在mysql上组合4个SELECT查询

Question

我试图结合这3个mysql语句。 我一直在使用UNION和JOIN关键字，但它没有提出我想要的输出。 有什么建议？ 这是我的3个陈述。

SELECT * FROM entrancequestion
WHERE Subject='Abstract Reasoning'
ORDER BY RAND()
LIMIT 10

SELECT * FROM entrancequestion
WHERE Subject='English'
ORDER BY RAND()
LIMIT 30

SELECT * FROM entrancequestion
WHERE Subject='Mathematics'
ORDER BY RAND()
LIMIT 30

SELECT * FROM entrancequestion
WHERE Subject='Science'
ORDER BY RAND()
LIMIT 30

我尝试结合前两个语句，如下所示：

SELECT * FROM entrancequestion
WHERE Subject='Abstract Reasoning'
LIMIT 10
UNION
SELECT * FROM entrancequestion
WHERE Subject='English'
ORDER BY RAND()
LIMIT 30;

但是它只读取第二个LIMIT查询，其中t仅输出30行。

我想创建一个查询，其中显示总共100行并根据索引随机化。 非常感谢您的帮助。

Answer 1

使用UNION和LIMIT时，需要在查询中使用括号，如：

(SELECT * FROM entrancequestion WHERE Subject='Abstract Reasoning' LIMIT 10)
UNION
(SELECT * FROM entrancequestion WHERE Subject='English' ORDER BY RAND() LIMIT 30);

Answer 2

引用文档 ，

要将ORDER BY或LIMIT应用于单个SELECT，请将子句放在包含SELECT的括号内：

(SELECT * FROM entrancequestion
WHERE Subject='Abstract Reasoning'
ORDER BY RAND()
LIMIT 10)

UNION

(SELECT * FROM entrancequestion
WHERE Subject='English'
ORDER BY RAND()
LIMIT 30)

UNION

(SELECT * FROM entrancequestion
WHERE Subject='Mathematics'
ORDER BY RAND()
LIMIT 30)

UNION

(SELECT * FROM entrancequestion
WHERE Subject='Science'
ORDER BY RAND()
LIMIT 30 )

Answer 3

试试这个：

 SELECT TOP 10 * FROM entrancequestion
 WHERE Subject='Abstract Reasoning'
 UNION
 SELECT TOP 30 * FROM entrancequestion
 WHERE Subject='English'
 ORDER BY RAND()

Answer 4

如果你对第一个有LIMIT 10人不太在意：

SELECT * FROM entrancequestion
WHERE
    Subject='Abstract Reasoning' OR
    Subject='English' OR
    Subject='Mathematics' OR
    Subject='Science'
ORDER BY RAND()
LIMIT 30