How to write a loop statement in SQL?

In SQL, there are no loop statements like ‘for’ or ‘while’ in other programming languages, but loops can be simulated using cursors and conditional statements. Here is an example of how to simulate a loop using cursors and conditional statements:

-- 创建游标
DECLARE @id INT
DECLARE @name VARCHAR(50)

DECLARE cursor_name CURSOR FOR
SELECT id, name FROM your_table

-- 打开游标
OPEN cursor_name

-- 获取第一行数据
FETCH NEXT FROM cursor_name INTO @id, @name

-- 循环处理数据
WHILE @@FETCH_STATUS = 0
BEGIN
    -- 在这里编写循环代码
    -- 使用变量 @id 和 @name 来处理当前行的数据

    -- 获取下一行数据
    FETCH NEXT FROM cursor_name INTO @id, @name
END

-- 关闭游标
CLOSE cursor_name
DEALLOCATE cursor_name

In a loop, use @@FETCH_STATUS to check if the cursor has more rows to process. If @@FETCH_STATUS is 0, it means there are more rows to fetch, otherwise there are no more rows left. In the loop, variables can be used to handle the data from the current row. In this example, @id and @name variables are used to process each row of the query result set.

It’s important to note that loop operations in SQL are typically executed on the database server rather than in the application itself. Therefore, when writing SQL loops, make sure they can handle large amounts of data without causing performance issues. In most cases, you can improve query performance by using set operations, subqueries, or recursive queries instead of loops.