行的'WHERE topic_name =（SELECT * FROM topic WHERE subject_name ='Maths'）'附近检查与您的MySQL服务器版本相对应的手册以使用正确的语法

Answer 1

您的陈述全乱了。 正确的声明是：

SELECT *
FROM question
WHERE topic_name = (SELECT * FROM topic WHERE subject_name = 'Maths')
ORDER BY question_id DESC LIMIT 3 ;

但是，由于子查询，这将无法工作。 也许您的意思是：

SELECT *
FROM question
WHERE topic_name in (SELECT topic_name FROM topic WHERE subject_name = 'Maths')
ORDER BY question_id DESC LIMIT 3 ;

Answer 2

您的WHERE子句需要放在ORDER BY子句之前：

SELECT * FROM question WHERE topic_name IN (SELECT topic_name FROM topic WHERE subject_name = 'Maths') ORDER BY question_id DESC LIMIT 3

Answer 3

指定哪一列并将结果限制为一或使用IN运算符

SELECT * FROM question 
WHERE topic_name = (SELECT topic FROM topic WHERE subject_name = 'Maths')
ORDER BY question_id DESC LIMIT 1 

SELECT * FROM question 
WHERE topic_name in(SELECT topic FROM topic WHERE subject_name = 'Maths')
ORDER BY question_id DESC LIMIT 3 

实际上，这不是最佳建议。 进行内部联接：

SELECT * FROM question q
INNER JOIN topic t 
ON q.topic_name = t.topic
WHERE subject_name = 'Maths'
ORDER BY question_id LIMIT 3