mysql支持select count('1') from (values (1, 2), (3, 4)) where false;吗

MySQL支持这个语法，但是结果会返回0，因为WHERE子句中的条件false将过滤掉所有的行，因此COUNT函数计算的行数为0。请注意，VALUES子句在MySQL 8.0版本之前不支持，需要使用UNION ALL子句替代。例如：SELECT COUNT('1') FROM (SELECT 1 AS a, 2 AS b UNION ALL SELECT 3, 4) AS t WHERE false;

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修改正确 if (select count(*) from operate where uid=0 and o1='+' and o2=1)=0 then insert into operate (o1,o2,uid) values ('+',1,0)

您好，针对您提供的代码，这是一段伪代码，并不是具体的编程语言。如果您想要将其转换成具体的编程语言的代码，需要根据具体的语言来编写。

假设您要将其转换成 MySQL 中的 SQL 语句，可以将其修改为如下形式：

INSERT INTO operate (o1, o2, uid)
SELECT '+', 1, 0
FROM dual
WHERE NOT EXISTS (SELECT * FROM operate WHERE uid = 0 AND o1 = '+' AND o2 = 1);

上述 SQL 语句的作用是：如果 operate 表中 uid 为 0，o1 为 '+'，o2 为 1 的数据行不存在，则插入一行数据，o1 为 '+'，o2 为 1，uid 为 0。

具体来说，这段 SQL 语句的逻辑如下：

1. 使用 SELECT * FROM operate WHERE uid = 0 AND o1 = '+' AND o2 = 1 查询 operate 表中 uid 为 0，o1 为 '+'，o2 为 1 的数据行。

2. 如果查询结果为空，则说明该数据行不存在，使用 INSERT INTO operate (o1, o2, uid) SELECT '+', 1, 0 FROM dual 插入一行数据，o1 为 '+'，o2 为 1，uid 为 0。

如果查询结果不为空，则说明该数据行已经存在，不进行插入操作。

需要注意的是，上述 SQL 语句中使用了 SELECT FROM dual 语法，这是 MySQL 特有的写法，用于在没有 FROM 子句的情况下执行 SELECT 语句。

希望这个回答能够帮助到您。

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'SELECT count(*) FROM haonana WHERE unitcode is not null )) t2 FROM

Apologies for the error in the previous response. Here's an updated SQL query to calculate the ratio of the occurrence of fields "a" and "b" together to the occurrence of field "a":

SELECT a, COUNT(*) AS total_count,
       (COUNT(*) / (SELECT COUNT(*) FROM YourTable WHERE a IS NOT NULL)) AS ratio
FROM YourTable
WHERE a = b
GROUP BY a
HAVING ratio > 1/3;

In this query, replace "YourTable" with the actual name of your table and "a" with the correct field name.

This query will count the occurrences of fields "a" and "b" together, calculate the ratio of that count to the total count of field "a" (excluding NULL values), and then filter the results to those with a ratio greater than one-third.

Please note that this is a sample query, and you may need to modify it according to your specific table structure and requirements.