如何将 SQL Server 游标转换为 MySQL 等价物-数据库问题

How to convert a SQL Server cursor to MySQL equivalent(如何将 SQL Server 游标转换为 MySQL 等价物)

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问题描述

如何将下面的程序转换为 MySQL 格式?

How can I convert the procedure below to MySQL format?

这是要转换的部分:

DECLARE @CurrentFirstName varchar(300)
DECLARE @CurrentAge INT

DECLARE CursorName CURSOR FAST_FORWARD FOR 
    SELECT Firstname,Age 
    FROM Customers

OPEN CursorName
FETCH NEXT FROM CursorName INTO @CurrentFirstName, @CurrentAge

WHILE @@FETCH_STATUS = 0
BEGIN
      IF @AGE>60 /*this is stupid but we can apply any complex condition here*/ BEGIN
    insert into ElderCustomers values (@CurrentFirstName,@CurrentAge)
      END


    FETCH NEXT FROM CursorName INTO @CurrentFirstname,@CurrentAge
END

CLOSE CursorName
DEALLOCATE CursorName

以上如有错误请见谅

推荐答案

MySQL 等价物是这样的:

The MySQL equivalent would be something like this:

BEGIN
  DECLARE CurrentFirstName VARCHAR(300);
  DECLARE CurrentAge INT;
  DECLARE done INT DEFAULT FALSE;
  DECLARE CursorName CURSOR FOR
    SELECT FirstName, Age FROM Customers;
  DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
  OPEN CursorName;
  myloop: LOOP
    FETCH CursorName INTO CurrentFirstName, CurrentAge;
    IF done THEN
      LEAVE myloop;
    END IF;
    IF CurrentAge > 60 THEN
      insert into ElderCustomers values (CurrentFirstName,CurrentAge);
    END IF;
  END LOOP;
  CLOSE CursorName;
END;

最大的区别在于循环，当没有更多行要获取时使用 CONTINUE HANDLER 设置标志，并在设置标志时退出循环.(这看起来很难看，但在 MySQL 中就是这样做的.)

The big difference is in the loop, using the CONTINUE HANDLER to set a flag when there are no more rows to fetch, and exiting the loop when the flag is set. (That looks ugly, but that's the way it's done in MySQL.)

这个例子引出了一个问题，为什么不把它写成(在 SQL Server 和 MySQL 中更有效):

This example begs the question why this isn't written (more efficiently, in both SQL Server and MySQL) as:

INSERT INTO ElderCustomers (FirstName, Age)
SELECT FirstName, Age
  FROM Customers
 WHERE Age > 60

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